255/45R18 vs 235/55R18
235/55R18 is 4.2% taller (+1.14 in) and 0.79 in narrower than 255/45R18.
235/55R18 is 4.2% taller than 255/45R18. It often still fits, but expect a noticeable speedometer error and check clearance at full steering lock.
Real speed on 235/55R18.
Change in height under the axle.
On the same wheel, split inboard and outboard.
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255/45R18 or 235/55R18?
Can I use 235/55R18 instead of 255/45R18?
235/55R18 is 4.2% taller than 255/45R18. It often still fits, but expect a noticeable speedometer error and check clearance at full steering lock. Also match or beat the original load index and speed rating, and check that the wheel width suits the new tire.
Will my speedometer be off?
At an indicated 60 mph on 235/55R18 you are really doing 62.5 mph if the speedometer was set up for 255/45R18. Going the other way, 255/45R18 would make it read 57.6 mph at a real 60.
How much ground clearance does it change?
Half the diameter difference: +0.57 in (+15 mm) under the axle.