315/70R17 vs 37x12.50R17 — largest metric size, factory 37
What a Bronco Raptor's factory tire has that the biggest metric alternative does not, and it is all height.
37x12.50R17 is 2.64 in taller than 315/70R17 (7.68%) and 0.10 in wider. A speedometer calibrated for 315/70R17 would under-read by 7.68%.
Measured against each other
Overall diameter
34.36 in37.00 in+2.64 in (+7.68%)Section width
12.40 in / 315 mm12.50 in / 318 mm+0.10 in (+0.79%)Sidewall height
8.68 in10.00 in+1.32 inCircumference
108.0 in116.2 in+8.3 inRevolutions per mile
586.9545.1-41.8Ride height at the axle
——+1.32 inSpeedometer reading
——+7.68% under-read
| Dimension | 315/70R17 | 37x12.50R17 | Change |
|---|---|---|---|
| Overall diameter | 34.36 in | 37.00 in | +2.64 in (+7.68%) |
| Section width | 12.40 in / 315 mm | 12.50 in / 318 mm | +0.10 in (+0.79%) |
| Sidewall height | 8.68 in | 10.00 in | +1.32 in |
| Circumference | 108.0 in | 116.2 in | +8.3 in |
| Revolutions per mile | 586.9 | 545.1 | -41.8 |
| Ride height at the axle | — | — | +1.32 in |
| Speedometer reading | — | — | +7.68% under-read |
Calibrated for 315/70R17, running 37x12.50R17
- Dial reads 30
- 32.3 mph
- Dial reads 50
- 53.8 mph
- Dial reads 70
- 75.4 mph
- Odometer counts 100
- 107.7 mi covered
Change either size in the comparison tool, or see the full speed table in the speedometer error calculator.
Width: near-identical. Height: two and a half inches apart. This is one of the purest height comparisons available at the top of the 17-inch range.
The widths are the same tire
12.40 in against 12.50 in — a tenth of an inch, 0.79%. Whatever room a vehicle has beside its wheels, both of these will use essentially the same amount of it.
That is unusual and it makes the comparison unusually clean: everything that separates these two sizes is vertical.
Where the 2.64 in lives
Sidewall. 8.68 in becomes 10.00 in — 1.32 in more per side, on a wheel that has not changed.
A 10.00 in sidewall on a 17-inch wheel means the rubber column is taller than the wheel's radius, which is a proportion almost nothing else in production reaches. The flotation tire's derived aspect ratio is 80; the metric size states 70.
What the difference does
Every one of the 1.32 in that separates the two sidewalls appears twice — once under the axle and once above the crown — which is why a size that adds almost no width is nevertheless the harder of the two to package.
A 4.10:1 axle converts to 4.41:1 if engine speed is to stay put, and the closest set manufactured is 4.30:1 — an undershoot rather than a match. Left uncorrected, 7.68% shows up as 86.1 mph at a dial reading of 80.
Why the cross-shop happens
Because 315/70R17 is what people reach for when they want something in the class without the flotation size's cost, weight or availability constraints. It is the largest common metric size on this wheel, and it lands 2.64 in short of the factory 37.
For a vehicle delivered on 37s, going to the metric size is a reduction rather than a substitution, and it is worth being explicit that the two are not alternatives at the same height.
Weight, which the codes do not carry
The flotation tire is the heavier object by a considerable margin — more casing, more tread depth in the patterns it is usually sold in, and 1.32 in more sidewall per side to build. Unsprung and rotating mass at each corner is one of the real costs of the step, and no dimension on this page measures it. The only way to compare two specific tires on that basis is to look up the individual products.
The comparison in the other direction
Starting from the 37 rather than the metric size changes the percentage, because the reference diameter changes — that page works the same swap the other way round and gets a different number.