Tire Compare
Tire sizes side by side: diameter, sidewall, speedo error, and sizes that keep your diameter.
255/35R18 is 2.7% shorter than 245/40R18: inside the ±3% rule. At an indicated 60 mph you'd be doing 58.4.
255/35R18 vs current−2.68%WITHIN ±3% · −17.5 mm diameter
| 245/40R18 | 255/35R18 | |
|---|---|---|
| Overall diameter | 653.2 mm (25.72″) | 635.7 mm (25.03″) −17.5 |
| Sidewall | 98.0 mm | 89.3 mm −8.7 |
| Section width | 245 mm (9.6″) | 255 mm (10.0″) +10 |
| Wheel diameter | 18″ | 18″ |
| Circumference | 2052 mm | 1997 mm |
| Revs per mile | 784 | 806 |
| Ground clearance | — | −8.7 mm |
| Typical rim widths | 8–9.5″ | 8.5–9.5″ |
SPEEDO SHOWS → YOU'RE ACTUALLY DOING
| SPEEDO | ON 255/35R18 |
|---|---|
| 30 mph | 29.2 mph |
| 45 mph | 43.8 mph |
| 60 mph | 58.4 mph |
| 75 mph | 73.0 mph |
| 90 mph | 87.6 mph |
SIZES THAT KEEP YOUR DIAMETER
Within ±1.5% of 245/40R18 (25.72″) on a 18″ wheel, closest first. Check it's made in your load index and speed rating.
- 215/45R18−0.38%25.62″
- 275/35R18−0.54%25.58″
- 225/45R18+1.00%25.97″
- 235/40R18−1.22%25.40″
- 255/40R18+1.22%26.03″
KNOW THE TERMS
- 245/40R18
- 245 mm section width / sidewall 40% of that width / Radial / fits an 18″ wheel. Letters before the R (ZR) and the load and speed code after it don't change the size.
- Overall diameter
- Wheel diameter plus two sidewalls. It decides speedo accuracy, gearing and ground clearance.
- The ±3% rule
- A common guideline: keep a new size within about 3% of stock diameter so the speedo, ABS and traction control stay in sensible range. Some cars are fussier, and AWD cars need all four tires matched far more closely.
- Speedo error
- Your speedo assumes the stock size. A taller tire covers more ground per turn, so you're going faster than it shows; a shorter one, slower.
- Revs per mile
- How many times the tire turns in a mile. Fewer revs means taller effective gearing.
- Plus sizing
- Going up a wheel size with a lower-profile tire so the overall diameter stays the same: same speedo, less sidewall.
ASSUMPTIONS
- Dimensions are nominal, from the size designation. Real tires vary a few mm by brand, model and wear.
- Width is the section width measured on the tire's standard rim; it grows or shrinks about 5 mm per ½″ of rim width.
- Speedo figures assume the speedometer is accurate on your current size. Many read a little high from the factory.
- Typical rim widths are a rule of thumb from approved-rim tables; the tire maker's list is the real authority.