275/40R21 vs 275/45R21 at a glance
| Measure | 275/40R21 (current) | 275/45R21 (new) | Difference |
|---|---|---|---|
| Overall height | 29.66 in | 30.74 in | +1.08 in (+3.65%) |
| Width | 10.83 in (275 mm) | 10.83 in (275 mm) | +0 mm |
| Sidewall | 4.33 in | 4.87 in | +0.54 in |
| Wheel | 21 in | 21 in | +0 in |
| Circumference | 93.2 in | 96.6 in | +3.4 in |
| Revs per mile | 679.9 | 656.0 | −23.9 |
Verdict: check clearance and speedometer
Over 3% and up to 5%: worth checking clearance, the speedometer and gearing before you buy. Going from 275/40R21 to 275/45R21 is a 3.7% increase in overall height (+1.08 in), and the vehicle sits about 0.54 in higher. The tire is the same width. The 3% rule of thumb comes from Discount Tire and 1010tires; it is not a guarantee of fit.
True speed and odometer
| Speedometer shows | You are really doing | Difference |
|---|---|---|
| 35 mph | 36.3 mph | +1.3 mph |
| 45 mph | 46.6 mph | +1.6 mph |
| 55 mph | 57.0 mph | +2.0 mph |
| 60 mph | 62.2 mph | +2.2 mph |
| 65 mph | 67.4 mph | +2.4 mph |
| 70 mph | 72.6 mph | +2.6 mph |
| 75 mph | 77.7 mph | +2.7 mph |
Odometer: 1,000 indicated miles is 1,037 real miles, and 10,000 is 10,365.
The math
275/40R21: 21 + 2 × (275 × 40% ÷ 25.4) = 29.66 in. 275/45R21: 21 + 2 × (275 × 45% ÷ 25.4) = 30.74 in. Change = 30.74 ÷ 29.66 − 1 = +3.65%. True speed at 60 = 60 × 30.74 ÷ 29.66 = 62.19 mph.
Check next
- Regear and RPM at cruise for this swap
- Speedometer and odometer correction
- Other sizes within ±3% of 275/40R21
Calculated from the size alone. Real tires vary a little by brand and model, and the loaded radius is a bit smaller. Confirm the wheel width range, clearance, load index and speed rating for your vehicle. See how to read a tire size, the load index chart and the speed rating chart.
Size pages: 275/40R21 · 275/45R21. More swaps: all tire size comparisons.