Tyre and wheel calculators
Change a size and the results follow; the address keeps your numbers, so you can share a result.
Compare two tyre sizes
| Measure | 205/70 R14 | 205/65 R15 | Change |
|---|---|---|---|
| Diameter | 642.6 mm25.30 in | 647.5 mm25.49 in | +4.9 mm+0.76 % |
| Sidewall | 143.5 mm | 133.3 mm | −10.3 mm |
| Circumference | 2019 mm | 2034 mm | +15 mm |
| Revs per km | 495.3 | 491.6 | −3.7 |
- When the speedometer reads 100 km/h you are doing 100.8 km/h.
- At a true 100 km/h it reads 99.2 km/h.
- The odometer records −7.6 km per 1,000 km driven.
Your speedometer will read low. With the larger tyre it shows less than your true speed. UN ECE Regulation 39 doesn’t allow a speedometer to show less than the true speed — check it against GPS, and allow for it.
Diameters are nominal — worked out from the size. A loaded tyre rolls about 3 % smaller; comparisons between two tyres aren't affected.
Closest tyre for a new rim
Only tyres that plausibly go on this width (approximate).
Target: 205/70 R14, 642.6 mm · 25.30 in across.
| Size | Diameter | Difference | Within |
|---|---|---|---|
| 175/75 R15 | 643.5 mm25.33 in | +0.14 % | ±1 % |
| 155/85 R15 | 644.5 mm25.37 in | +0.30 % | ±1 % |
| 165/80 R15 | 645.0 mm25.39 in | +0.37 % | ±1 % |
| 185/70 R15 | 640.0 mm25.20 in | −0.40 % | ±1 % |
| 235/55 R15 | 639.5 mm25.18 in | −0.48 % | ±1 % |
| 265/50 R15 | 646.0 mm25.43 in | +0.53 % | ±1 % |
| 215/60 R15 | 639.0 mm25.16 in | −0.56 % | ±1 % |
| 205/65 R15 | 647.5 mm25.49 in | +0.76 % | ±1 % |
| 285/45 R15 | 637.5 mm25.10 in | −0.79 % | ±1 % |
| 255/50 R15 | 636.0 mm25.04 in | −1.03 % | ±2 % |
| 155/82 R15 | 635.2 mm25.01 in | −1.15 % | ±2 % |
| 245/55 R15 | 650.5 mm25.61 in | +1.23 % | ±2 % |
| 195/65 R15 | 634.5 mm24.98 in | −1.26 % | ±2 % |
| 225/60 R15 | 651.0 mm25.63 in | +1.31 % | ±2 % |
| 165/82 R15 | 651.6 mm25.65 in | +1.40 % | ±2 % |
| 195/70 R15 | 654.0 mm25.75 in | +1.77 % | ±2 % |
| 275/50 R15 | 656.0 mm25.83 in | +2.09 % | ±3 % |
| 155/80 R15 | 629.0 mm24.76 in | −2.12 % | ±3 % |
| 165/75 R15 | 628.5 mm24.74 in | −2.19 % | ±3 % |
| 225/55 R15 | 628.5 mm24.74 in | −2.19 % | ±3 % |
9 more within ±3 %.
Sizes from 155 to 285 mm wide and aspects 30–85 % (and 82). Check the size is made, and its load and speed rating, before you buy. Diameters are nominal — worked out from the size. A loaded tyre rolls about 3 % smaller; comparisons between two tyres aren't affected.
Offset and position
- Outer face: 0.7 mm further in (−0.7 mm)
- Inner face: 13.3 mm further in, towards the strut (+13.3 mm)
- Tyre: 2.5 mm taller (radius, +2.5 mm)
- Backspacing: current 6½J ET 30 125.3 mm · 4.93 in; new 7J ET 37 138.6 mm · 5.46 in
Rim widths are nominal (between the bead seats); the flanges add about an inch overall, which backspacing includes.
Backspacing to ET
Measured backspacing is from the hub face to the inner edge of the rim.
135 mm (5.31 in) of backspacing on a 7 in rim is ET 33.4.
Wheel bolt length
Under the head to the tip.
The new wheel's mounting pad at the bolt hole, minus the old one's (+ if thicker).
How much thread reaches into the hub with the original bolt, if you've measured it.
- Bolt length that keeps the original engagement: 45.0 mm
- Minimum thread engagement: 12 mm (once the thread diameter, M12)
Keep the seat the wheel needs (Mercedes wheels take ball-seat bolts of the right radius).
Tyre age
The last group of digits after “DOT” on the sidewall: week, then year.