245/40R18 vs 255/35R19 — the two changes nearly cancel
A performance plus-one near the bottom of the aspect-ratio scale, where every step is small and none of them is quite enough.
255/35R19 is 0.31 in taller than 245/40R18 (1.21%) and 0.39 in wider. A speedometer calibrated for 245/40R18 would under-read by 1.21%.
Measured against each other
Overall diameter
25.72 in26.03 in+0.31 in (+1.21%)Section width
9.65 in / 245 mm10.04 in / 255 mm+0.39 in (+4.08%)Sidewall height
3.86 in3.51 in-0.34 inCircumference
80.8 in81.8 in+1.0 inRevolutions per mile
784.2774.9-9.4Ride height at the axle
——+0.16 inSpeedometer reading
——+1.21% under-read
| Dimension | 245/40R18 | 255/35R19 | Change |
|---|---|---|---|
| Overall diameter | 25.72 in | 26.03 in | +0.31 in (+1.21%) |
| Section width | 9.65 in / 245 mm | 10.04 in / 255 mm | +0.39 in (+4.08%) |
| Sidewall height | 3.86 in | 3.51 in | -0.34 in |
| Circumference | 80.8 in | 81.8 in | +1.0 in |
| Revolutions per mile | 784.2 | 774.9 | -9.4 |
| Ride height at the axle | — | — | +0.16 in |
| Speedometer reading | — | — | +1.21% under-read |
Calibrated for 245/40R18, running 255/35R19
- Dial reads 30
- 30.4 mph
- Dial reads 50
- 50.6 mph
- Dial reads 70
- 70.8 mph
- Odometer counts 100
- 101.2 mi covered
Change either size in the comparison tool, or see the full speed table in the speedometer error calculator.
Down at 35-series, the sidewall is barely three and a half inches tall and there is very little left to trade.
What each change contributes
The extra inch of wheel adds 1.00 in of diameter on its own. The move from 40-series to 35-series takes 0.34 in of sidewall away — 3.86 in becomes 3.51 in — and that is doubled, so it gives back 0.69 in. The 10 mm of extra width, meanwhile, adds a little sidewall of its own.
Net: 0.31 in of diameter gained, 1.21% up.
Why it never quite balances at this end of the scale
An aspect step is worth less and less in absolute terms as the number gets smaller, because it is a percentage of a casing that is barely growing. Five points of 245 mm is 0.48 in; five points of 255 mm is 0.50 in. Meanwhile the wheel keeps demanding a full inch. By the time you are stepping from 40 to 35 there is only one useful step left below, and it cannot cover another wheel size.
What the vehicle registers
One mile an hour at 80, and nothing else worth writing down. That is the correct answer for a factory-adjacent performance plus-one, and it is why manufacturers are willing to offer both.
The width is doing something the arithmetic will not show
10.04 in of section width against 9.65 in. On a car where these sizes appear, that 0.39 in changes the contact patch and the steering weight in ways that are entirely outside the scope of a dimension table. What this page establishes is only that it costs almost nothing in rolling diameter, so whatever changes about the car's behaviour is a property of the tire rather than of the gearing.
What the extra wheel leaves behind
An inch of rim diameter, paid for with the 0.34 in of sidewall above it. At 3.51 in the 19-inch fitment carries the shortest rubber column of any size in this comparison set, so there is less tire standing between the rim and a kerb here than anywhere else on the site. That is a consequence of the wheel size rather than of the tire, and it does not appear in any of the figures above.
Where the ladder started
Two rungs down, 205/55R16 against 225/45R17 shows the same exercise executed with plenty of sidewall available and a 0.09 in result. The contrast between that figure and this one is the whole story of what happens as the profile drops.