Best Speed for MPG: How Much MPG It Really Costs

Best Speed for MPG is one of those questions that sounds simple until you try to answer it for your own car. The number depends on the car, the journey, the weather and how you drive, so there is no single figure that fits every driver. What this page does instead is give you the method, so you can work out your own figure and know exactly what it means.

Below you will find the formula, a worked example with every step shown, a conversion table, and the factors that push the result up or down. Read the figures on this page about best speed for mpg as a worked illustration, and swap in your own miles, gallons and price to get your own answer. Fuel prices and efficiency figures used in examples are labelled as examples, because they change constantly and differ by place and by car.

The formula — best speed for mpg

The formula is short: MPG = miles driven ÷ gallons used. Drive 320 miles and use 8.9 gallons, and your MPG is 320 ÷ 8.9 = 36 MPG. That is the whole calculation. The skill is in measuring the two inputs honestly. Miles come from the trip meter or odometer, read at the same point in the fill-up cycle each time. Gallons come from the pump: fill the tank, drive, then fill it again, and the gallons that go in the second time are the gallons you used. That fill-to-fill approach is called the brim-to-brim method, and it is the most reliable way to measure real-world MPG without a trip computer. A single tank gives a rough figure; three or four tanks averaged together give a much steadier one, because one short fill or one long motorway run can tilt a single result.

Worked example — best speed for mpg

Suppose the car is used for a week of commuting plus a weekend trip. The trip meter reads 320 miles between fills, and the second fill takes 8.9 gallons. MPG = 320 ÷ 8.9 = 36 MPG. In metric terms that is 6.53 L/100km (US gallon) or 7.85 L/100km (UK imperial gallon). At an example fuel price of $3.45 per gallon, the fuel for that 320-mile stretch costs (320 ÷ 36) × $3.45 = $30.67, which is $0.10 per mile. That per-mile figure is the one worth remembering, because it turns any future journey into a cost: multiply the miles by $0.10 and you have the fuel bill for that trip at this example price. If your next tank gives a different MPG, do not be surprised — one tank is a sample, not a verdict. Log three or four tanks, add all the miles together, add all the gallons together, and divide the totals: that averaged figure is the one to trust.

MPG and fuel cost at a glance — best speed for mpg

The first table converts common MPG figures to litres per 100 km in both gallon types. Read the figures on this page about best speed for mpg as a worked illustration, and swap in your own miles, gallons and price to get your own answer. The second turns the example economy of 36 MPG into fuel costs at three example prices across three distances — replace the example price with the one you pay and the maths is identical.

MPG L/100km (US gallon) L/100km (UK imperial gallon)
20 11.76 14.12
25 9.41 11.30
30 7.84 9.42
35 6.72 8.07
40 5.88 7.06
45 5.23 6.28
50 4.70 5.65
Distance $3.05/gal $3.45/gal $3.85/gal
100 miles $8.47 $9.58 $10.69
250 miles $21.18 $23.96 $26.74
500 miles $42.36 $47.92 $53.47

How this factor moves your MPG — best speed for mpg

What matters about best speed for mpg is not whether it affects MPG in theory, but how much it affects yours in practice. Every factor that adds load, drag or cold running costs fuel, and the cost is proportional to how often it applies. Short journeys in cold weather are the worst combination, because the engine spends a large share of each trip warming up, when it is at its least efficient. Long steady motorway runs are the opposite: the engine is warm, the speed is steady, and small factors matter less. If you want a number rather than an opinion, measure brim-to-brim over several tanks before and after the change, keep everything else the same, and let the averaged figures speak.

Common mistakes — best speed for mpg

The most common mistake is mixing gallons: US and imperial gallons differ by about 20 percent, so a figure converted with the wrong gallon is wrong by the same margin. That is the point to carry into best speed for mpg: the method is the same, but the numbers are yours, so the example here is only a starting point. Next is trusting a single tank — one short fill or one unusual journey can tilt it badly — instead of averaging several. Third is reading the trip meter without resetting it at the fill, which mixes miles from before the fill into the calculation. Fourth is using the tank's capacity instead of the gallons actually pumped; the capacity is not the amount you used. And fifth is comparing a dashboard figure with a brim-to-brim figure as if they were the same measurement: the dashboard estimates from the engine's data, the brim-to-brim method measures what actually went in, and they disagree by a few percent in either direction.

Related calculations — best speed for mpg

If this page answered your question, these pages take the next step — the same method applied to a closely related question, or the calculator that does the arithmetic for you. Read the figures on this page about best speed for mpg as a worked illustration, and swap in your own miles, gallons and price to get your own answer.

Putting your best speed for mpg figure to work

Putting the figure to work is where the method pays off. For best speed for mpg, the same reasoning applies — measure brim-to-brim, average several tanks, and use your own figures rather than a quoted one. Once you know your own MPG, measured brim-to-brim over several tanks, you can price any journey before you set off: multiply the miles by the fuel price divided by your MPG, and the answer is the fuel cost of that trip at that example price. You can also compare cars honestly, because you are comparing measured figures rather than brochure claims. Keep a simple log — date, miles, gallons, price — and review it monthly. Patterns show up quickly: a winter dip, a motorway improvement, a slow decline that points to tyre pressures or a change in routes. The log turns a vague feeling about fuel use into a number you can act on, and that is the whole point of measuring in the first place.

Using your best speed for mpg figure day to day

A useful habit is to record the figure alongside the conditions. Read the figures on this page about best speed for mpg as a worked illustration, and swap in your own miles, gallons and price to get your own answer. Note the kind of driving each tank covered — mostly town, mostly motorway, or a mix — and the season, because both move the result enough to matter. A tank that looks disappointing after a week of short cold trips is not a problem with the car; it is a problem with the comparison. Compare motorway tanks with motorway tanks and winter with winter, and the averaged figure becomes a reliable baseline. Once you have that baseline, a genuine change stands out: a sudden drop after nothing else changed is worth a look at tyre pressures, a roof box left fitted, or a brake that is dragging. The measurement is simple; the discipline of recording it is what makes it useful.

Frequently asked questions

What is a good MPG figure?

It depends on the car and how it is used. That is the point to carry into best speed for mpg: the method is the same, but the numbers are yours, so the example here is only a starting point. A small petrol car on mixed driving, a large SUV and a diesel on motorways all have different sensible ranges, so a single number cannot judge them all. The useful comparison is your car against itself: measure brim-to-brim over several tanks and see whether the averaged figure is stable on similar journeys.

How do I calculate MPG from litres?

Divide the litres by 3.785 to get US gallons or by 4.546 to get imperial gallons, then divide the miles driven by that gallon figure. That is the point to carry into best speed for mpg: the method is the same, but the numbers are yours, so the example here is only a starting point. Say which gallon you used, because the two give MPG figures about 20 percent apart.

Why is my MPG lower than the dashboard says?

The dashboard estimates from engine data and recent driving, while the brim-to-brim method measures the fuel that actually went into the tank. Read the figures on this page about best speed for mpg as a worked illustration, and swap in your own miles, gallons and price to get your own answer. They commonly disagree by a few percent in either direction. Trust the brim-to-brim average over several tanks for planning and expenses.

How many tanks should I average for a reliable MPG?

Three or four tanks is a good minimum. That is the point to carry into best speed for mpg: the method is the same, but the numbers are yours, so the example here is only a starting point. One tank is noisy — a pump clicking off early or one unusual journey can tilt it — while averaging the totals over several tanks smooths those effects out and gives a figure you can rely on.

Does driving slower always improve MPG?

Up to a point. That is the point to carry into best speed for mpg: the method is the same, but the numbers are yours, so the example here is only a starting point. Very slow driving in low gears is inefficient, and very fast driving wastes fuel on aerodynamic drag. Most cars are at their most economical at a steady moderate cruise, with smooth acceleration and as little braking as possible. The exact sweet spot varies by car.