Tire RulerWhat changes when you switch to 305/55R20
- Height: +0.21″ (+0.6%), so ground clearance at the axle goes up 0.10″.
- Width: −0.49″ (-12 mm). Check inner clearance to the suspension and frame, and the fender liner at full lock.
- Sidewall: +0.10″. A taller sidewall rides a bit softer and protects the wheel off-road.
- Revolutions per mile: 611 → 607. Your gears act about 0.6% taller; at 65 mph in a 0.69 top gear with 3.73s, that’s 1,704 → 1,693 RPM.
Sizes are theoretical, from the sidewall code. The same size can differ by a few tenths of an inch between brands and tread patterns; check the maker’s spec sheet for the exact model.
Other comparisons
Frequently asked questions
How much bigger is 305/55R20 than 33x12.50R20?
305/55R20 is 0.21 inches (0.6%) taller and 0.49 inches narrower: 33.2" × 12.0" versus 33.0" × 12.5".
Will 305/55R20 change my speedometer?
Yes, by 0.6%. Going from 33x12.50R20 to 305/55R20, an indicated 60 mph is about 60.4 mph.
Can I replace 33x12.50R20 with 305/55R20?
Often, yes: the height difference is within the ±3% most tire shops accept. Both fit a 20" wheel; confirm the rim width, load index and clearance for your vehicle.
Do I need to regear going from 33x12.50R20 to 305/55R20?
Usually not for a 0.6% change. With 3.73 gears, the 305/55R20 acts like 3.71.