Tire Size Calculator
Compare two tire sizes: how much taller or shorter the new tire is, how far off it puts the speedometer, and what it does to ride height. For the sizes that fit a 1965 to 1970 Mustang, see the classic Mustang wheel and tire sizes.
Note: sizes are calculated from the numbers on the sidewall. Real tires of the same size differ by a few tenths of an inch between brands, so check the maker's published diameter before buying.
Current Tire
Calculated Diameter: 25.85"
New Tire
Calculated Diameter: 25.63"
Results
225/60R15 vs 215/70R14
−0.22"
−0.9% diameter (25.63" vs 25.85")
- Speedometer reads 60 mph
- 59.5 mph actual
- Ride height
- 0.11" lower
Within 3% of the current diameter, the change most tire guides call safe without other changes.
Size Comparison
| Measure | Current (215/70R14) | New (225/60R15) | Difference |
|---|---|---|---|
| Diameter | 25.85" | 25.63" | −0.22" |
| Section width | 8.46" | 8.86" | +0.39" |
| Sidewall height | 5.93" | 5.31" | −0.61" |
| Circumference | 81.21" | 80.52" | −0.69" |
| Revolutions per mile | 780.2 | 786.9 | +6.7 |
Speedometer Reading on 225/60R15
Assumes the speedometer read true on the 215/70R14.
| Speedometer reads | Actual speed |
|---|---|
| 30 mph | 29.7 mph |
| 45 mph | 44.6 mph |
| 60 mph | 59.5 mph |
| 75 mph | 74.4 mph |
On a T5, a different speedometer driven gear puts it right: the T5 speedometer gear calculator works it out for the 225/60R15.
Tire Size Formula
A metric size gives the section width in millimeters, the sidewall height as a percentage of that width, and the rim diameter in inches. The calculator works out the overall diameter as:
Diameter = rim + 2 × width × aspect ÷ 100 ÷ 25.4
25.4 turns millimeters into inches, and the sidewall counts twice because it's above and below the rim. An inch size like 27x10.50R15 gives the diameter and width in inches directly. The speedometer counts turns of the tire, so the actual speed is the reading × new diameter ÷ current diameter, and revolutions per mile are 63,360 inches ÷ the circumference.
Example (the calculator's defaults): a 215/70R14 is 14 + 2 × 215 × 70 ÷ 100 ÷ 25.4 = 25.85" tall, and a 225/60R15 is 15 + 2 × 225 × 60 ÷ 100 ÷ 25.4 = 25.63". The new tire is 0.22" shorter (−0.9%), so at an indicated 60 mph the car is doing 59.5 mph.
Frequently Asked Questions
Most tire guides keep a replacement within 3% of the original diameter. Inside that, the speedometer, gearing and clearances change little. Beyond it, expect a noticeably wrong speedometer, and check the clearance to the fenders, springs and control arms through full steering lock and suspension travel.
Yes. The speedometer counts turns of the transmission output, so a taller tire covers more road per turn and the car goes faster than the speedometer reads; a shorter tire does the opposite. On a classic Mustang the fix is a different driven gear in the transmission, which the T5 speedometer gear calculator works out.
In effect, yes. A taller tire acts like a numerically lower axle ratio: lower RPM at cruise and a little less acceleration. Going from a 215/70R14 to a 225/60R15 is under 1% shorter, which nobody will feel. The gear ratio calculator shows the engine speed at any road speed for the new size.
The R means radial construction; the Z marks a tire rated for over 149 mph. A 235/45ZR17 is the same size as a 235/45R17, so the calculator treats them the same. The P on a P-metric size means a passenger car tire and doesn't change the size either.