245/35R20 Tire Size: Dimensions, Alternatives & Speedometer Effect

Measured from the ground to the top of the tread, a new 245/35R20 tire stands 679.5 mm tall (26.8 inches). The sidewall — the rubber between the rim edge and the tread — is 85.8 mm high, the tire covers 2134.7 mm of road per revolution, and it completes 753.9 revolutions in a mile. Those numbers come straight from the three figures in the size code: 245 mm width, 35% aspect ratio and a 20-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.

Before the numbers, one practical point. A tire size is only correct for your car if it matches what the manufacturer specified, and the place to find that is the placard on the driver's door jamb, not the size currently fitted — a previous owner may have changed it. The dimensions on this page tell you exactly what a 245/35R20 measures, so you can compare it against the placard figure and against any alternative you are considering. If the two do not match, the tables further down show how far apart they are and whether the difference stays inside the commonly used plus-or-minus 3% guideline.

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 245/35R20 mean?

The code breaks into three parts. The first number, 245, is the section width in millimetres — the width of the tire from sidewall to sidewall when mounted on its measuring rim. The second number, 35, is the aspect ratio: the sidewall height expressed as a percentage of the width. Multiply them (245 x 35% = 85.8 mm) and you get the sidewall height. The final number, 20, is the rim diameter in inches — 20 x 25.4 = 508.0 mm. Add the rim diameter to twice the sidewall (508.0 + 2 x 85.8 = 679.5 mm) and you have the overall diameter. That is the whole formula, and it is why every size in the chart can be checked rather than trusted blindly.

245/35R20 dimensions

Measurement Value
Overall diameter 679.5 mm (26.8 in)
Sidewall height 85.8 mm
Circumference 2134.7 mm
Revolutions per mile 753.9
Rim diameter 20 in

The diameter of 679.5 mm (26.8 in) is the headline number because everything else follows from it. The circumference, 2134.7 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 753.9 revolutions per mile — the figure the speedometer and odometer effectively count. A tire with fewer revs per mile covers more ground per turn, so the car travels faster than the speedometer thinks if you fit a taller tire without recalibrating. All of these are nominal new-tire 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 tire.

Closest alternative sizes to 245/35R20

Diameter is the right way to compare candidate replacements, and the table ranks the nearest three. The best match is 245/40R19 at -0.1% (-0.9 mm). Sizes inside plus or minus 3% of the original diameter are the ones usually considered for a swap, because the speedometer error they introduce is small. Outside that band the error grows and so do the knock-on effects on gearing, odometer readings and, on some cars, the calibration of driver-assistance systems that assume a standard rolling circumference.

Alternative size Diameter Sidewall Diameter difference Within ±3% guideline?
245/40R19 678.6 mm (26.7 in) 98.0 mm -0.1% Within the guideline
285/35R19 682.1 mm (26.9 in) 99.8 mm +0.4% Within the guideline
275/35R19 675.1 mm (26.6 in) 96.2 mm -0.6% Within the guideline

Speedometer effect of changing from 245/35R20

Here is what the diameter difference does on the road. The speedometer assumes the original circumference, so a -0.1% diameter change from fitting 245/40R19 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
245/40R19 -0.1% 30.0 mph 49.9 mph 69.9 mph
285/35R19 +0.4% 30.1 mph 50.2 mph 70.3 mph
275/35R19 -0.6% 29.8 mph 49.7 mph 69.5 mph

Wider, narrower or taller than 245/35R20: what actually changes

Going wider than 245 mm while keeping the same aspect ratio also makes the tire taller, because the sidewall is a percentage of the width — that is the single most common surprise in a size change. Going wider and dropping the aspect ratio keeps the diameter closer to standard, which is the idea behind plus-sizing. A taller tire raises the car slightly and lengthens the gearing, so the engine turns a little slower at a given road speed; a shorter tire does the opposite. Wider rubber generally brings more dry grip and more road noise, and it can increase fuel use slightly through higher rolling resistance. None of these effects is dramatic for a small change, but they all point the same way: change one dimension at a time and keep the diameter close.

How to check 245/35R20 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 tire. Then confirm physical clearance at full steering lock and under load, and confirm the pressure-monitoring system still works. A tire professional can run through all of that quickly, and that confirmation is the step this page cannot replace. The 245/35R20 measures 679.5 mm across, so that is the diameter the placard figure has to be compared with.

Mistakes to avoid with 245/35R20

The most common mistake is reading the size off the tire already fitted and assuming it is the correct size — a previous owner may have changed it. The second is going wider without adjusting the aspect ratio, which quietly makes the tire taller and throws the speedometer off. The third is ignoring the load index and speed rating, which matter as much as the dimensions. The fourth is fitting different sizes on the same axle, which the car was not designed for. And the fifth is forgetting that the spare, the pressure-monitoring system and the speedometer all assume the standard size — change the size and all three need a thought. For a 245/35R20, that means checking the 245 mm width, the 35% aspect ratio and the 20-inch rim against the placard before you buy.

Related 245/35R20 sizes and tools

Frequently asked questions

What is the overall diameter of a 245/35R20 tire?

The overall diameter of a new 245/35R20 tire is 679.5 mm, which is 26.8 inches. That is the rim diameter (508.0 mm) plus twice the sidewall height (85.8 mm). It is a nominal figure, so an individual tire can vary slightly by brand, model, wear and inflation pressure.

How tall is the sidewall on a 245/35R20 tire?

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 tire gives over bumps.

How many revolutions per mile does a 245/35R20 tire do?

A 245/35R20 tire completes 753.9 revolutions per mile, based on its 2134.7 mm circumference. That figure matters because the speedometer and odometer count revolutions: fit a tire with a different circumference and the readings drift by the same percentage as the diameter change.

What is the closest alternative size to 245/35R20?

The closest size in the chart is 245/40R19, 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 245/35R20 affect my speedometer?

Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 245/40R19 instead of 245/35R20 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.