How do you compare two tire sizes?

Work out the overall diameter of each from its size code, then take the difference as a percentage of the original. That single percentage is also the speedometer error, the odometer error and the change in effective gearing, because all three are proportional to rolling circumference.

Is a wider tire always taller?

No. Aspect ratio is a percentage of width, so a wider tire with a lower profile is often shorter. 265/70R17 stands 31.61 inches tall; 285/60R18 is 20 mm wider and yet 31.46 inches — very slightly shorter, despite the bigger wheel.

How much does ground clearance change?

Exactly half the diameter change, because the axle sits at the center of the tire. Going up 1.10 inches in overall diameter raises the axle — and with it the body, the bumper and the differential — by 0.55 inches. The common mistake is to expect the full 1.10.

Does a size comparison tell me whether a tire will fit?

No. It tells you what changes dimensionally. Whether the tire clears the strut, the fender liner and the steering stops at full lock depends on wheel offset and suspension geometry that no size code contains.

Side by side

Tire Size Comparison

Enter the size you have and the size you are thinking about. This shows the difference in overall diameter, section width, sidewall height, ground clearance and revolutions per mile, and what your speedometer will read once the new tires are on. Both sizes are read straight from the code, so it works for any size, not just a list.

Current size

versus

New size

Common sizes

Difference in overall diameter

Outside 3%

+3.49%

285/70R17 is +1.10″ in overall diameter against 265/70R17, which lifts the axle by +0.55″ and changes the speedometer by +3.49%. That breaks the ±3% rule of thumb tire fitters use, so expect a noticeable speedometer error.

265/70R17265 mm

285/70R17285 mm

265/70R17 31.61″285/70R17 32.71″

Every dimension

Difference between 265/70R17 and 285/70R17
Dimension	265/70R17 → 285/70R17	Change
Overall diameter	31.61″ → 32.71″	+1.10″ (+3.49%)
Section width	265 mm → 285 mm	+20 mm (+7.55%)
Sidewall height	185.5 mm → 199.5 mm	+14.0 mm
Rolling circumference	99.29″ → 102.76″	+3.46″
Revolutions per mile	638.1 → 616.6	−21.5
Axle height / clearance	15.80″ → 16.35″	+0.55″

What the speedometer will read

Indicated versus actual speed after the size change
Speedometer reads	Actual speed	Error
30 mph	31.0 mph	+1.0 mph
50 mph	51.7 mph	+1.7 mph
60 mph	62.1 mph	+2.1 mph
70 mph	72.4 mph	+2.4 mph
80 mph	82.8 mph	+2.8 mph

Your odometer is affected the same way: every 100 miles it counts is really 103.5 miles on the ground.

There is a permanent page for this comparison: 265/70R17 vs 285/70R17

What actually changes when you change size?

Four numbers move, and only one of them is independent. Overall diameter is the driver; rolling circumference, revolutions per mile and speedometer error are all fixed functions of it. Section width is the one genuinely separate dimension, and it is the one that decides whether a tire rubs.

The percentages are not symmetric, which trips people up. Going from 265/70R17 to 285/70R17 is +3.49%; going back the other way is −3.37%. The denominator changes, so an increase and the decrease that undoes it are different numbers.

How much difference is too much?

±3% of overall diameter is the working limit. At 3% an indicated 60 mph is really 61.8 mph — small enough that the dash is still honest, and small enough that wheel-speed sensors feeding ABS and stability control keep producing ratios the car recognizes.

Section width has no equivalent rule, because there is no equivalent formula. A tire 20 mm wider needs roughly 10 mm more room on each side of the arch, but where that room has to be depends entirely on the wheel's offset. Two wheels of the same width and different offset put the same tire in two different places.

Ready-made comparisons

Each of these is a permanent page with the whole calculation worked out and a scale drawing of the two tires on the same ground line.

All 557 comparisons →

Common questions about tire size comparison

How do you compare two tire sizes?: Work out the overall diameter of each from its size code, then take the difference as a percentage of the original. That single percentage is also the speedometer error, the odometer error and the change in effective gearing, because all three are proportional to rolling circumference.
Is a wider tire always taller?: No. Aspect ratio is a percentage of width, so a wider tire with a lower profile is often shorter. 265/70R17 stands 31.61 inches tall; 285/60R18 is 20 mm wider and yet 31.46 inches — very slightly shorter, despite the bigger wheel.
How much does ground clearance change?: Exactly half the diameter change, because the axle sits at the center of the tire. Going up 1.10 inches in overall diameter raises the axle — and with it the body, the bumper and the differential — by 0.55 inches. The common mistake is to expect the full 1.10.
Does a size comparison tell me whether a tire will fit?: No. It tells you what changes dimensionally. Whether the tire clears the strut, the fender liner and the steering stops at full lock depends on wheel offset and suspension geometry that no size code contains.