235/80R18 vs 245/80R18
Going from one to the other changes the overall height by +1.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/80R18 | 245/80R18 | Change |
|---|---|---|---|
| Overall height | 32.8" | 33.4" | +1.9 % |
| Sidewall | 7.40" | 7.72" | +4.3 % |
| Section width | 235 mm | 245 mm | +4.3 % |
| Circumference | 103.1" | 105.0" | +1.9 % |
| Revs per mile | 615 | 603 | -1.9 % |
| Axle height | 16.40" | 16.72" | +8.0 mm |
| Approved rim width | 6.5–8.5" | 7–9" | 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 245/80R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 51.0 mph | +1.0 |
| 70 mph | 71.3 mph | +1.3 |
| 90 mph | 91.7 mph | +1.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,192 on the road, so it under-counts by 192. At a true 70 mph the first turns 717 times a minute, the second 704.