Tire Size Calculator
Enter a tire size the way it is written on the sidewall — 225/45R17 — to see its overall diameter, sidewall height and revolutions per mile, then compare it with a second size to check the speedometer error and the ride height change.
Dimensions are geometric values from the sidewall markings — real tires vary a little with rim width, load and inflation.
Formula and calculation
Worked example
Moving from 225/45R17 — 24.9724 inches tall — to 235/40R18 changes the diameter by 1.7%, so a speedometer reading of 60 would really be 61.0311. The taller tire lifts the body by 0.2146 inches and tightens the fender gap by the same amount.
The 3% guideline, and why it exists
Fitting tires more than about 3% taller or shorter than the factory size makes the speedometer and odometer drift, changes when the transmission shifts and how eagerly the car accelerates, and can confuse ABS and traction control that read wheel speed. It also moves the tire closer to the fender and suspension: a taller tire sits nearer the spring perch and can rub at full lock or over bumps. That is why tire shops settled on 3% as the practical limit for a stock vehicle — some trucks and SUVs have room for more with a lift, but that is a fitment question for the shop, not a rule of thumb.
What plus-sizing keeps and what it changes
Plus-sizing means going up a rim diameter while dropping the aspect ratio so the overall diameter stays close to stock: 225/45R17 to 235/40R18 is a plus-one step and only 1.7% taller. The diameter — and with it the speedometer, the gearing and the ride height — barely moves, while the shorter sidewall flexes less, so the steering feels quicker and the ride gets firmer. The wider tread that usually comes with it needs a rim wide enough for it, and it can rub on the inner fender at full lock. Check the vehicle placard or the tire shop's fitment guide before ordering.
What the results mean
- Overall diameter
- The tire's overall height, ground to top: rim diameter plus two sidewall heights. It sets your gearing, your speedometer calibration and how much room the tire needs.
- Diameter
- The same overall diameter in millimeters — the figure tire shops quote in metric markets.
- Sidewall height
- The rubber height between the rim and the tread. Plus-sizing (a bigger rim with a lower aspect ratio) always shortens the sidewall, which sharpens steering but firms up the ride.
- Circumference
- How far the tire rolls in one full turn, π × diameter. Revolutions × circumference is the distance the wheel has covered.
- Revolutions per mile
- How many times the wheel turns in a mile. ABS, traction control and speedometer calibrations are built around this number, which is why large changes cause problems.
- Diameter change vs your current tire
- How much taller (positive) or shorter (negative) the new tire is than your current one. Positive values raise the car and make the gearing numerically lower.
- Actual speed when the speedometer reads 60
- The speed you are really doing when the speedometer shows the reading you entered. A taller tire makes the speedometer read low; a shorter one makes it read high.
- Ride height and fender clearance change
- Half the diameter change, in inches. The body sits higher by this much, and the gap between the top of the tire and the fender shrinks by the same amount.
- Fitment check
- A one-line verdict on the change — inside or outside the 3% guideline that tire shops commonly use for a safe swap.
Frequently asked questions
How do I read a tire size like 225/45R17?
225 is the section width in millimeters, 45 is the aspect ratio — the sidewall is 45% of the width, so 101 mm tall — R means radial, and 17 is the rim diameter in inches. A size ending in something like 91V adds the load index and speed rating, which this calculator ignores.
Will a bigger tire make my speedometer wrong?
Yes, and by roughly the same percentage as the diameter change. A tire 3% taller than stock turns 3% slower at the same road speed, so the speedometer under-reads: it shows 60 when you are doing about 61.8. Shorter tires make it over-read.
What does the 3% rule mean for tires?
It is the industry rule of thumb that a replacement tire should stay within 3% of the factory diameter. On a stock 26.3-inch tire that is about 0.8 inches either way — enough room for most plus-size swaps, and past it the speedometer, gearing and clearance problems start adding up.
Do bigger tires hurt fuel economy?
They cut engine rpm at highway speed but add rotating weight and rolling resistance, and because they also change the odometer, hand-calculated mpg looks better than it really is. On most cars the net effect is small and hard to separate from driving conditions — measure a few tanks before and after if it matters to you.