275/65R18 vs 35x12.50R20 — load range ahead of diameter
A three-quarter-ton question in which the geometry is the easy part and the casing specification is the hard one.
35x12.50R20 is 2.93 in taller than 275/65R18 (9.12%) and 1.67 in wider. A speedometer calibrated for 275/65R18 would under-read by 9.12%.
Measured against each other
Overall diameter
32.07 in35.00 in+2.93 in (+9.12%)Section width
10.83 in / 275 mm12.50 in / 318 mm+1.67 in (+15.45%)Sidewall height
7.04 in7.50 in+0.46 inCircumference
100.8 in110.0 in+9.2 inRevolutions per mile
628.8576.2-52.6Ride height at the axle
——+1.46 inSpeedometer reading
——+9.12% under-read
| Dimension | 275/65R18 | 35x12.50R20 | Change |
|---|---|---|---|
| Overall diameter | 32.07 in | 35.00 in | +2.93 in (+9.12%) |
| Section width | 10.83 in / 275 mm | 12.50 in / 318 mm | +1.67 in (+15.45%) |
| Sidewall height | 7.04 in | 7.50 in | +0.46 in |
| Circumference | 100.8 in | 110.0 in | +9.2 in |
| Revolutions per mile | 628.8 | 576.2 | -52.6 |
| Ride height at the axle | — | — | +1.46 in |
| Speedometer reading | — | — | +9.12% under-read |
Calibrated for 275/65R18, running 35x12.50R20
- Dial reads 30
- 32.7 mph
- Dial reads 50
- 54.6 mph
- Dial reads 70
- 76.4 mph
- Odometer counts 100
- 109.1 mi covered
Change either size in the comparison tool, or see the full speed table in the speedometer error calculator.
On a Super Duty, the tire's construction is a more consequential choice than its size, and this pairing changes both at once.
The geometry, briefly
Nearly three inches of extra diameter — 9.12% — and less than half an inch of extra sidewall, because two of those three inches are wheel rather than tire. The casing widens 1.67 in at the same time, which on a Super Duty is the dimension the front end has most of and worries about least.
Left uncorrected, the height is what the instruments report — and on a truck bought to tow, the odometer half of that is the expensive half.
Why load range leads on this platform
A three-quarter-ton truck's rear axle rating is a large number, and the placard's load index reflects what the vehicle is certified to carry rather than what it usually does. A tire that clears geometrically and does not meet that index is not an option, and load index is not a function of size at all — it is a function of casing construction and reference inflation pressure.
Flotation sizes in this class are sold across several load ranges, and the difference between them at the same nominal size is substantial. The load range reference explains what the markings mean.
The pressure consequence
A heavier load range achieves its rating at a higher cold inflation pressure. Fitting one and inflating it to the pressure printed for the original tire under-inflates it for the certified load. That is a relationship between published load tables and pressure, not a recommendation, and it is the detail most often missed on heavy-duty fitments.
Where the height actually comes from
Of the 2.93 in gained, 2.00 in is the two extra inches of wheel and 0.93 in is the tire. Put differently: the buyer is paying for a set of 20-inch wheels and receiving two-thirds of the height increase from them. That is worth separating out, because the same money spent on a taller 18-inch tire buys a taller result with more sidewall.
The alternative that keeps the wheels
275/70R18 against 35x12.50R20 is the same destination from the Super Duty's other factory size, and 275/65R18 to 285/75R18 reaches 34.83 in on the original 18-inch wheels with a taller sidewall than either 20-inch option can offer.