TireSizeCheck

305/70R17 vs 285/70R17

Fit verdict

Borderline. Check clearance first

Diameter change

−3.3%

At 3.3% 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

32.71″

was 33.81″

True speed at an indicated 60

58.0 mph

your speedo reads high

Revolutions per mile

617

was 596, +20

Measurement 305/70R17 285/70R17 Change
Overall diameter 33.81″ 32.71″ −1.10″ (−3.3%)
Sidewall height 8.41″ (213.5 mm) 7.85″ (199.5 mm) −14.0 mm
Tread width 305 mm 285 mm −20 mm
Circumference 106.22″ 102.76″ −3.46″
Revs per mile 596 617 +20
Speedometer error baseline −3.3% reads high

Diameter overlay

305/70R17 285/70R17

Speedometer check

With 285/70R17 fitted, when the speedometer shows the indicated speed, your actual speed is:

25 mph shown

24.2 mph actual

35 mph shown

33.9 mph actual

45 mph shown

43.5 mph actual

55 mph shown

53.2 mph actual

65 mph shown

62.9 mph actual

75 mph shown

72.6 mph actual

Try other sizes in the calculator 285/70R17 details 305/70R17 details

Common questions

Will 285/70R17 fit instead of 305/70R17?
At 3.3% 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 33.81″ to 32.71″, a change of −3.3%. 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 285/70R17?
At an indicated 60 mph you are really doing about 58.0 mph, a −3.3% error. The speedometer reads high, so you are going slower than it shows.
What is the difference in revolutions per mile between 305/70R17 and 285/70R17?
305/70R17 turns about 596 times per mile and 285/70R17 turns about 617, a difference of +20. Your speedometer and odometer both count rotations, so this number is what throws them off.