Speedometer difference
With a speedometer calibrated for the 265/70R18, this is your real speed on 275/65R18 tires.
| Speedometer shows | Actual speed with 275/65R18 | Difference |
|---|---|---|
| 20 mph | 19.7 mph | −0.3 mph |
| 30 mph | 29.5 mph | −0.5 mph |
| 40 mph | 39.3 mph | −0.7 mph |
| 50 mph | 49.2 mph | −0.8 mph |
| 60 mph | 59.0 mph | −1.0 mph |
| 70 mph | 68.9 mph | −1.1 mph |
| 80 mph | 78.7 mph | −1.3 mph |
Fitment considerations
- Width: the 275/65R18 is 0.39 in wider in section width, so each sidewall sits roughly 0.20 in further out on the same wheel. Check clearance at full steering lock against the upper control arm, frame and fender liner, and confirm your wheel width is inside the tire maker’s approved rim-width range for the 275/65R18.
- Height: the 275/65R18 is shorter, which reduces ground clearance by about 0.27 in and makes the speedometer read high.
- Speedometer: a 1.6% change is small, but it adds up on the odometer: 1,000 miles shown would be about 984 real miles.
- Load rating: match or exceed the load index (and LT load range, if applicable) on your door placard. P-metric and LT tires of the same size are rated differently.
These checks are general; whether a size fits a particular vehicle depends on its suspension, wheels and body. Your door placard lists the approved original sizes, and a tire professional can confirm fitment.
How these numbers are calculated
Diameter = 2 × section width × aspect ratio ÷ 100 ÷ 25.4 + rim diameter (for inch sizes, the first number is the diameter). Revolutions per mile = 63,360 ÷ (π × diameter). Actual speed = indicated speed × 32.07 ÷ 32.61. Real tires vary by brand and model, so treat these as close estimates and use manufacturer specs for exact measurements.
Frequently asked questions
How much bigger is 275/65R18 than 265/70R18?
275/65R18 is 0.53 inches (1.6%) shorter in overall diameter (32.61 in vs 32.07 in) and +0.39 inches in section width.
Will my speedometer be off with 275/65R18 instead of 265/70R18?
Yes, by about 1.6%. If the speedometer is set up for 265/70R18, it will show 60 mph when you are really going about 59.0 mph.
Is 275/65R18 within 3% of 265/70R18?
Yes. The diameter difference is −1.6%.