Tire Size Comparison Tool

Popular tire size comparisons

These pages carry the full calculation for a specific pair, so you can send someone a link rather than a set of instructions. The calculator above handles any combination not listed.

Getting a number you can act on

1. 01
   Start from what is actually fitted

   Read the size off the sidewall rather than from paperwork. Previous owners change sizes, and comparing against the wrong baseline makes every figure below wrong too.

2. 02
   Enter the size you are considering

   Metric and flotation sizes can be mixed freely — comparing a 285/70R17 against a 33x12.50R17 is one of the more useful things this tool does.

3. 03
   Look at the drawing before the table

   The overlay is drawn to a shared scale. If the two circles look nearly identical, the change is minor regardless of how different the sizes read.

4. 04
   Check the diameter change against the 3% line

   Inside 3% is generally treated as a routine substitution. Outside it, treat the change as a modification and verify clearance physically.

5. 05
   Read the speedometer consequence

   The true-speed figure tells you how fast you are really going when the dial reads 60. That number matters for tickets, for fuel calculations and for warranty mileage.

What the calculator is actually doing

Nothing here is proprietary. If you would rather check it by hand, or explain it to someone at a counter, these are the same expressions the tool evaluates.

Why half the diameter change disappears

The most common mistake when planning a size change is assuming a one-inch bigger tire raises the vehicle by an inch. It does not. It raises it by half an inch.

The reason is simple once you picture it. Overall diameter is measured across the whole tire, but the vehicle sits on the axle at the centre. Half the extra diameter goes downward, pushing the axle further from the ground; the other half goes upward, into the wheel arch.

So a change from 32 inches to 34 inches — a substantial jump — yields one inch of extra ground clearance at the axle and one inch less space above the tire. That second half is where fitment problems live, and it is the half people forget to check.

Speedometer error is not a rounding issue

Speedometers infer road speed from how fast the wheels turn, using a rolling circumference figure fixed when the vehicle was calibrated. Change the tire and that assumption becomes wrong by exactly the proportion the diameter changed.

A taller tire covers more ground per revolution than the calibration expects, so the vehicle is travelling faster than the dial shows. A 4% increase means an indicated 60 mph is really about 62.4 mph — enough to matter where enforcement tolerances are tight.

The odometer inherits the same error. At 4% over, every indicated 1,000 miles is really 1,040. Across a three-year lease that is thousands of miles that never appear on the clock, and across a warranty period it works the other way if the tires are smaller than stock.

Some vehicles allow the tire size to be recalibrated in software, either at a dealer or with an aftermarket tool. Where that option exists it is worth taking, because it corrects the speedometer, the odometer and anything else that derives speed from wheel rotation.

What to check when the change is more than 3%

• Clearance at full steering lock in both directions, not just straight ahead
• Clearance at full suspension compression, which is where liners and mud flaps get caught
• Whether the section width change brings the tire closer to the strut or the control arm
• Whether the load rating still meets the vehicle's placard requirement
• Whether the vehicle allows a speedometer recalibration for the new size
• Whether the gearing still suits the vehicle, particularly on a truck that tows

Comparing metric against flotation sizes

This is where a comparison tool earns its keep, because the two notations are genuinely hard to compare mentally.

A flotation size states its overall diameter outright: 33x12.50R17 is a nominal 33 inches tall and 12.5 inches wide on a 17-inch rim. A metric size hides its diameter behind a percentage calculation, so 285/70R17 gives no hint that it works out at roughly 32.7 inches.

Put through the same arithmetic, those two sizes are within about a third of an inch of each other in height, while the flotation tire is a little wider. That is why they are so often cross-shopped — and why choosing between them usually comes down to width, load rating and what is available in the tread pattern you want, rather than height.