285/50R20 vs 31x10.50R15
| Overall diameter-0.22″ (-0.71%) | 31.22″ → 31″ |
|---|---|
| At an indicated 60 mphyou are going slower than the dash says | 59.6 mph |
| Within the ±3% ruleno recalibration normally needed | Yes |
| Ground clearancehalf the diameter change — the axle moves, not the whole tyre | -0.11″ |
| Section width-0.72″ — check clearance at full lock | 11.22″ → 10.5″ |
| Sidewall heightmore sidewall — softer ride, more roll in corners | 5.61″ → 8″ |
| Revolutions per mile5 per mile | 646 → 651 |
| Rim diameterdifferent wheel required | 20″ → 15″ |
| Odometer over 10,000 mierror accumulated on the reading | 71 mi |
Notes
- 0.71% 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 shorter tyre covers less ground per turn, so at an indicated 60 mph you are actually doing 59.6 — 0.4 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. -0.11″ here.
- 0.72″ narrower. Narrower tyres cut better in snow and slush and cost less; the trade is dry grip and a smaller contact patch under braking.
- 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 -0.71% — from 31.22″ to 31″. 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 tire is fitted. At an indicated 60 mph you would actually be doing 59.6 mph — 0.4 mph fast.
Revolutions per mile go from 646 to 651, 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 -0.11″.
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
- Load Index and Speed Rating — The Two Codes at the End of the SizeEveryone reads the size and stops. The two characters after it are the ones with a safety consequence.
- Speedometer Error From Tire Size — One Ratio, and It Runs Both WaysA one-inch increase in diameter on a 30-inch tire is a 3.3% error. At an indicated 70 you are doing a bit over 72.
- 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.