245/35R21 Tyre Size: Dimensions, Alternatives & Speedometer Effect
Measured from the ground to the top of the tread, a new 245/35R21 tyre stands 704.9 mm tall (27.8 inches). The sidewall — the rubber between the rim edge and the tread — is 85.8 mm high, the tyre covers 2214.5 mm of road per revolution, and it completes 726.7 revolutions in a mile. Those numbers come straight from the three figures in the size code: 245 mm width, 35% aspect ratio and a 21-inch rim. This guide explains the maths behind them, lists the sizes that match this one most closely, and works out what fitting a different size would do to your speedometer reading.
People usually land on this page for one of three reasons: checking a tyre already on the car, shopping for a replacement, or wondering whether a slightly different size will work. In all three cases the approach is the same — start with the exact measurements, then compare. A 245/35R21 is a mid-width size, a short, stiff sidewall that sharpens steering response but leaves less cushioning over potholes and kerbs, and the tables below let you hold it up against the nearest alternatives on diameter, sidewall and revs per mile. The disclaimer box just below is part of that comparison: these are calculations, and the final fitment check belongs to your placard and a tyre professional.
This is a calculation, not a safety approval. Always check your vehicle's door-jamb placard, load index and speed rating, and confirm fitment with a tyre professional before fitting a different tyre size.
What does 245/35R21 mean?
There are only three numbers to decode. Width: 245 mm. Aspect ratio: 35%, which fixes the sidewall at 245 x 35/100 = 85.8 mm. Rim: 21 inches, or 533.4 mm. The diameter is the rim diameter plus the sidewall top and bottom, giving 704.9 mm in total. Because the aspect ratio is a percentage of the width, a wider tyre with the same aspect ratio is also a taller tyre — a point that catches people out when they go wider without dropping the aspect ratio to compensate.
245/35R21 dimensions
| Measurement | Value |
|---|---|
| Overall diameter | 704.9 mm (27.8 in) |
| Sidewall height | 85.8 mm |
| Circumference | 2214.5 mm |
| Revolutions per mile | 726.7 |
| Rim diameter | 21 in |
The diameter of 704.9 mm (27.8 in) is the headline number because everything else follows from it. The circumference, 2214.5 mm, is pi times the diameter, and it is the distance the car travels each time the wheel turns once. Divide a mile (63,360 inches) by the circumference in inches and you get 726.7 revolutions per mile — the figure the speedometer and odometer effectively count. A tyre with fewer revs per mile covers more ground per turn, so the car travels faster than the speedometer thinks if you fit a taller tyre without recalibrating. All of these are nominal new-tyre figures: real tyres vary slightly by brand, model, wear and inflation pressure, so treat the decimals as precise maths on a nominal size, not a measurement of your individual tyre.
Closest alternative sizes to 245/35R21
If you are looking for a substitute, start here. These are the three nearest sizes by overall diameter, limited to rims within one inch of 21 inches so the wheel itself stays realistic. 245/40R20 is the nearest at -0.1% (-0.9 mm difference). Anything within plus or minus 3% is generally treated as acceptable from a speedometer point of view, but diameter is only one of the checks: the replacement also needs the right load index and speed rating, and enough clearance around the suspension and arches at full steering lock.
| Alternative size | Diameter | Sidewall | Diameter difference | Within ±3% guideline? |
|---|---|---|---|---|
| 245/40R20 | 704.0 mm (27.7 in) | 98.0 mm | -0.1% | Within the guideline |
| 255/40R20 | 712.0 mm (28.0 in) | 102.0 mm | +1.0% | Within the guideline |
| 255/35R21 | 711.9 mm (28.0 in) | 89.2 mm | +1.0% | Within the guideline |
Speedometer effect of changing from 245/35R21
Here is what the diameter difference does on the road. The speedometer assumes the original circumference, so a -0.1% diameter change from fitting 245/40R20 shifts every reading by the same percentage. Indicated 30 mph becomes 30.0 mph in reality, indicated 50 mph becomes 49.9 mph, and indicated 70 mph becomes 69.9 mph. The odometer drifts by the same percentage too, which also affects recorded mileage over time. If the difference were larger than 3%, recalibration would be worth discussing with a specialist rather than ignored.
| Replacement size | Diameter change | Actual speed at 30 mph indicated | Actual speed at 50 mph indicated | Actual speed at 70 mph indicated |
|---|---|---|---|---|
| 245/40R20 | -0.1% | 30.0 mph | 49.9 mph | 69.9 mph |
| 255/40R20 | +1.0% | 30.3 mph | 50.5 mph | 70.7 mph |
| 255/35R21 | +1.0% | 30.3 mph | 50.5 mph | 70.7 mph |
Wider, narrower or taller than 245/35R21: what actually changes
What actually changes if you move away from 245/35R21? Width first: a wider tyre puts more rubber on the road, which usually helps dry grip and can hurt fuel use a little through extra rolling resistance. Height second: because the sidewall is 35% of the width, widening without lowering the aspect ratio raises the diameter, lengthens the gearing and lifts the car slightly. A shorter sidewall sharpens steering response but transmits more of the road surface into the cabin. The practical rule is to hold the diameter as steady as you can — that is what the alternative table above is for — and to check that a wider tyre still clears the suspension and arches at full lock before committing.
How to check 245/35R21 fits your vehicle
Do the checks in this order. Read the placard and note the specified size, load index and speed rating for your exact vehicle. Compare the candidate size against the placard diameter using the plus-or-minus 3% guideline. Confirm the load index and speed rating are at least what the placard asks for. Confirm the rim width suits a 245 mm tyre. Then confirm physical clearance at full steering lock and under load, and confirm the pressure-monitoring system still works. A tyre professional can run through all of that quickly, and that confirmation is the step this page cannot replace. The 245/35R21 measures 704.9 mm across, so that is the diameter the placard figure has to be compared with.
Mistakes to avoid with 245/35R21
People go wrong in a handful of predictable ways. They trust the fitted tyre instead of the placard. They pick a wider tyre and forget the aspect ratio, so the diameter creeps up. They match the size code but buy a lower load index or speed rating. They mix sizes across an axle. They forget to check clearance at full steering lock, where a tyre that clears going straight can still rub. Each of these is easy to avoid once you know to look for it, and the tables on this page give you the numbers to check against. On a 245/35R21 the figures to check are a 704.9 mm diameter, a 85.8 mm sidewall and 726.7 revs per mile.
Related 245/35R21 sizes and tools
- 255/35R21 tyre size
- 255/40R21 tyre size
- Use the Tyre Size Calculator to compare 245/35R21 against any other size.
- Use the Plus Sizing Calculator to find a wider size that keeps the diameter close to 245/35R21.
Frequently asked questions
What is the overall diameter of a 245/35R21 tyre?
The overall diameter of a new 245/35R21 tyre is 704.9 mm, which is 27.8 inches. That is the rim diameter (533.4 mm) plus twice the sidewall height (85.8 mm). It is a nominal figure, so an individual tyre can vary slightly by brand, model, wear and inflation pressure.
How tall is the sidewall on a 245/35R21 tyre?
Taking 245/35r21 tyre size as the case in point: The sidewall is 85.8 mm tall, because the aspect ratio is 35% of the 245 mm width (245 x 35/100 = 85.8 mm). A low-profile sidewall like this one means a short, stiff sidewall that sharpens steering response but leaves less cushioning over potholes and kerbs. The sidewall height is the main thing that decides how much cushioning the tyre gives over bumps.
How many revolutions per mile does a 245/35R21 tyre do?
A 245/35R21 tyre completes 726.7 revolutions per mile, based on its 2214.5 mm circumference. That figure matters because the speedometer and odometer count revolutions: fit a tyre with a different circumference and the readings drift by the same percentage as the diameter change.
What is the closest alternative size to 245/35R21?
The closest size in the chart is 245/40R20, which differs in diameter by -0.1% (-0.9 mm). The commonly used guideline is to stay within plus or minus 3% of the original diameter, and the alternative table on this page shows which sizes meet that. Diameter is only one check, though — load index, speed rating and physical clearance still need confirming.
Will changing from 245/35R21 affect my speedometer?
Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 245/40R20 instead of 245/35R21 changes the diameter by -0.1%, so an indicated 70 mph would really be 69.9 mph. Keep the diameter within plus or minus 3% to keep that error small, and check the placard before fitting anything different.