What is the diameter difference between 225/45R18 and 245/35R20?

225/45R18 has an overall diameter of 25.97 inches (660 mm). 245/35R20 is 26.75 inches (680 mm). That's a 0.78 inch (20 mm) difference — 245/35R20 is taller.

Will 245/35R20 cause speedometer error on a car fitted with 225/45R18?

Swapping 225/45R18 for 245/35R20 produces a speedometer error of about 3%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 61.8 mph. Anything under 3% is usually acceptable without recalibration.

How many revolutions per mile does each size do?

225/45R18 turns 777 revs per mile. 245/35R20 turns 754 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.

What's the sidewall height difference?

225/45R18 has a sidewall of 101 mm (3.99 in). 245/35R20 sidewall is 86 mm (3.38 in). Lower-profile sidewalls give sharper steering response but less bump compliance.

Is 245/35R20 a safe plus-size upgrade from 225/45R18?

A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 3%. This swap is outside the typical ±3% safe range and may require speedometer recalibration or affect ABS / traction control behaviour.

Tire size comparison

225/45R18 vs 245/35R20

Overall diameter, sidewall, revs per mile, and speedometer error — 245/35R20 is the taller tire, with a 3% speedo delta when swapping.

225/45R18

25.97in

660 mm overall diameter

245/35R20

26.75in

680 mm overall diameter

Diameter Δ

0.78 in

Speedo error

225/45R18 revs/mi

777

245/35R20 revs/mi

754

Outside the typical ±3% safe plus-size band — you may want speedometer recalibration, and this change can affect ABS or traction control behaviour.

Full spec

Side by side

Metric	225/45R18	245/35R20
Section width	225 mm	245 mm
Aspect ratio	45	35
Wheel diameter	18″	20″
Sidewall height	101 mm	86 mm
Overall diameter	25.97 in (660 mm)	26.75 in (680 mm)
Circumference	81.59 in	84.04 in
Revolutions / mile	777	754

Other comparisons for these sizes

205/55R16 vs 225/45R18 215/50R17 vs 225/45R18 225/50R17 vs 225/45R18 225/40R18 vs 225/45R18 225/45R18 vs 225/40R19 225/45R18 vs 235/45R18 225/45R18 vs 245/45R18 225/45R18 vs 235/40R18 245/40R19 vs 245/35R20 245/35R20 vs 255/35R20

FAQ

Frequently asked

What is the diameter difference between 225/45R18 and 245/35R20?: 225/45R18 has an overall diameter of 25.97 inches (660 mm). 245/35R20 is 26.75 inches (680 mm). That's a 0.78 inch (20 mm) difference — 245/35R20 is taller.
Will 245/35R20 cause speedometer error on a car fitted with 225/45R18?: Swapping 225/45R18 for 245/35R20 produces a speedometer error of about 3%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 61.8 mph. Anything under 3% is usually acceptable without recalibration.
How many revolutions per mile does each size do?: 225/45R18 turns 777 revs per mile. 245/35R20 turns 754 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.
What's the sidewall height difference?: 225/45R18 has a sidewall of 101 mm (3.99 in). 245/35R20 sidewall is 86 mm (3.38 in). Lower-profile sidewalls give sharper steering response but less bump compliance.
Is 245/35R20 a safe plus-size upgrade from 225/45R18?: A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 3%. This swap is outside the typical ±3% safe range and may require speedometer recalibration or affect ABS / traction control behaviour.