A change this small is generally treated as a straight substitution. Speedometer error stays under three percent and the clearance designed around 35X12.50R20 is largely preserved. The section width still changes by 0.1 inches, so confirm your rim width suits 315/70R17 before ordering.
Want to try other sizes against these? Open this comparison in the interactive tool.
This swap changes the wheel as well as the tire
35X12.50R20 and 315/70R17 sit on different rims — 20 inches against 17. That makes this a wheel purchase rather than a tire purchase, and the cost difference between the two is substantial.
Because the aspect ratio has not dropped to compensate, the overall diameter moves by −1.8%. Check that figure against the 3% guidance before treating this as a straight swap.
Also confirm the new wheel's offset and width suit the vehicle. A different rim diameter says nothing about where the wheel sits laterally, and that is decided separately by offset and width.
35X12.50R20 and 315/70R17 measured against each other
Both sizes reduced to the same units. The diameter row is the one that drives everything else on this page — ride height, gearing, speedometer accuracy and odometer readings all follow from it.
| Measurement | 35X12.50R20 | 315/70R17 | Difference |
|---|---|---|---|
| Overall diameter | 35" | 34.36" | −0.64" (−1.8%) |
| Section width | 12.5" (318 mm) | 12.4" (315 mm) | −0.1" |
| Sidewall height | 7.5" | 8.68" | +1.18" |
| Rim diameter | 20" | 17" | −3" |
| Circumference | 109.96" | 107.95" | −2" |
| Revolutions per mile | 576 | 587 | +11 |
| Ride height at axle | Baseline | −0.32" | −0.32" |
What the dial shows against how fast you are going
Your speedometer was calibrated to 35X12.50R20, which turns about 576 times per mile. Fit 315/70R17 at about 587 revolutions per mile and the same wheel speed now represents a smaller road speed than the instrument assumes. The error is 1.8% at every speed, so it grows in absolute terms the faster you go.
| Speedometer reads | You are actually doing | Difference |
|---|---|---|
| 30 mph | 29.5 mph | −0.5 mph |
| 45 mph | 44.2 mph | −0.8 mph |
| 60 mph | 58.9 mph | −1.1 mph |
| 75 mph | 73.6 mph | −1.4 mph |
Over distance the same proportion applies to the odometer. Every indicated 1,000 miles on 315/70R17 is really about 982 miles covered — so the vehicle accumulates recorded mileage faster than it actually travels.
What this particular change asks you to check
- Section width drops by 0.1 inches, which gains inboard clearance but reduces contact patch.
- Overall height drops by 0.64 inches, lowering the vehicle 0.32 inches at the axle.
- The rim diameter changes from 20 inches to 17 inches, so this swap needs different wheels as well as different tires.
- Speedometer error of 1.8% — check whether your vehicle supports recalibrating for a new tire size.
- Confirm the load index on 315/70R17meets or exceeds the figure on the vehicle's tire placard, particularly if the vehicle tows or carries load.
- On all-wheel-drive and four-wheel-drive vehicles, change all four together — mismatched rolling circumferences load the differentials and transfer case continuously.
35X12.50R20 vs 315/70R17 FAQ
Is 315/70R17 bigger than 35X12.50R20?+
315/70R17 measures 34.36 inches in overall diameter against 35 inches for 35X12.50R20. That makes it 0.64 inches shorter, a change of −1.8%. It is also 0.1 inches narrower in section width.
Will 315/70R17 fit in place of 35X12.50R20?+
The diameter change is −1.8%, which falls inside the 3% envelope generally treated as a routine substitution. That makes it a plausible swap, but the section width also changes by 0.1 inches, so confirm the rim width suits the new size and check clearance at full lock.
How much lift does 315/70R17 give over 35X12.50R20?+
0.32 inches at the axle — half the diameter difference. The other half of the change goes upward into the wheel arch, which is where fitment problems usually appear.
How far off will my speedometer be?+
The speedometer will read high by about 1.8%. At an indicated 60 mph you would actually be travelling about 58.9 mph, so the vehicle covers less ground than the dial claims.
Do I need to regear for 315/70R17?+
A −1.8% change is small enough that most vehicles absorb it without complaint. Regearing is usually considered from around 10% upward, or sooner on a vehicle that tows regularly.
What is the revolutions-per-mile difference?+
35X12.50R20 turns about 576 times per mile and 315/70R17 about 587 — a difference of +11 revolutions. That figure is what the speedometer and odometer are calibrated against, which is why both readings shift.
These are nominal figures. Every number above is calculated from the marked size. Real tires vary from nominal by a few millimetres between manufacturers, and by more as tread wears or pressure changes. Use this to plan and to check a quote — measure the actual tire before committing to a tight fitment.