235/80R18 vs 255/70R18
Going from one to the other changes the overall height by -2.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/80R18 | 255/70R18 | Change |
|---|---|---|---|
| Overall height | 32.8" | 32.1" | -2.3 % |
| Sidewall | 7.40" | 7.03" | -5.1 % |
| Section width | 235 mm | 255 mm | +8.5 % |
| Circumference | 103.1" | 100.7" | -2.3 % |
| Revs per mile | 615 | 629 | +2.3 % |
| Axle height | 16.40" | 16.03" | -9.5 mm |
| Approved rim width | 6.5–8.5" | 7–9.5" | overlap 7–8.5" |
Both are approved on a rim 7" to 8.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 235/80R18. Fit 255/70R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.9 mph | -1.1 |
| 70 mph | 68.4 mph | -1.6 |
| 90 mph | 87.9 mph | -2.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,772 on the road, so it over-counts by 228. At a true 70 mph the first turns 717 times a minute, the second 734.
What drivers reported
Nobody has filed a complaint naming 235/80R18. 255/70R18 carries 23.