245/75R16 vs 255/55R18
| Overall diameter-1.43″ (-4.68%) | 30.47″ → 29.04″ |
|---|---|
| At an indicated 60 mphyou are going slower than the dash says | 57.2 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 | -0.71″ |
| Section width+0.39″ — check clearance at full lock | 9.65″ → 10.04″ |
| Sidewall heightless sidewall — firmer ride, more wheel damage risk on potholes | 7.23″ → 5.52″ |
| Revolutions per mile32 per mile | 662 → 694 |
| Rim diameterdifferent wheel required | 16″ → 18″ |
| Odometer over 10,000 mierror accumulated on the reading | 491 mi |
Notes
- 4.68% 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 57.2 — 2.8 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. -0.71″ 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.
The one number that matters
Overall diameter moves -4.68% — from 30.47″ to 29.04″. 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 tire is fitted. At an indicated 60 mph you would actually be doing 57.2 mph — 2.8 mph fast.
Revolutions per mile go from 662 to 694, which is the same fact stated the way tire manufacturers state it.
Clearance changes by half, not all
The tire grows at the top and at the bottom; only the top half lifts the axle. Ground clearance moves -0.71″.
This is the point most lift-and-tire discussions get muddled, because the tire 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 molded sizes vary by a few millimeters between brands, and a worn tire is up to ⅜″ smaller in diameter than a new one. Both of those are smaller than the difference this page is showing you.
Nearby sizes
Read more
- Recalibrating a Speedometer After a Tire Change: Four Routes, and How to Verify It WorkedOn most modern vehicles the tire size is a stored number, not a mechanical ratio. Where that is true, correcting it takes minutes; where it is not, the options get physical.
- Plus Sizing Chart: Wider Tire, Shorter Sidewall, Same Rolling DiameterPlus sizing trades sidewall for wheel and keeps the overall diameter where it was. Done properly nothing on the vehicle notices — which is the entire point.
- Bigger Tires Change Your Gearing — Here Is How Much, and What to Re-Gear ToA larger tire is a gear change nobody installed. The effect is the same as swapping the differential for a taller one, and the arithmetic is a single ratio.
- Will 33s Fit Without a Lift? Often Yes — the Rubbing Happens Where You Are Not LookingMost stock trucks clear a 33-inch tire driving in a straight line. Fitment is decided at full lock over a bump, which is the one condition nobody tests in the parking lot.