Tire Size Calculator
Compare two tire sizes — diameter, sidewall, circumference, revolutions per mile — and see how far off your speedometer will read.
Diameter difference
+1.72%
- Current tire diameter
- 24.97 in (634.3 mm)
- New tire diameter
- 25.4 in (645.2 mm)
- True speed when the speedometer shows 60
- 61
Nominal sizes — real tires vary a little by brand, tread and inflation. Check your car's door placard and owner's manual before changing sizes.
Side by side
| Current | New | |
|---|---|---|
| Width | 225 mm (8.86 in) | 235 mm (9.25 in) |
| Sidewall height | 101.3 mm (3.99 in) | 94 mm (3.7 in) |
| Overall diameter | 24.97 in (634.3 mm) | 25.4 in (645.2 mm) |
| Circumference | 78.5 in | 79.8 in |
| Revolutions per mile | 808 | 794 |
Show the work
Current sidewall
sidewall = width × aspect ÷ 100
225 × 45 ÷ 100 = 101.25 mm
Current diameter
D = rim × 25.4 + 2 × sidewall
17 × 25.4 + 2 × 101.25 = 634.3 mm
New sidewall
sidewall = width × aspect ÷ 100
235 × 40 ÷ 100 = 94 mm
New diameter
D = rim × 25.4 + 2 × sidewall
18 × 25.4 + 2 × 94 = 645.2 mm
Diameter difference
(Dnew ÷ Dcurrent − 1) × 100
(645.2 ÷ 634.3 − 1) × 100 = +1.72%
How it works
Sidewall = width × aspect ratio ÷ 100; diameter = rim × 25.4 + 2 × sidewall (mm); true speed = speedometer reading × new diameter ÷ old diameter
Questions
- How does the tire size calculator work?
- Sidewall = width × aspect ratio ÷ 100; diameter = rim × 25.4 + 2 × sidewall (mm); true speed = speedometer reading × new diameter ÷ old diameter.
- What do I need to enter?
- Current tire size, New tire size, Speedometer reading. Optional fields can be left blank.
Related tools
- Light Bulb Watts to LumensFind the lumens an LED needs to replace an old 40–150 W bulb, or the old-bulb equivalent of an LED's lumens, with the energy saved.
- Ring Size ConverterConvert ring sizes between US/Canada, UK/Australia letters, inner diameter and inner circumference (ISO 8653).
- Hat Size ConverterTurn a head measurement into US fitted hat size (in eighths) and metric hat size.
- Coffee to Water Ratio CalculatorFind grams of coffee for your water (or water for your coffee) at the SCA golden ratio or your own brew ratio.
- Baker's Percentage CalculatorTurn a dough's weights into baker's percentages and hydration — or percentages back into grams for any flour amount.
- Turkey Cooking Time CalculatorUSDA roasting time for a turkey by weight at 325 °F, stuffed or not, plus the safe internal temperature and thawing time.
Want this on your site? Embed the tire size calculator.