155/70R18 vs 155/65R18
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 | 155/70R18 | 155/65R18 | Change |
|---|---|---|---|
| Overall height | 26.5" | 25.9" | -2.3 % |
| Sidewall | 4.27" | 3.97" | -7.1 % |
| Section width | 155 mm | 155 mm | 0.0 % |
| Circumference | 83.4" | 81.5" | -2.3 % |
| Revs per mile | 760 | 778 | +2.3 % |
| Axle height | 13.27" | 12.97" | -7.8 mm |
| Approved rim width | 4–5.5" | 4–5.5" | overlap 4–5.5" |
Both are approved on a rim 4" to 5.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 155/70R18. Fit 155/65R18 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 9,770 on the road, so it over-counts by 230. At a true 70 mph the first turns 886 times a minute, the second 907.