235/45R18 vs 245/40R19 — the same climb, one wheel higher
A performance plus-one on a heavier car, and proof that the overshoot is a property of the ladder rather than of any one car.
245/40R19 is 0.39 in taller than 235/45R18 (1.48%) and 0.39 in wider. A speedometer calibrated for 235/45R18 would under-read by 1.48%.
Measured against each other
Overall diameter
26.33 in26.72 in+0.39 in (+1.48%)Section width
9.25 in / 235 mm9.65 in / 245 mm+0.39 in (+4.26%)Sidewall height
4.16 in3.86 in-0.31 inCircumference
82.7 in83.9 in+1.2 inRevolutions per mile
766.1754.9-11.2Ride height at the axle
——+0.19 inSpeedometer reading
——+1.48% under-read
| Dimension | 235/45R18 | 245/40R19 | Change |
|---|---|---|---|
| Overall diameter | 26.33 in | 26.72 in | +0.39 in (+1.48%) |
| Section width | 9.25 in / 235 mm | 9.65 in / 245 mm | +0.39 in (+4.26%) |
| Sidewall height | 4.16 in | 3.86 in | -0.31 in |
| Circumference | 82.7 in | 83.9 in | +1.2 in |
| Revolutions per mile | 766.1 | 754.9 | -11.2 |
| Ride height at the axle | — | — | +0.19 in |
| Speedometer reading | — | — | +1.48% under-read |
Calibrated for 235/45R18, running 245/40R19
- Dial reads 30
- 30.4 mph
- Dial reads 50
- 50.7 mph
- Dial reads 70
- 71.0 mph
- Odometer counts 100
- 101.5 mi covered
Change either size in the comparison tool, or see the full speed table in the speedometer error calculator.
Move the whole exercise up a wheel size and the same shortfall appears in the same place.
The shortfall, quantified
An extra inch of wheel diameter needs half an inch of sidewall surrendered at each end. Going from 45-series on a 235 to 40-series on a 245 releases only 0.31 in — 4.16 in becomes 3.86 in — so the books do not balance and 0.39 in of extra overall diameter is the result.
That is 1.48%, and it is the same failure mode as the 17-to-18 version of this step, arriving one rung later and with slightly different numbers.
Why this ladder cannot be fixed
Because the two quantities scale differently. The wheel grows in fixed one-inch steps. The sidewall you have to pay with is a percentage of a casing that is only growing 10 mm at a time. The higher up the ladder you go, the worse the exchange rate gets, and by the 19-inch rung a 10 mm width increment plus a 5-point aspect drop is no longer enough.
What it costs in practice
Very little on the road. The wheel turns 754.9 times per mile rather than 766.1, and at a dial reading of 60 the car is travelling 60.9. The instruments will not tell you anything happened.
It costs more at the wheel. A 3.86 in sidewall is a thin margin between a 19-inch rim and a road surface, and the wheel itself is heavier and considerably more expensive to replace than the 18.
Where the extra width goes
0.39 in, from 9.25 in to 9.65 in — a modest 4.26%. Section width on this class of car is usually the easy constraint; the difficult one is what the extra radius does at full compression, and the answer is 0.19 in of it.
The comparison worth reading alongside
225/45R17 against 235/40R18 is the identical exercise one rung below, with a 0.43 in overshoot. The two pages together show the drift is systematic rather than a quirk of either pairing, and the plus-sizing reference sets out the arithmetic the ladder is trying to satisfy.