How to use the tire size calculator
- Current tire: choose Metric and enter the size printed on your sidewall. For 225/65R17, type 225 in Width (mm), 65 in Aspect ratio (%) and 17 in Rim diameter (in).
- New tire: enter the size you are thinking of buying the same way. For truck sizes marked like 33x12.50R15, pick Flotation and fill in Overall diameter (in), Section width (in) and Rim diameter (in).
- Speedometer reads: type the speed you want to check and choose mph or km/h.
- Read the verdict, table and speedometer lines under Your results. They update as you type.
How the numbers are worked out
A metric size gives the sidewall as a percentage of the width. Sidewall height is width ร aspect ratio รท 100 (in millimeters), and overall diameter is the rim diameter plus two sidewalls, converted at 25.4 mm per inch. Circumference is ฯ ร diameter, and revolutions per mile is 63,360 inches divided by that circumference. Your speedometer counts wheel turns, so real speed equals the speedometer reading ร new diameter รท current diameter. Flotation sizes already state the overall diameter, so it is used as given. The diameter formula is the one published in the Wikipedia Tire code article, which notes that the results are theoretical.
Worked example with the starting numbers
The current 225/65R17 has a 146.25 mm sidewall (5.76 in), so its overall diameter is 28.52 in and it turns 707 times per mile. The new 235/60R18 has a 141 mm sidewall (5.55 in), which gives 29.10 in and 693 turns per mile. That is +0.59 in, or +2.06%, so it passes the 3% rule of thumb. When your speedometer reads 60 mph you are really going 61.2 mph, and after 100 real miles the odometer shows 97.98 miles. Engine speed at the same road speed is 2.02% lower.
Limits and edge cases
- Sizes give theoretical diameters. Measured diameters differ by brand and model, and tread wear shrinks a tire by roughly 2% over its life (Wikipedia, Speedometer).
- The 3% limit is a rule of thumb, not a law. Also check the approved rim width range for the new tire, keep the load index and speed rating at or above the originals, and confirm fender and suspension clearance.
- The results compare the new tire with your current one, not with true speed. EU and UNECE rules let a speedometer read up to 10% plus 4 km/h above true speed, so check yours against a GPS app.
- A taller tire also lowers engine RPM at a given speed. The gear ratio calculator shows the exact RPM, and the speed converter switches between mph and km/h. The fuel cost calculator prices a change in mileage.
Tire size questions
How do I read a tire size like 225/65R17?
225 is the section width in millimeters, 65 means the sidewall is 65% as tall as the tire is wide, R marks radial construction, and 17 is the wheel diameter in inches.
How much bigger can my new tires be?
A common rule of thumb is to keep the overall diameter within 3% of the original. For 225/65R17 that is 28.52 in plus or minus 0.86 in, so about 27.66 to 29.37 in. Beyond that, speedometer error and gearing changes become noticeable and clearance problems are more likely.
Will bigger tires make my speedometer wrong?
Yes. A taller tire covers more ground on each turn, so the speedometer reads lower than your real speed. A tire that is 2.06% taller turns 60 mph on the dial into 61.2 mph on the road.
Does tire width change the speedometer?
Not by itself, because only overall diameter matters. But a wider tire at the same aspect ratio is also taller, since the sidewall is a percentage of the width. Lower the aspect ratio when you go wider to keep the diameter close to the original.
Sources: Wikipedia, Tire code; Wikipedia, Speedometer (UNECE Regulation 39 tolerances). Results are calculated from nominal sizes and are not a fitment guarantee.