225/65R17 vs 245/40R18
| Overall diameter-2.8″ (-9.82%) | 28.52″ → 25.72″ |
|---|---|
| At an indicated 60 mphyou are going slower than the dash says | 54.1 mph |
| Within the ±3% ruleABS, traction control and the odometer all read off this | No |
| Ground clearancehalf the diameter change — the axle moves, not the whole tyre | -1.4″ |
| Section width+0.79″ — check clearance at full lock | 8.86″ → 9.65″ |
| Sidewall heightless sidewall — firmer ride, more wheel damage risk on potholes | 5.76″ → 3.86″ |
| Revolutions per mile77 per mile | 707 → 784 |
| Rim diameterdifferent wheel required | 17″ → 18″ |
| Odometer over 10,000 mierror accumulated on the reading | 1,088 mi |
- Current size
- 225/65R17
- New size
- 245/40R18
The one number that matters
Overall diameter moves -9.82% — from 28.52″ to 25.72″. Everything else on this page follows from that.
The industry works to a ±3% window, and this change falls outside it. Inside 3%, the speedometer, odometer, ABS and stability control all keep working off their original calibration. Outside it, they do not.
Your speedometer will lie, and in a specific direction
A speedometer counts wheel rotations. It has no idea what tyre is fitted. At an indicated 60 mph you would actually be doing 54.1 mph — 5.9 mph fast.
Revolutions per mile go from 707 to 784, which is the same fact stated the way tyre manufacturers state it.
Clearance changes by half, not all
The tyre grows at the top and at the bottom; only the top half lifts the axle. Ground clearance moves -1.4″.
This is the point most lift-and-tyre discussions get muddled, because the tyre diameter change gets quoted as if all of it went into ride height.
Geometry only
No manufacturer fudge factors here — just the arithmetic of the sidewall. Real moulded sizes vary by a few millimetres between brands, and a worn tyre is up to ⅜″ smaller in diameter than a new one. Both of those are smaller than the difference this page is showing you.
Notes
- 9.82% is outside the ±3% window. Beyond that the speedometer error becomes a real number, the odometer under-reads, and on many vehicles the transmission shift points and stability control are calibrated to the original rolling diameter. Budget for a recalibration.
- The speedometer reads off wheel rotation. A shorter tyre covers less ground per turn, so at an indicated 60 mph you are actually doing 54.1 — 5.9 mph slower.
- Ground clearance changes by half the diameter change, not all of it. The tyre grows at the top and the bottom; only the top half lifts the axle. -1.4″ here.
- 0.79″ of extra section width is where fitment problems usually start — not at ride height, but at full steering lock and full suspension compression. Check both before buying, and remember section width is the widest point of the sidewall, not the tread.
- Geometry only — no manufacturer fudge factors. Real moulded sizes vary by a few millimetres between brands, and a worn tyre is smaller than a new one by up to ⅜″ in diameter.