255/30R22 vs 305/30R21
305/30R21 is a safe swap for 255/30R22 by the usual ±3% rule: the diameter differs by +0.65%.
Speedometer and odometer
With 305/30R21 on a car calibrated for 255/30R22, the speedometer reads 59.6 mph when you're really doing 60, and the odometer under-counts by 0.6%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.2 mph |
| 45 mph | 45.3 mph |
| 60 mph | 60.4 mph |
| 70 mph | 70.5 mph |
| 80 mph | 80.5 mph |
Other sizes close to 255/30R22
Questions
Can I replace 255/30R22 with 305/30R21?
305/30R21 is a safe swap for 255/30R22 by the usual ±3% rule: the diameter differs by +0.65%.
What's the difference between 255/30R22 and 305/30R21?
305/30R21 is 0.18" taller and 50 mm wider; the sidewall is +0.59". At an indicated 60 mph you'd actually be doing 60.4 mph.
Will 305/30R21 fit on 255/30R22 wheels?
No — 255/30R22 needs a 22" wheel and 305/30R21 a 21" wheel. This is a plus/minus-size swap that changes wheels too.