Tire size calculators

Free tools that turn tire size codes into real dimensions. Try a quick comparison below, or pick a calculator, a size page or a reference chart.

Metric (265/70R17) or inch (33x12.50R20). Axle ratio is used for the RPM table only.

285/65R18 is 1.02 in (26 mm) taller than 265/65R18 (+3.2%). With a speedometer calibrated for 265/65R18, an indicated 60 mph is really 61.9 mph. The diameter difference is more than ±3%, a range often quoted as a rule of thumb, so speedometer, gearing and clearance effects are larger.

Geometry only: this does not confirm that a size fits, is approved for or is safe on a particular vehicle. Wheel width and offset, suspension, clearances, load index, speed rating, the vehicle maker's specification and local regulations all apply — check with your owner's manual and a qualified tire professional.

265/65R18 vs 285/65R18
	265/65R18	285/65R18	Difference
Diameter	31.56 in	32.59 in	+1.02 in (+3.24%)
Diameter (mm)	802 mm	828 mm	+26 mm
Width	10.43 in	11.22 in	+0.79 in (+20 mm)
Sidewall	6.78 in	7.29 in	+0.51 in
Rim	18 in	18 in	±0.0 in
Circumference	99.2 in	102.4 in	+3.22 in
Revs per mile	639	619	−20
Ground clearance change	—	—	+0.51 in

Speedometer error

Indicated vs true speed (mph)
Speedometer shows	True speed on 285/65R18	Difference
20 mph	20.6 mph	+0.6 mph
30 mph	31.0 mph	+1.0 mph
40 mph	41.3 mph	+1.3 mph
50 mph	51.6 mph	+1.6 mph
60 mph	61.9 mph	+1.9 mph
70 mph	72.3 mph	+2.3 mph
80 mph	82.6 mph	+2.6 mph

Indicated vs true speed (km/h)
Speedometer shows	True speed on 285/65R18	Difference
30 km/h	31.0 km/h	+1.0 km/h
50 km/h	51.6 km/h	+1.6 km/h
80 km/h	82.6 km/h	+2.6 km/h
100 km/h	103.2 km/h	+3.2 km/h
120 km/h	123.9 km/h	+3.9 km/h

Assumes the speedometer was accurate on 265/65R18. Factory speedometers often read slightly high on purpose; check against GPS.

Engine RPM and gearing

Engine RPM in a 1:1 gear with a 3.73 axle ratio
Road speed	265/65R18	285/65R18	Change
45 mph	1,788 rpm	1,731 rpm	−57 rpm
55 mph	2,185 rpm	2,116 rpm	−69 rpm
65 mph	2,582 rpm	2,501 rpm	−81 rpm
75 mph	2,979 rpm	2,886 rpm	−93 rpm

To restore the original effective gearing on 285/65R18 you would need an axle ratio of about 3.85 (nearest ratios on our reference list: 3.73 or 3.91). For overdrive gears multiply the RPM by the gear ratio (e.g. ×0.69).

Tire calculators

Tire size calculator
Diameter, width, sidewall and revolutions for any size.
Tire size comparison
Two sizes side by side, to scale, with speedometer and RPM tables.
Speedometer error calculator
How fast you are really going after a tire change.
Inch ↔ metric converter
33x12.50R17 to metric and back, with closest equivalents.
Plus size calculator
Sizes that keep your diameter on a bigger or smaller wheel.
Gear ratio & RPM
Engine RPM and the regear needed for bigger tires.
DOT date code decoder
Find out when a tire was made and how old it is.
How to read a tire size
Every part of a sidewall code explained.
Tire size chart
Common sizes by wheel diameter with heights and widths.
Load index chart
Every load index from 60 to 150 in kg and lb.
Speed rating chart
Speed symbols J to Y in km/h and mph.

Tire size comparisons

All comparisons →

Tire sizes

Light truck & off-road (metric)

Flotation (inch) sizes

Crossover & SUV

Passenger car

How the numbers are worked out

A metric size like 265/70R17 describes a tire 265 mm wide whose sidewall is 70% of that width (185.5 mm), fitted to a 17-inch wheel. Overall diameter is the wheel plus two sidewalls: 17 in + 2 × 7.303 in = 31.61 in. Inch (flotation) sizes like 33x12.50R17 state the diameter and width directly.

Everything on RimRatio is computed from those published dimensions with the formulas on our methodology page. Real tires vary a little by brand, tread depth, wheel width and load, so treat the results as planning numbers and confirm fitment with your vehicle documentation and installer.