Is 265/65R18 bigger than 255/70R17?

265/65R18 is 0.51 inches taller and 10 mm wider. In overall diameter that is +1.64%, which stays inside the ±3% rule of thumb for a substitution.

How much will 265/65R18 throw off my speedometer?

Every reading scales by 1.01635. An indicated 60 mph becomes 61.0 mph, and every 100 miles on the odometer is really 101.6 miles.

How much does 265/65R18 change ground clearance over 255/70R17?

The axle centerline moves by 0.25 inches — half the diameter change, because half the extra height sits below the hub and half above it.

Will 265/65R18 fit where 255/70R17 fits?

Nothing derived from the size code can answer that. Clearance depends on wheel offset, suspension travel and body dimensions that no tire size contains. This page tells you exactly what changes dimensionally, which is the part that can be calculated.

255/70R17 vs 265/65R18

265/65R18 is +0.51″ in overall diameter and +10 mm in section width against 255/70R17 — a diameter change of +1.64%, which stays inside the ±3% substitution rule. An indicated 60 mph would really be 61.0 mph.

Within 3%

Every dimension

Difference between 255/70R17 and 265/65R18
Dimension	255/70R17 → 265/65R18	Change
Overall diameter	31.06″ → 31.56″	+0.51″ (+1.64%)
Section width	255 mm → 265 mm	+10 mm (+3.92%)
Sidewall height	178.5 mm → 172.3 mm	−6.2 mm
Rolling circumference	97.56″ → 99.16″	+1.60″
Revolutions per mile	649.4 → 639.0	−10.4
Axle height / clearance	15.53″ → 15.78″	+0.25″

255/70R17255 mm

265/65R18265 mm

255/70R17 31.06″265/65R18 31.56″

Which is taller, 255/70R17 or 265/65R18?

265/65R18 is the taller of the two, at 31.56″ against 31.06″. The gap is 0.51 inches, or 12.9 mm.

Both are metric sizes, so neither states its height directly. Both figures below are derived from the section width, aspect ratio and wheel diameter in the code. 255/70R17 is 255 mm wide with a 178.5 mm sidewall on a 17″ wheel; 265/65R18 is 265 mm wide with a 172.3 mm sidewall on a 18″ wheel.

What the speedometer will read

Indicated versus actual speed after the size change
Speedometer reads	Actual speed	Error
25 mph	25.4 mph	+0.4 mph
40 mph	40.7 mph	+0.7 mph
55 mph	55.9 mph	+0.9 mph
60 mph	61.0 mph	+1.0 mph
70 mph	71.1 mph	+1.1 mph
80 mph	81.3 mph	+1.3 mph

Fitting 265/65R18 in place of 255/70R17 multiplies every speed and distance reading by 1.01635. Over 10,000 indicated miles the odometer is out by +164 miles.

Effective gearing moves by the same proportion. A vehicle with 3.73 axles running 255/70R17 would need 3.79 to keep the same engine speed on 265/65R18. Work it out for your own axle ratio.

Full specification of each size

255/70R17

Derived dimensions for 255/70R17
Dimension	Metric	Imperial
Section width	255.0 mm	10.04″
Sidewall height	178.5 mm	7.03″
Wheel diameter	431.8 mm	17.0″
Overall diameter	788.8 mm	31.06″
Rolling circumference	2,478.1 mm	97.56″
Revolutions per mile	403.5 / km	649.4 / mile
Measuring rim (estimate)	191 mm	7.5″

265/65R18

Derived dimensions for 265/65R18
Dimension	Metric	Imperial
Section width	265.0 mm	10.43″
Sidewall height	172.3 mm	6.78″
Wheel diameter	457.2 mm	18.0″
Overall diameter	801.7 mm	31.56″
Rolling circumference	2,518.6 mm	99.16″
Revolutions per mile	397.0 / km	639.0 / mile
Measuring rim (estimate)	203 mm	8″

Rim widths differ too: 255/70R17 is designed for 7″–8.5″ and 265/65R18 for 7.5″–9″. Their measuring rims differ, so the same wheel will not be ideal for both.

Can you swap one for the other?

Inside the ±3% rule

The diameter difference of +1.64% is within the ±3% that fitters use as the boundary for a substitution, so the speedometer, odometer and wheel-speed-derived systems stay close enough to calibration. The section width difference of +10 mm is a separate question and is the one that decides whether it rubs.

Neither figure says anything about physical fitment. Whether 265/65R18 clears the strut, the fender liner and the steering stops depends on wheel offset and suspension geometry that a size code does not encode. For maintenance rather than sizing, Car Basics covers tire care.