235/40R18 vs 245/45R17
| Overall diameter+0.28″ (1.1%) | 25.4″ → 25.68″ |
|---|---|
| At an indicated 60 mphyou are going faster than the dash says | 60.7 mph |
| Within the ±3% ruleno recalibration normally needed | Yes |
| Ground clearancehalf the diameter change — the axle moves, not the whole tyre | +0.14″ |
| Section width+0.39″ — check clearance at full lock | 9.25″ → 9.65″ |
| Sidewall heightmore sidewall — softer ride, more roll in corners | 3.7″ → 4.34″ |
| Revolutions per mile-9 per mile | 794 → 785 |
| Rim diameterdifferent wheel required | 18″ → 17″ |
| Odometer over 10,000 mierror accumulated on the reading | -109 mi |
- Current size
- 235/40R18
- New size
- 245/45R17
The one number that matters
Overall diameter moves 1.1% — from 25.4″ to 25.68″. Everything else on this page follows from that.
The industry works to a ±3% window, and this change falls inside 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 60.7 mph — 0.7 mph slow.
Revolutions per mile go from 794 to 785, 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 0.14″.
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
- 1.1% is inside the ±3% window the industry works to. The speedometer, odometer, ABS and stability control all take their reference from wheel rotation, and within 3% none of them need recalibrating.
- The speedometer reads off wheel rotation. A taller tyre covers more ground per turn, so at an indicated 60 mph you are actually doing 60.7 — 0.7 mph faster, which is the direction that collects tickets.
- 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. +0.14″ here.
- 0.39″ 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.