TireSizeCheck

305/50R20 vs 265/50R20

Fit verdict

Borderline. Check clearance first

Diameter change

−4.9%

At 4.9% this lands between the 3% and 5% marks. Plenty of people run swaps like this, but you will see real speedometer error, and you want to check the wheel well at full lock and full compression before you buy. Your speedometer will read high, so you are going slower than it says.

New overall diameter

30.43″

was 32.01″

True speed at an indicated 60

57.0 mph

your speedo reads high

Revolutions per mile

663

was 630, +33

Measurement 305/50R20 265/50R20 Change
Overall diameter 32.01″ 30.43″ −1.57″ (−4.9%)
Sidewall height 6.00″ (152.5 mm) 5.22″ (132.5 mm) −20.0 mm
Tread width 305 mm 265 mm −40 mm
Circumference 100.56″ 95.61″ −4.95″
Revs per mile 630 663 +33
Speedometer error baseline −4.9% reads high

Diameter overlay

305/50R20 265/50R20

Speedometer check

With 265/50R20 fitted, when the speedometer shows the indicated speed, your actual speed is:

25 mph shown

23.8 mph actual

35 mph shown

33.3 mph actual

45 mph shown

42.8 mph actual

55 mph shown

52.3 mph actual

65 mph shown

61.8 mph actual

75 mph shown

71.3 mph actual

Try other sizes in the calculator 265/50R20 details 305/50R20 details

Common questions

Will 265/50R20 fit instead of 305/50R20?
At 4.9% this lands between the 3% and 5% marks. Plenty of people run swaps like this, but you will see real speedometer error, and you want to check the wheel well at full lock and full compression before you buy. Your speedometer will read high, so you are going slower than it says. Overall diameter goes from 32.01″ to 30.43″, a change of −4.9%. The rule of thumb most shops use: under 3% is fine, 3 to 5% needs a clearance check, over 5% needs recalibration.
How far off will my speedometer read on 265/50R20?
At an indicated 60 mph you are really doing about 57.0 mph, a −4.9% error. The speedometer reads high, so you are going slower than it shows.
What is the difference in revolutions per mile between 305/50R20 and 265/50R20?
305/50R20 turns about 630 times per mile and 265/50R20 turns about 663, a difference of +33. Your speedometer and odometer both count rotations, so this number is what throws them off.