275/70R18 vs 37x12.50R20 — the largest common Super Duty fitment
A factory heavy-duty size against the biggest package the aftermarket routinely sells for it, with both variables moving hard.
37x12.50R20 is 3.84 in taller than 275/70R18 (11.59%) and 1.67 in wider. A speedometer calibrated for 275/70R18 would under-read by 11.59%.
Measured against each other
Overall diameter
33.16 in37.00 in+3.84 in (+11.59%)Section width
10.83 in / 275 mm12.50 in / 318 mm+1.67 in (+15.45%)Sidewall height
7.58 in8.50 in+0.92 inCircumference
104.2 in116.2 in+12.1 inRevolutions per mile
608.3545.1-63.2Ride height at the axle
——+1.92 inSpeedometer reading
——+11.59% under-read
| Dimension | 275/70R18 | 37x12.50R20 | Change |
|---|---|---|---|
| Overall diameter | 33.16 in | 37.00 in | +3.84 in (+11.59%) |
| Section width | 10.83 in / 275 mm | 12.50 in / 318 mm | +1.67 in (+15.45%) |
| Sidewall height | 7.58 in | 8.50 in | +0.92 in |
| Circumference | 104.2 in | 116.2 in | +12.1 in |
| Revolutions per mile | 608.3 | 545.1 | -63.2 |
| Ride height at the axle | — | — | +1.92 in |
| Speedometer reading | — | — | +11.59% under-read |
Calibrated for 275/70R18, running 37x12.50R20
- Dial reads 30
- 33.5 mph
- Dial reads 50
- 55.8 mph
- Dial reads 70
- 78.1 mph
- Odometer counts 100
- 111.6 mi covered
Change either size in the comparison tool, or see the full speed table in the speedometer error calculator.
Nearly four inches of diameter, two inches of wheel, and — unusually for a 20-inch package — almost an inch more sidewall than the truck started with.
Both variables push the same way
Unlike most 20-inch conversions, this one does not sacrifice sidewall. 7.58 in becomes 8.50 in, because a 37 is tall enough that even a 20-inch wheel leaves a substantial column of rubber above it.
The derived aspect ratio of 37x12.50R20 is 68, against the metric size's stated 70 — remarkably close, which means the two tires are close to the same shape at very different scales.
What eleven and a half per cent does
Expressed in the units the driveline works in rather than the ones on the dial, engine speed at any given road speed falls by 11.59% — the same proportion as every reading in the panel above, and precisely the quantity the gearing conversion below is written to undo.
The gearing conversion
From 3.73:1 the arithmetic asks for 4.16:1, with 4.10:1 the nearest ratio manufactured; from 4.10:1 it asks for 4.58:1 against an available 4.56:1. Heavy-duty axles are expensive to re-gear and most of these trucks carry the error instead, helped by a diesel's torque and by the fact that a Super Duty spends much of its life at part throttle.
The load question, which is the real one
Four tires of this size on a truck rated to carry and tow a great deal is a load-index question before it is a geometry question. Flotation sizes in this class span several load ranges at the same nominal dimensions, and the placard states the index the vehicle is certified against.
Why the sidewall survives here and not on a 35
Simple arithmetic. A 20-inch wheel takes twenty of the available inches whatever tire is wrapped around it, so what remains for sidewall is (nominal diameter − 20) ÷ 2. On a 35 that leaves 7.50 in; on a 37 it leaves 8.50 in. Two inches of extra nominal diameter buys a full inch of extra sidewall, and that is the only way a 20-inch package recovers compliance.
The intermediate package
275/70R18 against 35x12.50R20 is the same wheel change with two inches less tire — 1.84 in rather than 3.84 in — and it is where most of these trucks stop.