235/35R20 Tyre Size: Dimensions, Alternatives & Speedometer Effect

If you are looking up the 235/35R20 tyre size, the figures that matter are the diameter (672.5 mm, or 26.5 inches), the sidewall (82.2 mm), the circumference (2112.7 mm) and the revolutions per mile (761.7). The size code itself tells you the ingredients — 235 mm of width, an aspect ratio of 35%, a 20-inch rim — and this page shows how those ingredients combine into the measurements above, which sizes sit closest to it, and what happens to your speedometer if you fit one of them instead. Every figure here is computed from the size code, not estimated, so you can use it to check a tyre you already own or one you are thinking of buying.

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 235/35R20 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 235/35R20 mean?

The code breaks into three parts. The first number, 235, is the section width in millimetres — the width of the tyre 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 (235 x 35% = 82.2 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 82.2 = 672.5 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.

235/35R20 dimensions

Measurement Value
Overall diameter 672.5 mm (26.5 in)
Sidewall height 82.2 mm
Circumference 2112.7 mm
Revolutions per mile 761.7
Rim diameter 20 in

Start with the circumference: 2112.7 mm per revolution. That is the distance your car covers every time this tyre turns once, and it is what makes the revs-per-mile figure (761.7) what it is — a mile divided by the circumference. Fit a tyre with a larger circumference and each revolution carries you further, so at the same wheel speed the car is moving faster than the instruments report. The sidewall (82.2 mm) is the flexible part: it absorbs the first shock from bumps and potholes, and a low-profile sidewall like this one means a short, stiff sidewall that sharpens steering response but leaves less cushioning over potholes and kerbs. These are nominal new-tyre dimensions, and real tyres vary slightly by brand, model, wear and inflation pressure.

Closest alternative sizes to 235/35R20

The three sizes below are the closest matches to 235/35R20 in the chart, drawn from the same rim or one inch either side and ranked by diameter difference. The closest, 235/40R19, differs by -0.3% (-1.9 mm). The commonly used guideline is to stay within plus or minus 3% of the original diameter, which keeps the speedometer error small and the gearing close to standard. That guideline is about the speedometer and the car's calibration, not about whether the tyre physically clears the arches — a size can be within 3% on diameter and still rub if it is much wider, so width and clearance need their own check.

Alternative size Diameter Sidewall Diameter difference Within ±3% guideline?
235/40R19 670.6 mm (26.4 in) 94.0 mm -0.3% Within the guideline
275/35R19 675.1 mm (26.6 in) 96.2 mm +0.4% Within the guideline
265/35R19 668.1 mm (26.3 in) 92.8 mm -0.7% Within the guideline

Speedometer effect of changing from 235/35R20

The speedometer effect is a straight ratio. Fitting 235/40R19 (670.6 mm) in place of 235/35R20 (672.5 mm) changes the diameter by -0.3%, so every indicated speed is multiplied by 0.997 to get the real speed. At 30 mph on the dial the true speed is 29.9 mph, at 50 mph it is 49.9 mph, and at 70 mph it is 69.8 mph. The table below shows the same working for all three alternatives, so you can see at a glance which swap keeps the error negligible and which one starts to matter.

Replacement size Diameter change Actual speed at 30 mph indicated Actual speed at 50 mph indicated Actual speed at 70 mph indicated
235/40R19 -0.3% 29.9 mph 49.9 mph 69.8 mph
275/35R19 +0.4% 30.1 mph 50.2 mph 70.3 mph
265/35R19 -0.7% 29.8 mph 49.7 mph 69.5 mph

Wider, narrower or taller than 235/35R20: what actually changes

Taking 235/35r20 tyre size as the case in point: Going wider than 235 mm while keeping the same aspect ratio also makes the tyre 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 tyre raises the car slightly and lengthens the gearing, so the engine turns a little slower at a given road speed; a shorter tyre 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 235/35R20 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 235 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 235/35R20 measures 672.5 mm across, so that is the diameter the placard figure has to be compared with.

Mistakes to avoid with 235/35R20

The most common mistake is reading the size off the tyre 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 tyre 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 235/35R20, that means checking the 235 mm width, the 35% aspect ratio and the 20-inch rim against the placard before you buy.

Related 235/35R20 sizes and tools

Frequently asked questions

What is the overall diameter of a 235/35R20 tyre?

The overall diameter of a new 235/35R20 tyre is 672.5 mm, which is 26.5 inches. That is the rim diameter (508.0 mm) plus twice the sidewall height (82.2 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 235/35R20 tyre?

The sidewall is 82.2 mm tall, because the aspect ratio is 35% of the 235 mm width (235 x 35/100 = 82.2 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 235/35R20 tyre do?

A 235/35R20 tyre completes 761.7 revolutions per mile, based on its 2112.7 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 235/35R20?

The closest size in the chart is 235/40R19, which differs in diameter by -0.3% (-1.9 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 235/35R20 affect my speedometer?

Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 235/40R19 instead of 235/35R20 changes the diameter by -0.3%, so an indicated 70 mph would really be 69.8 mph. Keep the diameter within plus or minus 3% to keep that error small, and check the placard before fitting anything different.