How much does an inch of tire height change the speedometer?
Answer
+3.41%
A speedometer calibrated for 265/65R17 and fitted with 265/70R17 reads low by 3.41%. An indicated 70 mph is an actual 72.4 mph.
The working
- Indicated 30 mph
- 31.0 mph actual
- Indicated 50 mph
- 51.7 mph actual
- Indicated 70 mph
- 72.4 mph actual
- Indicated 100 miles
- 103.4 miles actual
- Diameter change
- +1.04 in
An inch is not a fixed amount of error
The error is a ratio, not a subtraction: the new diameter divided by the old one. So one inch added to a short tire is proportionally more than one inch added to a tall one, and the question has no single answer until the starting size is named.
Four real one-inch steps, each computed from sizes that exist:
- 245/75R16 to 245/75R17 — exactly 1.00 in, and 3.28%.
- 255/75R17 to 255/80R17 — exactly 1.00 in, and 3.13%.
- 265/65R17 to 265/70R17 — 1.04 in, and 3.41%.
- 275/65R18 to 275/70R18 — 1.08 in, and 3.38%.
Two of those are the same inch to the hundredth and they differ by 0.15 percentage points, purely because one started at 30.47 in and the other at 32.06 in.
The rule underneath it
Divide one inch by the original diameter. On a 30-inch tire that is about 3.3%; on a 37 it is about 2.7%; on a 25-inch car tire it is about 4%. Nothing else enters — not width, not aspect ratio, not wheel diameter except through the height it produces.
The other inch, which the instruments never see
An inch of diameter is only half an inch at the axle, and the two halves have completely separate consequences. The speedometer responds to the full inch, because it is counting rotations of a circumference that grew by the whole amount. Ride height responds to half of it. Arch clearance responds to the other half. People frequently quote a lift figure and a speedometer figure as though they were the same measurement, and they differ by a factor of two.
What the percentage buys you
Whatever it is, it applies unchanged to three things: the speed shown, the distance logged, and the axle ratio that would restore the original engine speed. That is why a single figure does so much work on this site.
Check your own pair rather than the rule of thumb
Round numbers of inches almost never appear in real size changes — the sizes you can buy land where they land. Enter both of yours and the exact figure comes back with a full speed table, rather than an approximation from a nominal inch.