Why overall diameter is the number that matters
A tire size looks like a width and a wheel size, but the figure that changes how your car drives is the one that isn't written on the sidewall: the overall diameter. Two of the three numbers in a metric size set it — the section width and the aspect ratio give you the sidewall height, and the rim diameter adds the middle — so a change that keeps the same rim can still make the tire noticeably taller or shorter. That overall height is what the speedometer, odometer and final drive were calibrated around, and on modern cars it also feeds the wheel-speed signal the ABS and stability control read. Change it and all of those read from slightly wrong numbers.
This is why the 3% guideline exists. Keep the new diameter within about 3% of the old one and the speedometer error stays small, the clearance change is minor, and the safety systems are close enough to happy. That is the target of a proper plus-size: step up in rim diameter, drop the aspect ratio to match, and keep the overall height near stock. Go beyond 3% on purpose — for a lift, for real off-road tires — and the trade is a speedometer that reads optimistically, an odometer and trip computer that drift, and clearance you have to check rather than assume. The calculator shows the exact percentage and the speed error at real-world speeds so the decision is deliberate.
The other figures fill in the picture. Revolutions per mile is effectively what the odometer counts, so matching it between two tires is the cleanest proof that a size swap is speedometer-neutral. Section width tells you how much wider the tire sits — roughly half the difference hanging over each side — which is where clearance and poke come from, though the measured width grows with rim width, so treat it as nominal. Use these numbers to shortlist a size and rule out an obvious mistake; they describe the tire's geometry, not whether it clears your particular car.
FAQ
How do I read a tire size like 275/65R18?
The first number, 275, is the section width in millimeters. The second, 65, is the aspect ratio — the sidewall height as a percentage of the width, so here the sidewall is 65% of 275, about 179 mm. The R means radial construction, and 18 is the rim diameter in inches. Overall diameter is the rim plus two sidewalls. Flotation sizes such as 35x12.50R17 are simpler: 35 is the overall diameter in inches, 12.50 is the width in inches, and 17 is the rim.
What is the 3% rule for changing tire size?
It is the common guideline that your new tire's overall diameter should stay within about 3% of the original. Overall diameter is what your speedometer, odometer and gearing were calibrated around, and on modern cars it also feeds the ABS and stability-control systems that read wheel speed. Stay within 3% and the speedometer error and clearance change stay small; go beyond it deliberately, for lift and clearance, knowing the readings drift and the systems are working from different numbers.
How does a taller tire change my speedometer?
A taller tire travels farther per revolution, so the car actually moves faster than the speedometer reads, and the odometer counts fewer miles than you drive. The error is proportional to the diameter change: a tire 5% taller means that at an indicated 60 mph you are really doing about 63 mph. This tool prints the actual speed at 30, 45, 60, 70 and 80 mph, with the km/h equivalents, so you can see the error across the range rather than at one point.
What are revolutions per mile, and why do they matter?
Revolutions per mile is how many times the tire turns to cover a mile — the mile length in millimeters divided by the tire's circumference. It is the number the speedometer and odometer effectively count, so a change in revolutions per mile is a direct measure of how far your readings will drift. Two tires with the same revolutions per mile read identically even if the sizes look different, which is exactly what plus-sizing aims for.
What tire sizes are close to mine?
The list under the comparison is calculated from your current size rather than taken from a catalog. It starts on the same rim — one or two steps in section width, 10 mm each, and one or two steps in aspect ratio, 5 points each — and keeps only the sizes whose overall diameter stays within 3% of your current tire, the one-step neighbors first. For one rim size up, two up and one down it tries widths from 10 mm narrower to 20 mm wider with every aspect ratio, and again keeps only what stays within 3% — which is what plus-one and plus-two fitments are: less sidewall on a bigger rim, the same overall height. Every row shows the diameter, the change in millimeters and percent, how much wider it sits, how much sidewall it gains or loses, and the real speed at an indicated 60 mph. Because they are calculated, check that the size is actually made, that it suits your rim width and that its load index is right for your vehicle.
Will a different tire size fit my wheel and car?
This tool compares the sizes; it does not confirm a fit. A tire has to suit the rim width — each rim has an approved range — and the taller or wider result has to clear the fender, suspension and brakes at full lock and full droop. Section width also grows on a wider rim, so the real width can exceed the printed number. Use the alternative-size list and the speedometer figures to shortlist a size, then check the rim-width range and clearance for your specific car before buying.
Tire fitment by vehicle
These are the factory tire sizes the presets above are built from — the baseline the 3% guideline is measured against. Each vehicle page carries the full wheel and tire specs and lets you try real wheels on that car. On the staggered cars (BMW M3, GR Supra, Mustang GT) the figure is the front tire.
- Ford F-150 tire size — 275/65R18 on 18×8.5
- Toyota 4Runner tire size — 265/70R17 on 17×7.5
- Jeep Wrangler tire size — 245/75R17 on 17×7.5
- Chevrolet Silverado 1500 tire size — 265/65R18 on 18×8.5
- Toyota Tacoma tire size — 265/65R17 on 17×7.5
- Ram 1500 tire size — 275/65R18 on 18×8
- Toyota RAV4 tire size — 225/65R17 on 17×7
- Tesla Model 3 tire size — 235/45R18 on 18×8.5
- Tesla Model Y tire size — 255/45R19 on 19×9.5
- BMW M3 tire size — 275/35R19 on 19×9.5
- Honda Civic Si tire size — 235/40R18 on 18×8
- Toyota GR Supra tire size — 255/35R19 on 19×9
- Subaru WRX tire size — 245/40R18 on 18×8.5
- Ford Mustang GT tire size — 255/40R19 on 19×9