Bigger Tires: Speedometer, Odometer and MPG
Updated 8 November 2026. Every figure on this page comes from the same formulas as our calculators.
Fitting taller tires changes more than the look. Your speedometer, odometer and the fuel economy you calculate all depend on the tire size the car was set up for. This guide shows what changes and by how much.
Why the speedometer reads low
The speedometer does not measure speed directly. It counts how fast the wheels (or the gearbox output) turn and multiplies by the distance one turn of the original tire covers. A taller tire covers more ground per turn, so at the same wheel speed you are going faster than the speedometer shows.
The error is the same percentage as the change in overall diameter.
Example: 265/65R18 to 275/70R18 and 275/75R18
Many full-size pickups come on 265/65R18 (31.56 in tall). Two popular upgrades:
| Size | Diameter | Change | Speedometer shows 60 mph |
|---|---|---|---|
| 265/65R18 (stock) | 31.56 in | — | 60 mph |
| 275/70R18 | 33.16 in | +5.05 % | 63.0 mph |
| 275/75R18 | 34.24 in | +8.48 % | 65.1 mph |
With 275/70R18, an indicated 30 mph is really 31.5 mph, and an indicated 60 is 63.0. See the full speed table.
The odometer under-counts
The odometer uses the same wheel-turn count, so it under-records distance by the same percentage. With 275/70R18 on a truck set up for 265/65R18, 1,000 miles on the odometer is about 1,050.5 miles actually driven. Over a year that adds up for service intervals, warranties and mileage logs.
Your MPG looks worse than it is
If you work out fuel economy from the odometer, the miles are under-counted and the result is too low. A truck that seems to do 20 MPG on 275/70R18 is really doing about 21.0 MPG over the distance actually covered. The tire size and fuel economy calculator corrects the figure for any pair of sizes.
That correction only fixes the measurement. Bigger tires are usually heavier and often have more rolling resistance and an aggressive tread, so real fuel use can still go up even after the correction.
Gearing and engine speed
A taller tire also acts like a taller final drive: the engine turns more slowly at a given road speed, and the truck can feel slower to accelerate. The gear ratio calculator shows the RPM change and which axle ratio would bring it back.
How much is too much?
- Tire fitters often use ±3 % of overall diameter as a rule of thumb for a size change that needs no other work. It is a guideline, not a law.
- Bigger changes can affect ABS, stability control, transmission shift points and clearance, depending on the vehicle.
- Many vehicles can have the tire size reprogrammed by a dealer or with a programmer, which corrects the speedometer and odometer. Whether yours can depends on the make and model.
- A taller tire also moves the top of the tire closer to the fender and liner. Check for rubbing at full steering lock and with the suspension compressed.