37x12.50R17 vs 40x13.50R17 — the top of the flotation range
Where the common flotation scale ends, and where the arithmetic becomes extreme enough to be instructive.
40x13.50R17 is 3.00 in taller than 37x12.50R17 (8.11%) and 1.00 in wider. A speedometer calibrated for 37x12.50R17 would under-read by 8.11%.
Measured against each other
Overall diameter
37.00 in40.00 in+3.00 in (+8.11%)Section width
12.50 in / 318 mm13.50 in / 343 mm+1.00 in (+8.00%)Sidewall height
10.00 in11.50 in+1.50 inCircumference
116.2 in125.7 in+9.4 inRevolutions per mile
545.1504.2-40.9Ride height at the axle
——+1.50 inSpeedometer reading
——+8.11% under-read
| Dimension | 37x12.50R17 | 40x13.50R17 | Change |
|---|---|---|---|
| Overall diameter | 37.00 in | 40.00 in | +3.00 in (+8.11%) |
| Section width | 12.50 in / 318 mm | 13.50 in / 343 mm | +1.00 in (+8.00%) |
| Sidewall height | 10.00 in | 11.50 in | +1.50 in |
| Circumference | 116.2 in | 125.7 in | +9.4 in |
| Revolutions per mile | 545.1 | 504.2 | -40.9 |
| Ride height at the axle | — | — | +1.50 in |
| Speedometer reading | — | — | +8.11% under-read |
Calibrated for 37x12.50R17, running 40x13.50R17
- Dial reads 30
- 32.4 mph
- Dial reads 50
- 54.1 mph
- Dial reads 70
- 75.7 mph
- Odometer counts 100
- 108.1 mi covered
Change either size in the comparison tool, or see the full speed table in the speedometer error calculator.
Forty inches is effectively the ceiling of what is made in volume. Past it the sizes exist but the market does not.
The step
Three inches of diameter, one inch of width. Sidewall goes from 10.00 in to 11.50 in — a rubber column two-thirds as tall as the wheel is wide across.
Unusually, the two dimensions move by almost the same proportion: 8.11% of diameter against 8.00% of width. At the lower rungs of the flotation ladder a one-inch step changes the width by three times as much proportionally as it changes the height; up here the two have converged, so the 40 is close to a straight enlargement of the 37 rather than a differently shaped tire.
What 504 revolutions per mile means
It is the lowest figure anywhere in this catalogue. A common passenger tire turns around 800 times per mile; a 40 turns 504. The wheel is completing barely five hundred rotations over a mile, which is why gearing on vehicles running tires this size bears no resemblance to anything factory.
A 4.10:1 axle converts to 4.43:1, and the closest set sold is 4.56:1. Vehicles on 40s have generally been re-geared twice by this point, and the arithmetic here is the last step of a long sequence rather than a standalone decision.
The derived proportions
37x12.50R17 works out to an 80-series. 40x13.50R17 works out to an 85 — the taller profile of the two, because the diameter grew three inches while the width grew only one.
At that proportion the sidewall dominates the tire completely, and the 17-inch wheel inside it is a small part of the assembly.
Why the scale stops here
Because packaging, weight and axle strength all become binding at once, and because the number of vehicles that can use a 40-inch tire without extensive modification is very small. Both of these sizes are made; only one of them is stocked anywhere ordinary.
What the odometer does at this scale
At 8.11%, a vehicle showing ten thousand indicated miles has covered 10,811. That is eight hundred miles of unrecorded use per ten thousand, and on a vehicle that has already been through one or two tire-size changes the errors compound rather than reset — each one is calculated against whatever the speedometer was last calibrated for, not against the factory tire.
The rung below
285/70R17 against 37x12.50R17 is how most vehicles arrive at the smaller of these two, and it is 4.29 in from a factory 33.