235/65R17 vs 255/40R21

255/40R21 is 0.0% taller (+0.00 in) and 0.79 in wider than 235/65R17.

Same height

255/40R21 and 235/65R17 are the same overall diameter, so the speedometer stays accurate.

	235/65R17	255/40R21	Change
Diameter	29.03″	29.03″	+0.00″+0.0%
Width	9.25″	10.04″	+0.79″+8.5%
Sidewall	6.01″	4.02″	−2.00″−33.2%
Circumference	91.2″	91.2″	+0.0″+0.0%
Wheel	17″	21″	+4″
Revs per mile	695	695	−0

235/65R17255/40R21

Change sizes Speedometer table Gearing

Speedometer shows 60 mph60.0mph

Real speed on 255/40R21.

Ground clearance+0.00in

Change in height under the axle.

Width per side+0.39in

On the same wheel, split inboard and outboard.

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235/65R17 or 255/40R21?

Can I use 255/40R21 instead of 235/65R17?

255/40R21 and 235/65R17 are the same overall diameter, so the speedometer stays accurate. 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 255/40R21 you are really doing 60.0 mph if the speedometer was set up for 235/65R17. Going the other way, 235/65R17 would make it read 60.0 mph at a real 60.

How much ground clearance does it change?

Half the diameter difference: +0.00 in (+0 mm) under the axle.

Other comparisons

235/70R16 vs 235/65R17 215/65R17 vs 235/65R17 225/60R17 vs 235/65R17 225/65R17 vs 235/65R17 235/60R17 vs 235/65R17 235/65R17 vs 245/65R17 235/65R17 vs 235/70R17 235/65R17 vs 255/65R17 235/65R17 vs 245/70R17 235/65R17 vs 255/45R20 235/65R17 vs 255/50R19 235/65R17 vs 255/55R18 235/70R16 vs 255/40R21 255/40R21 vs 255/50R19 255/40R21 vs 255/45R20 245/35R21 vs 255/40R21 255/40R21 vs 295/35R21 285/35R21 vs 255/40R21