37x12.50R18 vs 37x12.50R20

37x12.50R20 is the taller of the two, at 37.00″ against 37.00″. The gap is 0.00 inches, or 0.0 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. 37x12.50R18 is 317.5 mm wide with a 241.3 mm sidewall on a 18″ wheel; 37x12.50R20 is 317.5 mm wide with a 215.9 mm sidewall on a 20″ wheel.

Neither figure says anything about physical fitment. Whether 37x12.50R20 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.

Is 37x12.50R20 bigger than 37x12.50R18?

37x12.50R20 is 0.00 inches taller and 0 mm wider. In overall diameter that is 0.00%, which stays inside the ±3% rule of thumb for a substitution.

How much will 37x12.50R20 throw off my speedometer?

Every reading scales by 1.00000. An indicated 60 mph becomes 60.0 mph, and every 100 miles on the odometer is really 100.0 miles.

How much does 37x12.50R20 change ground clearance over 37x12.50R18?

The axle centerline moves by 0.00 inches — half the diameter change, because half the extra height sits below the hub and half above it.

Will 37x12.50R20 fit where 37x12.50R18 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.