Tire RulerWhat changes when you switch to 305/55R20
- Height: +0.87″ (+2.7%), so ground clearance at the axle goes up 0.43″.
- Width: +0.79″ (20 mm). Check inner clearance to the suspension and frame, and the fender liner at full lock.
- Sidewall: +0.43″. A taller sidewall rides a bit softer and protects the wheel off-road.
- Revolutions per mile: 624 → 607. Your gears act about 2.7% taller; at 65 mph in a 0.69 top gear with 3.73s, that’s 1,739 → 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.
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Frequently asked questions
How much bigger is 305/55R20 than 285/55R20?
305/55R20 is 0.87 inches (2.7%) taller and 0.79 inches wider: 33.2" × 12.0" versus 32.3" × 11.2".
Will 305/55R20 change my speedometer?
Yes, by 2.7%. Going from 285/55R20 to 305/55R20, an indicated 60 mph is about 61.6 mph.
Can I replace 285/55R20 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 285/55R20 to 305/55R20?
Usually not for a 2.7% change. With 3.73 gears, the 305/55R20 acts like 3.63.