275/70R18 vs 35x12.50R20 — a Ram 2500's usual upgrade
A heavy-duty factory size against the most common package fitted over it, and the sidewall arithmetic that comes with a 20-inch wheel.
35x12.50R20 is 1.84 in taller than 275/70R18 (5.56%) and 1.67 in wider. A speedometer calibrated for 275/70R18 would under-read by 5.56%.
Measured against each other
Overall diameter
33.16 in35.00 in+1.84 in (+5.56%)Section width
10.83 in / 275 mm12.50 in / 318 mm+1.67 in (+15.45%)Sidewall height
7.58 in7.50 in-0.08 inCircumference
104.2 in110.0 in+5.8 inRevolutions per mile
608.3576.2-32.0Ride height at the axle
——+0.92 inSpeedometer reading
——+5.56% under-read
| Dimension | 275/70R18 | 35x12.50R20 | Change |
|---|---|---|---|
| Overall diameter | 33.16 in | 35.00 in | +1.84 in (+5.56%) |
| Section width | 10.83 in / 275 mm | 12.50 in / 318 mm | +1.67 in (+15.45%) |
| Sidewall height | 7.58 in | 7.50 in | -0.08 in |
| Circumference | 104.2 in | 110.0 in | +5.8 in |
| Revolutions per mile | 608.3 | 576.2 | -32.0 |
| Ride height at the axle | — | — | +0.92 in |
| Speedometer reading | — | — | +5.56% under-read |
Calibrated for 275/70R18, running 35x12.50R20
- Dial reads 30
- 31.7 mph
- Dial reads 50
- 52.8 mph
- Dial reads 70
- 73.9 mph
- Odometer counts 100
- 105.6 mi covered
Change either size in the comparison tool, or see the full speed table in the speedometer error calculator.
The finished assembly is nearly two inches taller and has slightly less sidewall than it started with. That is what a two-inch wheel increase does to a nominally larger tire.
Sidewall goes the wrong way
Eight hundredths of an inch less rubber between rim and road — 7.58 in down to 7.50 in — on a tire whose overall diameter has grown 1.84 in.
All of the height gain — and then a fraction more — is the wheel. This is the defining trade of 20-inch heavy-duty packages, and it is worth naming because "going to 35s" sounds like a move toward more sidewall and is precisely the opposite here.
What the height is worth
5.56%. On a truck bought to tow, the consequential half of that is the odometer rather than the needle: service intervals, warranty periods and resale mileage are all counted in miles the cluster is now under-reporting by better than five in every hundred, and unlike the needle nobody is watching it.
Restoring the gearing, if anyone wanted to
From 3.73:1 the arithmetic asks for 3.94:1, with 3.91:1 the nearest ratio manufactured. On a diesel three-quarter-ton with a broad torque band and a six-speed automatic, a five and a half per cent change is generally absorbed rather than corrected — and re-gearing a heavy-duty axle is a substantially larger undertaking than doing the same to a half-ton.
The aspect ratios tell the story
The metric size states 70. The flotation size on a 20-inch wheel works out to 60. That ten-series gap is the sidewall the wheel took, expressed as a proportion, and it is the number that would tell a buyer what they were actually getting if flotation codes printed it. They do not, which is why the trade discusses these packages purely in terms of the first number.
What the truck has to accommodate
1.67 in of extra casing, at full lock, on a wheel whose offset determines where it goes. On a three-quarter-ton the front arch has more room than on a half-ton, which is why this package is common on Ram 2500s and less so on 1500s — but the specific answer is measured on the vehicle with the wheels turned.
The taller version of the same package
275/70R18 against 37x12.50R20 takes the same wheel change to a 37, at 3.84 in and 11.59% — roughly twice the diameter change and the point at which the gearing question stops being rhetorical.