37x12.50R20 vs 40x13.50R20
40x13.50R20 is +3.00″ in overall diameter and +25 mm in section width against 37x12.50R20 — a diameter change of +8.11%, which breaks the ±3% substitution rule. An indicated 60 mph would really be 64.9 mph.
Every dimension
| Dimension | 37x12.50R20 → 40x13.50R20 | Change |
|---|---|---|
| Overall diameter | 37.00″ → 40.00″ | +3.00″ (+8.11%) |
| Section width | 318 mm → 343 mm | +25 mm (+8.00%) |
| Sidewall height | 215.9 mm → 254.0 mm | +38.1 mm |
| Rolling circumference | 116.24″ → 125.66″ | +9.42″ |
| Revolutions per mile | 545.1 → 504.2 | −40.9 |
| Axle height / clearance | 18.50″ → 20.00″ | +1.50″ |
Which is taller, 37x12.50R20 or 40x13.50R20?
40x13.50R20 is the taller of the two, at 40.00″ against 37.00″. The gap is 3.00 inches, or 76.2 mm.
One of these is a flotation size, which states its overall diameter on the sidewall, while the metric size only implies one — so the comparison is between a stated height and a derived one. 37x12.50R20 is 317.5 mm wide with a 215.9 mm sidewall on a 20″ wheel; 40x13.50R20 is 342.9 mm wide with a 254.0 mm sidewall on a 20″ wheel.
What the speedometer will read
| Speedometer reads | Actual speed | Error |
|---|---|---|
| 25 mph | 27.0 mph | +2.0 mph |
| 40 mph | 43.2 mph | +3.2 mph |
| 55 mph | 59.5 mph | +4.5 mph |
| 60 mph | 64.9 mph | +4.9 mph |
| 70 mph | 75.7 mph | +5.7 mph |
| 80 mph | 86.5 mph | +6.5 mph |
Fitting 40x13.50R20 in place of 37x12.50R20 multiplies every speed and distance reading by 1.08108. Over 10,000 indicated miles the odometer is out by +811 miles.
Effective gearing moves by the same proportion. A vehicle with 3.73 axles running 37x12.50R20 would need 4.03 to keep the same engine speed on 40x13.50R20. Work it out for your own axle ratio.
Full specification of each size
37x12.50R20
| Dimension | Metric | Imperial |
|---|---|---|
| Section width | 317.5 mm | 12.50″ |
| Sidewall height | 215.9 mm | 8.50″ |
| Wheel diameter | 508.0 mm | 20.0″ |
| Overall diameter | 939.8 mm | 37.00″ |
| Rolling circumference | 2,952.5 mm | 116.24″ |
| Revolutions per mile | 338.7 / km | 545.1 / mile |
| Measuring rim (estimate) | 254 mm | 10″ |
40x13.50R20
| Dimension | Metric | Imperial |
|---|---|---|
| Section width | 342.9 mm | 13.50″ |
| Sidewall height | 254.0 mm | 10.00″ |
| Wheel diameter | 508.0 mm | 20.0″ |
| Overall diameter | 1,016.0 mm | 40.00″ |
| Rolling circumference | 3,191.9 mm | 125.66″ |
| Revolutions per mile | 313.3 / km | 504.2 / mile |
| Measuring rim (estimate) | 279 mm | 11″ |
Rim widths differ too: 37x12.50R20 is designed for 9″–11″ and 40x13.50R20 for 10″–12″. Their measuring rims differ, so the same wheel will not be ideal for both.
Can you swap one for the other?
Outside the ±3% rule
The diameter difference of +8.11% exceeds the ±3% rule of thumb. Expect a speedometer error of about 4.9 mph at an indicated 60, and an odometer that is off by the same proportion. It is a common swap even so — it just needs the instruments recalibrated to stay honest.
Neither figure says anything about physical fitment. Whether 40x13.50R20 clears the strut, the fender liner and the steering stops depends on wheel offset and suspension geometry that a size code does not encode. For maintenance rather than sizing, Car Basics covers tire care.
37x12.50R20 vs 40x13.50R20: common questions
- Is 40x13.50R20 bigger than 37x12.50R20?
- 40x13.50R20 is 3.00 inches taller and 25 mm wider. In overall diameter that is +8.11%, which exceeds the ±3% rule of thumb for a substitution.
- How much will 40x13.50R20 throw off my speedometer?
- Every reading scales by 1.08108. An indicated 60 mph becomes 64.9 mph, and every 100 miles on the odometer is really 108.1 miles.
- How much does 40x13.50R20 change ground clearance over 37x12.50R20?
- The axle centerline moves by 1.50 inches — half the diameter change, because half the extra height sits below the hub and half above it.
- Will 40x13.50R20 fit where 37x12.50R20 fits?
- Nothing derived from the size code can answer that. Clearance depends on wheel offset, suspension travel and body dimensions that no tire size contains. This page tells you exactly what changes dimensionally, which is the part that can be calculated.
Related comparisons
Other tire maths on this site
- tire size calculatorRead any tire size code and get overall diameter, sidewall height, circumference and revolutions per mile, with the arithmetic shown.
- tire size comparisonPut two sizes side by side and see the difference in diameter, width, sidewall, ground clearance and speedometer reading.
- tire size chartEvery common size sorted by wheel diameter, with overall diameter in inches and millimeters and revolutions per mile.
- tire diameter calculatorOverall diameter, radius and rolling circumference from the size code, and the reverse — the sizes that produce a diameter you want.
- speedometer error calculatorHow far out your speedometer and odometer read after a tire size change, at any indicated speed.
- plus sizing calculatorLarger wheel, same rolling diameter. Plus one and plus two candidates ranked by how close they hold the original diameter.
- gear ratio calculatorThe axle ratio that restores stock engine rpm after a tire diameter change, plus cruising rpm at any speed.
- metric to inch tire conversionConvert a metric size such as 285/70R17 to its inch equivalent, and an inch size such as 33x12.50R17 back to metric.
- rim width rangeThe rim widths a size is designed to mount on, and how section width changes as the rim gets wider.
- tire size meaningWhat every character in 265/70R17 stands for, digit by digit, including the load index and speed rating.