Tire Size, Speedometer & Gearing
What a tire size change does to your speedometer, odometer, clearance and effective axle ratio — with the regear that puts it back.
Both formats work — metric like 285/75R16 and flotation like 33x12.50R15, with or without an LT or P prefix.
What the speedometer shows against what you are doing
| Speedometer reads | You are actually doing | Difference |
|---|
Getting the gearing back
Bigger tires make the needle lie low
A speedometer counts driveshaft or wheel revolutions and multiplies by whatever rolling distance the factory assumed. Fit a taller tire and each revolution covers more ground, so you are travelling faster than the needle claims — which is the direction that collects tickets rather than the one that protects you from them.
The odometer under-counts by the same mechanism, and note that the two percentages are not the same number: the speedometer error is the diameter ratio, and the odometer shortfall is its reciprocal. Service intervals arrive later than they should, and a vehicle sold on indicated mileage has done more than the dash says.
Things worth knowing
- Lift is HALF the change in diameter, because the axle sits at the centre of the wheel. Going from 31 to 33 inch tires is one inch of clearance, not two.
- A bigger tire makes the speedometer under-read, so you are going faster than the needle claims. That is the direction that collects tickets, and it also means the odometer under-counts the miles between services.
- Bigger tires are taller gearing whether you wanted it or not. A 10% larger diameter is a 10% taller final drive, which is why a regear so often follows a tire upgrade rather than preceding it.
- Flotation sizes state the diameter outright — a 33x12.50R15 is 33 inches. Metric sizes hide it, and 285/75R16 works out at 32.8, so the two are closer than the numbers suggest.
- Tires run slightly smaller than their stated size once they are loaded and worn. Treat every diameter here as the unloaded figure, because that is what the size code describes.
How to use
- Enter your current and new tire sizes in either metric or flotation format.
- Set your axle ratio and the rpm you see at 70 mph now.
- Read the speedometer error and the ratio that restores your original gearing.
Frequently asked questions
Does the speedometer read high or low with bigger tires?
Low. Each wheel revolution covers more ground than the factory assumed, so you are travelling faster than the needle says. That is the direction that collects speeding tickets rather than protecting you from them.
How much lift do bigger tires give?
Half the change in diameter, because the axle sits at the centre of the wheel. Going from 31 to 33 inch tires is one inch of clearance, not two.
What gears do I need for 35 inch tires?
Multiply your current ratio by the diameter increase. A truck on 3.73s and 31s needs 4.21 to keep the same effective gearing on 35s, and 4.30 is the nearest ratio that is actually manufactured.
Is a 285/75R16 the same as a 33?
Very nearly — 32.83 inches against 33. Metric sizes hide the diameter in the sidewall percentage while flotation sizes state it outright, which makes the two look further apart than they are.
Does the odometer change too?
Yes, and not by the same percentage. The speedometer error is the diameter ratio and the odometer shortfall is its reciprocal, so they differ slightly. Either way you cover more miles than the dash records, and service intervals arrive later than they should.
Do I have to regear?
Not have to, but a 10 per cent larger tire is a 10 per cent taller final drive whether you wanted it or not. Whether that matters depends on how much torque you had spare — which is why a regear so often follows a tire upgrade rather than preceding it.
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