Is 265/70R18 bigger than 315/70R17?

265/70R18 is 1.76 inches shorter and 50 mm narrower. In overall diameter that is −5.11%, which exceeds the ±3% rule of thumb for a substitution.

How much will 265/70R18 throw off my speedometer?

Every reading scales by 0.94890. An indicated 60 mph becomes 56.9 mph, and every 100 miles on the odometer is really 94.9 miles.

How much does 265/70R18 change ground clearance over 315/70R17?

The axle centerline moves by 0.88 inches — half the diameter change, because half the extra height sits below the hub and half above it.

Will 265/70R18 fit where 315/70R17 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.

315/70R17 vs 265/70R18

265/70R18 is −1.76″ in overall diameter and −50 mm in section width against 315/70R17 — a diameter change of −5.11%, which breaks the ±3% substitution rule. An indicated 60 mph would really be 56.9 mph.

Outside 3%

Every dimension

Difference between 315/70R17 and 265/70R18
Dimension	315/70R17 → 265/70R18	Change
Overall diameter	34.36″ → 32.61″	−1.76″ (−5.11%)
Section width	315 mm → 265 mm	−50 mm (−15.87%)
Sidewall height	220.5 mm → 185.5 mm	−35.0 mm
Rolling circumference	107.95″ → 102.44″	−5.52″
Revolutions per mile	586.9 → 618.5	+31.6
Axle height / clearance	17.18″ → 16.30″	−0.88″

315/70R17315 mm

265/70R18265 mm

315/70R17 34.36″265/70R18 32.61″

Which is taller, 315/70R17 or 265/70R18?

315/70R17 is the taller of the two, at 34.36″ against 32.61″. The gap is 1.76 inches, or 44.6 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. 315/70R17 is 315 mm wide with a 220.5 mm sidewall on a 17″ wheel; 265/70R18 is 265 mm wide with a 185.5 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	23.7 mph	−1.3 mph
40 mph	38.0 mph	−2.0 mph
55 mph	52.2 mph	−2.8 mph
60 mph	56.9 mph	−3.1 mph
70 mph	66.4 mph	−3.6 mph
80 mph	75.9 mph	−4.1 mph

Fitting 265/70R18 in place of 315/70R17 multiplies every speed and distance reading by 0.94890. Over 10,000 indicated miles the odometer is out by −511 miles.

Effective gearing moves by the same proportion. A vehicle with 3.73 axles running 315/70R17 would need 3.54 to keep the same engine speed on 265/70R18. Work it out for your own axle ratio.

Full specification of each size

315/70R17

Derived dimensions for 315/70R17
Dimension	Metric	Imperial
Section width	315.0 mm	12.40″
Sidewall height	220.5 mm	8.68″
Wheel diameter	431.8 mm	17.0″
Overall diameter	872.8 mm	34.36″
Rolling circumference	2,742.0 mm	107.95″
Revolutions per mile	364.7 / km	586.9 / mile
Measuring rim (estimate)	241 mm	9.5″

265/70R18

Derived dimensions for 265/70R18
Dimension	Metric	Imperial
Section width	265.0 mm	10.43″
Sidewall height	185.5 mm	7.30″
Wheel diameter	457.2 mm	18.0″
Overall diameter	828.2 mm	32.61″
Rolling circumference	2,601.9 mm	102.44″
Revolutions per mile	384.3 / km	618.5 / mile
Measuring rim (estimate)	203 mm	8″

Rim widths differ too: 315/70R17 is designed for 9″–10.5″ and 265/70R18 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?

Outside the ±3% rule

The diameter difference of −5.11% exceeds the ±3% rule of thumb. Expect a speedometer error of about 3.1 mph at an indicated 60, and an odometer that is off by the same proportion. It is a common swap even so — it just needs the instruments recalibrated to stay honest.

Neither figure says anything about physical fitment. Whether 265/70R18 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.