Fuel economy: what maintenance buys, and the figures that were withdrawn

Search for fuel-saving tips and you'll meet the same numbers again and again. A new air filter gives you 10 per cent. Correct tyre (tire) pressures give you 3 per cent. Fixing a faulty oxygen sensor gives you 40 per cent. They sound official, and they were: each one was printed by fueleconomy.gov, the US government's fuel economy site. Each one was later withdrawn or cut, and you can date when. Here you'll see what maintenance actually buys in fuel on the current page, and learn to date a figure and tell a best case from an average, which is useful well beyond cars.

Most of this lesson's figures are American, because the source that has measured and revised them is American, and they're in US units (miles per gallon, US gallons, dollars). One section states Great Britain's law on leaving an engine running. That section opens with a short form of this label: This is general information, not legal advice. The rules differ between England and Wales, Scotland, Northern Ireland and each US state, and they change.

One page, read over nineteen years

fueleconomy.gov is run for the US Department of Energy and the Environmental Protection Agency (EPA) and administered for them by Oak Ridge National Laboratory. Its page "Keeping Your Vehicle in Shape" gives a figure for what each kind of maintenance does for fuel economy. This course read the current page and six copies of it saved by the Internet Archive between December 2007 and July 2023.¹² The page said this, capture by capture:

Sources: ¹². The dates are when the Internet Archive saved each copy; a change happened somewhere between two captures, except the tyre revision, which the page itself dates.

Four of the five figures fell or disappeared, in three changes. Only the oil grade stayed.

The air filter: 10 per cent, then none

In December 2007 the page said: "Replacing a clogged air filter can improve your car's gas mileage by as much as 10 percent."² By December 2010 it carried a heading, "NEW INFORMATION", and a different answer: "A new study shows that replacing a clogged air filter on cars with fuel-injected, computer-controlled gasoline engines does not improve fuel economy but it can improve acceleration time by around 6 to 11 percent."² (Gasoline is petrol.) The study it named was from Oak Ridge in 2009.² For older cars with a carburettor, the older way of mixing fuel and air, the 2010 page said a new filter "may improve fuel economy 2 to 6 percent under normal replacement conditions or up to 14 percent if the filter is so clogged that it significantly affects drivability." And it added: "The effect of a clogged air filter on diesel vehicles will be tested in the near future."²

Today's page includes diesels in the no-gain line: replacing a clogged filter on fuel-injected, computer-controlled petrol engines, "such as those manufactured from the early 1980s to the present", or on diesel engines, "does not improve fuel economy, but it can improve acceleration."¹

Why would a filter matter to one kind of engine and not the other? fueleconomy.gov's myths page gives the reason: modern fuel-injected engines "have on-board computers that automatically adjust the fuel-air ratio to the proper level."³ The page does not take the explanation further, and nor does this course. A carburettor is the case where fueleconomy.gov says the filter still affects fuel economy.¹³

Tyre pressure: cut more than fivefold

In 2007 the page said correct pressures could improve mileage "by around 3.3 percent", and that under-inflated tyres lowered it "by 0.4 percent for every 1 psi drop in pressure of all four tires" (1 psi is about 0.07 bar). The 2010 and 2013 pages said "up to 3.3 percent" and 0.3 per cent per psi.² Today's page says: "You can improve your gas mileage by 0.6% on average" (it goes on, "up to 3% in some cases"), and "Under-inflated tires can lower gas mileage by about 0.2% for every 1 psi drop in the average pressure of all tires."¹ A footnote dates the change: "Revised June 27, 2016."¹

The headline figure went from about 3.3 to 0.6, a cut of about five and a half times (this course's arithmetic). The old figure didn't vanish. It became the top of the new range.

Tune-up and the oxygen sensor: gone without a word

From 2007 to 2021 the page carried these two sentences (quoted here as in 2007; later copies write % for percent): "Fixing a car that is noticeably out of tune or has failed an emissions test can improve its gas mileage by an average of 4 percent, though results vary based on the kind of repair and how well it is done. Fixing a serious maintenance problem, such as a faulty oxygen sensor, can improve your mileage by as much as 40 percent."² The capture of 8 July 2021 still has both sentences. The capture of 6 July 2023 doesn't; in their place is a paragraph headed "Address Engine Issues Promptly", about the check-engine light, which lesson 11, "Dashboard warning lights", quoted.² None of the pages read says why the figures went, so this course does not guess. You may still meet the 40 per cent figure, and it was never an average: the page itself said "as much as".

Oil grade: the one that stayed

"You can improve your gas mileage by 1%–2% by using the manufacturer's recommended grade of motor oil", the page says now, and it said the same in 2007. Its example: using 10W-30 "in an engine designed to use 5W-30 can lower your gas mileage by 1%–2%."¹² The current page gives a second: "Using 5W-30 in an engine designed for 5W-20 can lower your gas mileage by 1%–1.5%."¹ The grade is in your handbook; checking the oil level is lesson 9's subject, "Under the bonnet: oil, coolant, brake fluid and screenwash".

Best case and average

Look at how the current page words the tyre figure: 0.6 per cent "on average", up to 3 per cent "in some cases". The footnote gives the assumption behind each. The estimates "assume a vehicle with an average under-inflation rate of 10% across all tires (25% for worst-case tire inflation scenarios)."¹ So 0.6 per cent is what correcting the average amount of under-inflation buys, and 3 per cent is what correcting a car a quarter under buys.

That tells you something the headline doesn't. Both figures are savings from fixing under-inflation. A car whose tyres are already at the placard figure has nothing to fix, and on this course's reading of that footnote, checking them saves no fuel at all, though it's still the check lesson 7, "Tyres I: pressure, the placard and TPMS", said to do, for other reasons.

Now put a second government source beside it. NHTSA, the US road safety regulator, says: "Properly inflating your tires can save you as much as 11 cents per gallon on fuel."⁴ fueleconomy.gov, from the same government, puts the average benefit at "$0.03/gallon".¹ The two don't contradict each other. "As much as" is a best case; "on average" is an average. When you meet a figure, the first question is which kind it is, and the second is what it assumes.

The dollar figures move for another reason, too. Every version of the page converts its percentages into money at an assumed fuel price, and the assumption changes: $3.10 a gallon in 2007, $2.87 in 2010, $3.25 in 2013, $2.37 in 2019, $3.09 in 2021, $3.58 in 2023, and $4.32 now.¹² A saving in cents per gallon is a percentage multiplied by a price, and it's only as current as the price.

A year's saving, worked

The arithmetic for one car follows. It is this course's, with the assumptions stated.

A driver in Mesa does 12,000 miles a year in a car that averages 30 miles per gallon. That's 400 gallons a year (12,000 divided by 30). Suppose her tyres are, like fueleconomy.gov's average car, about 10 per cent under-inflated, and she starts keeping them at the placard figure.

• 0.6 per cent of 400 gallons is 2.4 gallons.
• At the page's assumed $4.32 a gallon, 2.4 gallons is about $10 a year.

The page's own table rounds the saving to $0.03 a gallon, which over 400 gallons would be $12. The difference is rounding: 0.6 per cent of $4.32 is about 2.6 cents, and the table shows 3.¹

If her tyres had been a quarter under, the worst case, the saving would be up to 3 per cent: about 12 gallons, roughly $50 a year at the same price. That is real money, and it's also a car whose tyres were a quarter under their placard figure, which is the better reason to fix them.

Now a UK case with a gap. A driver in Wellingborough spends, say, £1,500 a year on fuel (this course's round number, not a price). He reads the 0.6 per cent figure and decides pressures aren't worth checking, because the saving is small.

A figure in the wild

The other skill is dating a claim. You see a post that says: "Changed my air filter, got 10% better mpg!" The car is a 2015 hatchback.

When the air filter is changed is your handbook's service schedule, not a fuel-saving decision. This course does not teach changing filters, for the reasons lesson 1 gave.

Octane: the handbook's number

Octane is the other place people spend money for fuel economy. fueleconomy.gov's octane page defines it: "Octane rating is the measure of a fuel's ability to resist "knocking" or "pinging" during combustion, caused by the air/fuel mixture detonating prematurely in the engine."⁵ Lesson 2, "The engine: petrol, diesel, and the drive to the wheels", showed the compression stroke. The page doesn't explain how knock happens beyond that definition, and nor does this course.

In the US, the page says, regular is typically 87, midgrade 88 to 90 and premium 91 to 94. Those are US pump numbers. This course read no official UK page on octane, so it gives no UK figures; match your handbook's requirement to what the pump says.

What the page says to use: "You should use the octane rating required for your vehicle by the manufacturer. So, check your owner's manual. Most gasoline vehicles are designed to run on 87 octane, but others are designed to use higher octane fuel."⁵ It gives the reason some engines need more: they squeeze the mixture harder (a higher compression ratio), or they have a turbocharger or supercharger, which the page says forces "more air into the engine". And "Increasing pressure in the cylinder allows an engine to extract more mechanical energy from a given air/fuel mixture but requires higher octane fuel to keep the mixture from pre-detonating."⁵

Going below what the maker requires is the costly mistake. The page: "Using a lower octane fuel than required can cause the engine to run poorly and can damage the engine and emissions control system over time. It may also void your warranty." Many newer vehicles can adjust their spark timing to reduce knock, the page says, "but engine power and fuel economy will still suffer."⁵

Going above it mostly buys nothing: "under normal driving conditions, you may get little to no benefit." The page distinguishes a handbook that requires premium from one that only recommends it: "If your vehicle requires midgrade or premium fuel, absolutely." If the manual says premium isn't required but the car will run better on it, "it's really up to you. The cost increase is typically higher than the fuel savings."⁵ The myths page adds the exception: under severe duty, "such as towing or hauling heavy loads (especially in hot weather), higher octane fuel might improve performance and gas mileage and reduce carbon dioxide (CO2) emissions by a few percent".³

Which fuel to buy is Yours, on conditions, and the condition is the handbook's word, required or recommended.

Warming up, idling and restarting

Many people let the engine idle before setting off, especially on a cold morning. fueleconomy.gov's myths page answers that directly: "Modern vehicles can be driven within seconds of being started, though the engine should not be subjected to extreme loads until it has reached its normal operating temperature. Plus, the quickest way to warm up a vehicle's engine is to drive it."³ Its cold-weather page puts a number on it, attributed to the makers: "Most manufacturers recommend driving off gently after about 30 seconds."⁶ Your handbook may give its own advice, and it governs.

The second belief is that restarting burns more fuel than idling. The myths page: "Modern fuel-injected engines start very efficiently, especially when warmed up. Idling can use a quarter to a half gallon of fuel per hour, depending on your vehicle's engine size, costing you about 1 to 2 cents per minute."³ Its driving page adds: "It only takes about 10 seconds worth of fuel to restart your vehicle."⁷ The advice follows: "Turn off your engine when your vehicle is sitting still, except when you are waiting in traffic or waiting in a line where you would need to turn it on and off frequently."³

The UK's Energy Saving Trust, an independent organisation funded largely by government contracts, gives similar advice with a shorter trigger: "Many newer cars automatically turn off when stationary in neutral. If yours doesn't, turn off your engine if you're going to be stationary for more than a minute."⁸ (This course read its page from an Internet Archive capture of July 2026.)

Leaving an engine running in Great Britain: the rule

This is general information, not legal advice.

In Great Britain, idling is also a legal matter. Rule 123 of the Highway Code says, in its MUST line: "You MUST NOT leave a parked vehicle unattended with the engine running or leave a vehicle engine running unnecessarily while that vehicle is stationary on a public road."⁹ That's law; the rule cites regulations 98 and 107 of the Road Vehicles (Construction and Use) Regulations 1986, which apply in England, Wales and Scotland.

The rule's next sentence is a should, which is advice: "Generally, if the vehicle is stationary and is likely to remain so for more than a couple of minutes, you should apply the parking brake and switch off the engine to reduce emissions and noise pollution." And then the exceptions: "However it is permissible to leave the engine running if the vehicle is stationary in traffic or for diagnosing faults."⁹

The regulations say it more fully. Regulation 98 requires a driver, when the vehicle is stationary, to stop its machinery so far as necessary to prevent noise or exhaust emissions, and it doesn't apply when the vehicle is stationary "owing to the necessities of traffic", or where the machinery has to run so that a failure can be examined, or where it has to run for a purpose other than driving the vehicle.¹⁰ Regulation 107 is about a car left on a road with no licensed driver attending it: that isn't allowed "unless the engine is stopped and any parking brake with which the vehicle is required to be equipped is effectively set", with exceptions for fire, police and ambulance use, and for a vehicle safely placed whose engine drives machinery on it.¹¹ Northern Ireland has its own rules, which this course didn't read, and it didn't research US anti-idling rules either.

So there are three voices on idling here, and they line up. The Highway Code's MUST is the law: not unnecessarily. Its should says switch off after a couple of minutes. The Energy Saving Trust says after a minute. fueleconomy.gov says off whenever the car is sitting still, except in traffic or in a line where you'd restart often. No source read explains why one says a minute and another a couple of minutes, and there's no need to choose: all three point the same way. Leaving a car running to defrost on a cold morning comes back in lesson 15, "Winter and summer".

Weight, roof boxes, and windows against air conditioning

Two things you carry cost fuel, and fueleconomy.gov measures both. Weight: "An extra 100 pounds in your vehicle could reduce your MPG by about 1%." (100 pounds is about 45 kg, this course's conversion.) The page adds that the effect is relative to the car's weight and "affects smaller vehicles more than larger ones."⁷ And a roof box: "A large, blunt roof-top cargo box, for example, can reduce fuel economy by around 2% to 8% in city driving, 6% to 17% on the highway, and 10% to 25% at Interstate speeds (65 mph to 75 mph)."⁷ (65 to 75 mph is about 105 to 120 km/h, this course's conversion.) The Energy Saving Trust gives the UK version without numbers: roof racks, boxes and bars "increase drag and fuel costs, especially at higher speeds, so try to take them off when they're not being used."⁸ The roof box's figure is bigger than any maintenance figure the current page gives, and it costs nothing to take off.

The last one is a real disagreement. The Energy Saving Trust: "It's more fuel efficient to open a window than use air conditioning when driving, even at higher speeds."⁸ fueleconomy.gov: "Roll the windows down at lower speeds; use the AC at highway speeds." Its reason: "Open windows increase aerodynamic drag (wind resistance), making your vehicle use more energy to push through the air. This effect is quite small at low speeds but increases at highway speeds."¹² fueleconomy.gov cites two published studies for that, which this course did not read; the Energy Saving Trust's page gives no reason or test. Neither page read gives a figure for how much one beats the other at speed, and this course does not settle it. Both pages agree on one thing. The Energy Saving Trust says open windows "are not always pleasant on very hot days or at higher speeds, so if you need to use air conditioning, try to use it sparingly", and fueleconomy.gov says not to use the AC "more than needed".⁸¹² It is not a rule of any kind.

What people get wrong

"A new air filter saves fuel." Not on a fuel-injected engine, fueleconomy.gov has said since 2010; it can help acceleration. The 10 per cent was its 2007 wording.¹²

"Correct tyre pressures give you 3 per cent." That's the worst case since 2016. The average is 0.6 per cent, and a car already at its placard figure gains nothing.¹

"Fixing an oxygen sensor can improve mileage by 40 per cent." It was an "as much as" figure, and it was dropped between July 2021 and July 2023.²

"Premium fuel is better for any car." Only if the maker requires it, or under severe duty; "little to no benefit" otherwise.³⁵

"Warm the engine up before you drive." "The quickest way to warm up a vehicle's engine is to drive it"; in Great Britain the Highway Code also has a rule on it (see "Leaving an engine running in Great Britain: the rule" above).³⁹

"Restarting uses more fuel than idling." About 10 seconds' worth, says fueleconomy.gov.⁷

"Fuel additives and gadgets improve economy." The myths page: "Excluding full conversions that meet all EPA certification standards, tests have shown that such devices and additives do not improve fuel economy and may damage your engine and/or increase your tailpipe emissions."³ This course recommends no product of any kind.

"An older car's economy just gets worse." "Vehicles that are 10 or even 15 years old will experience little decrease in fuel economy if properly maintained."³

"Change the oil every 3,000 miles." Lesson 1 quoted CalRecycle, California's recycling agency, calling that "out of date", and lesson 24, "Choosing a garage, servicing and warranties", comes back to service intervals.

Practice

For your plan

This lesson adds a fuel page to your car file: the handbook's fuel requirement in its own words (required or recommended, and the grade), the oil grade, and one line on what the tyre pressure check is for, which is mainly safety and tyre life, with a little fuel. If you carry a roof box, add a line about taking it off between trips.

Connections

Lesson 2, "The engine: petrol, diesel, and the drive to the wheels", has the compression stroke behind knock. Lesson 7, "Tyres I: pressure, the placard and TPMS", is where the placard figure and the check come from, and lesson 9 covers the oil. Lesson 11, "Dashboard warning lights", covers the check-engine light that replaced the tune-up paragraph. Lesson 15, "Winter and summer", comes back to fuel economy in the cold and to idling to defrost, and lesson 24, "Choosing a garage, servicing and warranties", to service intervals.

Go deeper

• fueleconomy.gov, "Keeping Your Vehicle in Shape", free. Read here in full. Short, and its data sources are listed under the figures; worth reading with this lesson's table beside it.
• fueleconomy.gov, "Fuel Economy Myths and Misconceptions", free. Read here in full. Eleven common beliefs, each with a one-paragraph government answer.
• fueleconomy.gov, "Selecting the Right Octane Fuel", free. Read here in full. Explains what octane is and when it matters, with US pump numbers throughout.
• The Internet Archive's December 2007 copy of the maintenance page, free. Read here in full. Put it beside today's page and read them together. It shows more plainly than anything else in this lesson how official advice changes.

Sources

1. fueleconomy.gov (US Department of Energy and EPA, administered by Oak Ridge National Laboratory), "Keeping Your Vehicle in Shape", undated; tyre estimates "Revised June 27, 2016". Read: full body text, including the data sources, re-read 26 September 2026.
2. The same page as saved by the Internet Archive: captures 20071226033438, 20101226073534, 20130102005358, 20190104073529, 20210708122652 and 20230706032230. Read: the maintenance section of each capture, re-read 26 September 2026. The study the 2010 page names, from Oak Ridge in 2009, was not read.
3. fueleconomy.gov, "Fuel Economy Myths and Misconceptions", undated (it refers to the 2023 model year). Read: full body text, re-read 26 September 2026.
4. NHTSA, "Tires", read from Internet Archive capture 20260924154655 (nhtsa.gov refuses automated readers). US federal. Read: full page from the capture, re-read 26 September 2026. The live page is linked.
5. fueleconomy.gov, "Selecting the Right Octane Fuel", undated. Read: full body text, re-read 26 September 2026. The studies it lists were not read.
6. fueleconomy.gov, "Fuel Economy in Cold Weather", undated. Read: full body text, re-read 26 September 2026.
7. fueleconomy.gov, "Driving More Efficiently", undated. Read: full body text, re-read 26 September 2026. Only its maintenance-adjacent items are used; driving technique is outside this course.
8. Energy Saving Trust, "Reduce emissions through efficient driving", read from Internet Archive capture 20260731174146 (the live site refuses automated readers). UK; an independent organisation funded largely by government contracts. Read: full text from the capture, re-read 26 September 2026. The live page is linked.
9. The Highway Code, "Rules for drivers and motorcyclists: general rules, techniques and advice for all drivers and riders (103 to 158)", Rule 123, updated 17 September 2023. Great Britain. Read: full text of the page, re-read 26 September 2026 through the GOV.UK content API.
10. The Road Vehicles (Construction and Use) Regulations 1986 (SI 1986/1078), regulation 98, as amended. England, Wales and Scotland. Read: the whole regulation on the live page, 26 September 2026.
11. The Road Vehicles (Construction and Use) Regulations 1986, regulation 107, as amended. England, Wales and Scotland. Read: the whole regulation on the live page, 26 September 2026.
12. fueleconomy.gov, "Fuel Economy in Hot Weather", undated. Read: full body text, re-read 26 September 2026. The studies it cites were not read.