Speedometer error calculator
Changing tire size changes how far you travel with every turn of the wheel, so the speedometer is no longer right. Enter your original tire size and the new one to see each tire's overall diameter and revolutions per mile, the percent error, and what your real speed is at each speedometer reading.
Not sure where to start? Try 265/70R17 changed to 285/70R17.
Why a different tire size changes the speedometer
A speedometer does not measure speed directly. It counts how fast the wheels turn and multiplies that by the distance one turn covers on the tire the vehicle was built with. Put on a tire with a different circumference and every turn covers a different distance, so the reading is off by the same percentage as the change in overall diameter.
A larger tire reads slow. A taller tire travels farther on each turn, so you are going faster than the dial says. A smaller tire does the opposite and reads fast. The odometer counts turns in the same way, so a larger tire under-counts the miles you have driven and a smaller one over-counts them.
Going from 265/70R17 (31.6 in tall) to 285/70R17 (32.7 in tall) makes the tire 3.49% taller. When the speedometer shows 60 mph you are really going about 62.1 mph, and after 100 miles on the odometer you have really driven about 103.5 miles.
The formula is simple: actual speed equals the speedometer reading times the new overall diameter divided by the original overall diameter. Overall diameter is the wheel diameter plus two sidewalls, and a sidewall is the tire's width times its aspect ratio. The tire size calculator shows the full set of dimensions for any two sizes, and the guide to reading a tire sidewall explains where each number comes from.
The ±3% guideline, and what it leaves out
A common general guideline is to keep a replacement tire's overall diameter, and so the speedometer error, within about ±3% of the original. It is a rule of thumb, not a law and not a safety check. Some vehicle makers allow less, and the legal limits on speedometer accuracy vary by region.
The guideline covers diameter only. It does not cover whether the new size fits the wheel and the wheel well, whether its load index is high enough, or whether the vehicle's stability control and all-wheel-drive system are calibrated for it. The guide to plus sizing wheels and tires walks through those checks.
Many factory speedometers are built to read a little high rather than low, so your real error can differ slightly from these figures, which assume an accurate dial. For a real-world check, compare your speedometer with a GPS speed reading on a steady stretch of road. If you run different sizes front and rear, use the staggered tire size checker.
Speedometer and tire size questions
Does a bigger tire make my speedometer read slow?
Yes. A taller tire has a longer circumference, so it covers more ground on each turn. The speedometer counts turns as if the tire were still the original size, so the number on the dial is lower than your real speed. A shorter tire does the opposite and makes the speedometer read high.
How do I calculate speedometer error from tire size?
Work out the overall diameter of each size (the wheel diameter plus two sidewalls), then divide the new diameter by the original. Multiply the speed on the dial by that ratio to get your actual speed. For 265/70R17 changed to 285/70R17 the ratio is about 1.035, so 60 mph on the dial is about 62.1 mph for real.
Does the odometer change too?
Yes, by the same percentage. The odometer counts wheel turns just like the speedometer, so a taller tire makes it under-count the distance you have driven and a shorter tire makes it over-count. That affects trip mileage, fuel economy figures and service intervals.
How much speedometer error is too much?
There is no single rule. A common general guideline is to keep the overall diameter, and so the speedometer error, within about 3% of the original. It is a guideline rather than a law or a safety check, and some vehicle makers are stricter. Speed limits are enforced against your actual speed, so a speedometer that reads low can put you over the limit without you noticing.
Can I fix the speedometer after changing tire size?
Often, yes. Many vehicles let a dealer or a specialist reprogram the speedometer for a new tire size, and some aftermarket devices do the same job. It depends on the vehicle. Ask before you buy if you plan to go well beyond the guideline.
Does tire wear change the speedometer reading?
A little. A tire loses up to about half an inch of overall diameter between new and worn out, so the same tire reads slightly different when new than when worn. The calculator uses nominal new-tire dimensions, and your GPS speed is the best check of what your speedometer really shows.