Frequently asked questions

Why does my speedometer read low with bigger tires, and by how much?

A bigger tire covers more distance per wheel rotation than your speedometer's factory calibration assumes, so the dash reads low. Go from 265/70R17 to 285/70R17 and it under-reads by 3.5% — an indicated 60mph is actually 62.1mph. Full mechanism and formula: speedometer error.

Does a bigger tire affect my odometer?

Yes, by the same ratio as the speedometer error. Over a real 12,000-mile year, that same 265→285 swap leaves the odometer showing about 11,596 miles — 404 miles it never counted. How that number is derived: speedometer error.

Is "Plus One" always diameter-neutral?

No. 205/55R16 to 225/50R17 is a commonly cited textbook Plus One pairing, but it's actually 3.9% larger in diameter — enough to push an indicated 60 to 62.4mph actual. Why the label doesn't guarantee a match: plus-sizing.

How much diameter change before the speedo is meaningfully off?

There's no official cutoff — "meaningful" is a judgment call. Here's how the error actually scales, across real size changes, at an indicated 60mph:

Roughly a mile per hour at 1%, several miles per hour once you're past 8–9%. Run your own pairing in the calculator instead of interpolating from this table.

What's the difference between offset and backspacing?

They're two units for nearly the same measurement, and they move together — not oppositely. Drop a wheel's offset from +18mm to 0mm on an 8in-wide wheel and backspacing drops right along with it, from 5.21in to 4.50in. Full conversion formula and clearance math: offset and backspacing.

Does a wider wheel change my tire's actual width?

Yes. A tire's spec'd section width — the 285 in 285/70R17 — is only exact on the rim width it's rated for. Mount it on a wider or narrower rim and the real measured width shifts a few millimeters, which is why this site's clearance figures are estimates, not guarantees. More on how clearance math handles tire width: offset and backspacing.