This is a modification rather than a substitution. Check clearance at full steering lock and full suspension compression, confirm the load rating meets the placard, and expect the speedometer to read high by about 10.4% unless it can be recalibrated for the new size.
Want to try other sizes against these? Open this comparison in the interactive tool.
This is a plus-size: bigger wheel, shorter sidewall
Going from 37X12.50R17 to 275/70R18 moves the rim from 17 inches to 18 and drops the aspect ratio from 80 to 70. That is the classic plus-size pattern, and the intention behind it is usually appearance and steering response rather than clearance.
The sidewall shrinks from 10 inches to 7.58 — 2.42 inches less rubber between the rim and the road. Less sidewall flex sharpens turn-in and makes the steering feel more immediate, which is the point.
The trade is real and worth stating plainly. A shorter sidewall transmits more of the road surface into the cabin, and it leaves the rim far more exposed to pothole damage. Replacement cost also rises, because both the larger wheel and the lower-profile tire are more expensive than what they replace.
37X12.50R17 and 275/70R18 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 | 37X12.50R17 | 275/70R18 | Difference |
|---|---|---|---|
| Overall diameter | 37" | 33.16" | −3.84" (−10.4%) |
| Section width | 12.5" (318 mm) | 10.83" (275 mm) | −1.67" |
| Sidewall height | 10" | 7.58" | −2.42" |
| Rim diameter | 17" | 18" | +1" |
| Circumference | 116.24" | 104.17" | −12.07" |
| Revolutions per mile | 545 | 608 | +63 |
| Ride height at axle | Baseline | −1.92" | −1.92" |
What the dial shows against how fast you are going
Your speedometer was calibrated to 37X12.50R17, which turns about 545 times per mile. Fit 275/70R18 at about 608 revolutions per mile and the same wheel speed now represents a smaller road speed than the instrument assumes. The error is 10.4% at every speed, so it grows in absolute terms the faster you go.
| Speedometer reads | You are actually doing | Difference |
|---|---|---|
| 30 mph | 26.9 mph | −3.1 mph |
| 45 mph | 40.3 mph | −4.7 mph |
| 60 mph | 53.8 mph | −6.2 mph |
| 75 mph | 67.2 mph | −7.8 mph |
Over distance the same proportion applies to the odometer. Every indicated 1,000 miles on 275/70R18 is really about 896 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 1.67 inches, which gains inboard clearance but reduces contact patch.
- Overall height drops by 3.84 inches, lowering the vehicle 1.92 inches at the axle.
- The rim diameter changes from 17 inches to 18 inches, so this swap needs different wheels as well as different tires.
- Speedometer error of 10.4% — check whether your vehicle supports recalibrating for a new tire size.
- Confirm the load index on 275/70R18meets 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.
37X12.50R17 vs 275/70R18 FAQ
Is 275/70R18 bigger than 37X12.50R17?+
275/70R18 measures 33.16 inches in overall diameter against 37 inches for 37X12.50R17. That makes it 3.84 inches shorter, a change of −10.4%. It is also 1.67 inches narrower in section width.
Will 275/70R18 fit in place of 37X12.50R17?+
The diameter change is −10.4%, which is outside the 3% envelope usually treated as a routine substitution. Treat this as a modification: verify clearance at full steering lock and full suspension compression, confirm the load rating meets the vehicle placard, and expect the speedometer to read incorrectly unless it can be recalibrated.
How much lift does 275/70R18 give over 37X12.50R17?+
1.92 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 10.4%. At an indicated 60 mph you would actually be travelling about 53.8 mph, so the vehicle covers less ground than the dial claims.
Do I need to regear for 275/70R18?+
A −10.4% change shifts the effective gearing noticeably. To restore the original engine speed you would multiply the current axle ratio by 0.896. Whether that is worth doing depends on how the vehicle is used — towing and heavily loaded driving make the case sooner than commuting does.
What is the revolutions-per-mile difference?+
37X12.50R17 turns about 545 times per mile and 275/70R18 about 608 — a difference of +63 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.