ReckonBoxReading a tire size
A tire diameter is the rim plus twice the sidewall, where the sidewall is the width in millimeters times the aspect ratio. A size like 265/70R17 packs all three numbers in: 265mm wide, a sidewall 70% of that width, on a 17 inch rim.
Do the arithmetic. The sidewall is 265 times 0.70, or 185.5mm, which is 7.3 inches. Two sidewalls plus the 17 inch rim gives an overall diameter of about 31.61 inches. A 225/45R17 works out smaller, about 24.97 inches, because the lower 45 aspect ratio means a much shorter sidewall on the same rim. That diameter is the number that matters when you change sizes.
How a size change moves the speedometer
A speedometer counts wheel revolutions, not distance, so a taller tire travels farther per turn and makes your true speed higher than the dial shows. The correction is the ratio of the diameters.
Put 285/70R17 tires on in place of the 265/70R17 stock size and the diameter grows from 31.61 to 32.71 inches, a 3.5% jump. Your true speed is the reading times the new diameter over the old, so an indicated 60 mph is really about 62.1 mph. The speedometer reads 3.5% slow, and the odometer undercounts by the same 3.5%, so 100 real miles show as roughly 96.5. That shifts mileage-based service intervals and every per-mile figure quietly in the background.
| 265/70R17 to 285/70R17 | Value |
|---|---|
| Old diameter | 31.61 in |
| New diameter | 32.71 in |
| Diameter change | 3.49% |
| True speed at an indicated 60 | 62.1 mph |
Plus sizing keeps the diameter honest
Plus sizing is the trick of fitting a bigger rim with a lower-profile tire so the overall diameter barely changes. Go up an inch of rim and drop the aspect ratio, and a 205/55R16 and a 225/45R17 end up within a fraction of a percent of each other in height. That keeps the speedometer accurate and the gearing intact while changing the look and the handling feel. The common rule of thumb is to hold the overall diameter within about 3% of stock. Past that the speedometer error grows, the effective gear ratio shifts, and a taller tire can rub the fender or suspension at full lock.
What this does not cover
This compares the geometry of two tire sizes and the speed effect, not whether a size physically fits your vehicle. Fitment depends on the wheel width, offset, and backspacing, the fender and suspension clearance, and the load and speed ratings the vehicle requires, none of which a size string alone tells you. Load index and speed rating are stamped after the size for a reason. Before buying, a tire shop or the vehicle placard confirms what actually bolts on and clears.
Match the diameter within a few percent and the only thing that changes is the part you meant to change.
Frequently asked questions
How do I calculate a tire diameter from its size? Overall diameter in inches is the rim size plus twice the sidewall height. The sidewall is the width in millimeters times the aspect ratio percent, converted to inches by dividing by 25.4. A 225/45R17 is 17 plus 2 times (225 times 0.45 divided by 25.4), which is about 24.97 inches. A 265/70R17 works out to about 31.61 inches.
How does a tire size change affect the speedometer? A speedometer counts wheel revolutions, so a taller tire travels farther per turn and makes your true speed higher than the dial shows. True speed is the reading times the new diameter divided by the old. Going from a 31.61 inch to a 32.71 inch tire, an indicated 60 mph is really about 62.1 mph, a 3.5% underread.
What is plus sizing? Plus sizing is fitting a larger rim with a lower-profile tire so the overall diameter stays about the same. Going up a rim inch while dropping the aspect ratio keeps the diameter close, which preserves the speedometer accuracy and gearing while changing the look and handling. Matching the diameter within a couple of percent is the goal.
How much larger a tire can I safely fit? A common rule of thumb is to keep the overall diameter within about 3% of the original so the speedometer, gearing, and clearance stay close. Beyond that, the speedometer error grows, the effective gear ratio changes, and the tire can rub the fender or suspension. This tool shows the diameter change so you can see where you land.
Does a bigger tire make the odometer wrong too? Yes, by the same ratio as the speedometer. A tire 3.5% larger means the odometer undercounts distance by about 3.5%, so 100 actual miles read as roughly 96.5. That affects mileage-based service intervals and any per-mile figure. The revolutions per mile the tool reports is what the odometer and speedometer both count.