Why new tires change your speedometer
Your speedometer counts how fast the wheels (or the driveshaft) turn, then converts that to speed using the circumference of the original tire. A taller tire covers more ground on every turn, so you travel further and faster than the dial shows. A shorter tire does the opposite. The odometer counts the same turns, so it is off by the same factor.
What to enter
- Old tire size: the size the speedometer is calibrated for, usually the one on the door jamb placard.
- New tire size: the size you now run. Metric (285/75R17) and inch sizes (35x12.50R20) both work.
- Axle ratios (optional): only if you also changed ring and pinion gears and your speedometer takes its signal before the axle, for example from the transmission output. If the vehicle reads speed from the wheels, a gear change alone does not move the speedometer. Check how your vehicle measures speed.
The formula
real speed = shown speed × (new diameter ÷ old diameter) × (old ratio ÷ new ratio)
speedometer error = (1 ÷ factor − 1) × 100%Leave the ratios empty and the last part is 1.
Worked example
From 265/70R17 (31.61 in) to 285/75R17 (33.83 in). The factor is 33.83 ÷ 31.61 = 1.0704. When the speedometer shows 60 mph you are doing 64.2 mph. The speedometer reads 6.6% low, and for every 100 miles on the odometer you really drove 107.0 miles. The other way round: at a real 65 mph the dial shows 60.7 mph, and at a real 70 mph it shows 65.4 mph.
Regearing brings it back
Going from 245/75R16 (30.47 in) to 285/75R16 (32.83 in) makes the speedometer read 7.2% low. If the axle gears change from 3.73 to 4.10 at the same time and the speedometer reads before the axle, the factor becomes 32.83 ÷ 30.47 × 3.73 ÷ 4.10 = 0.980. At an indicated 60 mph you are then doing 58.8 mph, and the speedometer reads 2.0% high.
How to fix the reading
Many vehicles can be recalibrated for a new tire size with a handheld programmer or at a dealer; older ones use a different speedometer drive gear. Which method applies depends on your vehicle, so check the service information for it. Until then, use the tables above, or a phone GPS, to know your real speed.
Questions
How much will bigger tires throw off my speedometer?
By the ratio of the diameters. Going from 265/70R17 (31.61 in) to 285/75R17 (33.83 in), the speedometer reads 6.6% low: 60 mph on the dial is 64.2 mph.
Is a speedometer reading low or high after bigger tires?
Low. A taller tire covers more ground per turn, so your real speed is higher than the reading, and the odometer counts fewer miles than you drive.
What does my speedometer show at a real 70 mph?
It depends on the change. After 265/70R17 to 285/75R17, the dial shows 65.4 mph at a real 70 mph. Enter your sizes to see yours.
Do new axle gears fix the speedometer?
Only if the speedometer reads speed before the axle. Then a lower (numerically higher) ratio makes it read higher: 245/75R16 to 285/75R16 with 3.73 to 4.10 gears leaves it reading 2.0% high.