235/40R20 Tyre Size: Dimensions, Alternatives & Speedometer Effect

A 235/40R20 tyre is 235 mm wide, has an aspect ratio of 40% and fits a 20-inch rim. Worked through, that gives an overall diameter of 696.0 mm (27.4 inches), a sidewall height of 94.0 mm, a circumference of 2186.5 mm and 736.0 revolutions per mile. Those four numbers are the whole story of this size: they decide how tall the tyre stands, how far the car travels on each revolution, and how far the speedometer reading can drift if you change size. This page lays out the exact figures, explains what each one means in plain language, and shows the closest alternative sizes so you can judge a swap with numbers instead of guesswork.

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 235/40R20 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 235/40R20 mean?

There are only three numbers to decode. Width: 235 mm. Aspect ratio: 40%, which fixes the sidewall at 235 x 40/100 = 94.0 mm. Rim: 20 inches, or 508.0 mm. The diameter is the rim diameter plus the sidewall top and bottom, giving 696.0 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.

235/40R20 dimensions

Measurement Value
Overall diameter 696.0 mm (27.4 in)
Sidewall height 94.0 mm
Circumference 2186.5 mm
Revolutions per mile 736.0
Rim diameter 20 in

The revs-per-mile figure, 736.0, deserves attention because it is the number your car actually senses. Speedometers and odometers count wheel revolutions and multiply by an assumed circumference; change the circumference and the maths drifts. With a 2186.5 mm circumference, this tyre turns 736.0 times per mile, and any replacement with a different circumference will turn a different number of times over the same mile. The diameter (696.0 mm, 27.4 in) and the sidewall (94.0 mm) complete the picture. Remember these are nominal new-tyre figures — real tyres vary slightly by brand, model, wear and inflation pressure, so two tyres with the same size code can differ by a millimetre or two in practice.

Closest alternative sizes to 235/40R20

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 20 inches so the wheel itself stays realistic. 265/40R19 is the nearest at -0.2% (-1.4 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?
265/40R19 694.6 mm (27.3 in) 106.0 mm -0.2% Within the guideline
235/45R19 694.1 mm (27.3 in) 105.8 mm -0.3% Within the guideline
275/40R19 702.6 mm (27.7 in) 110.0 mm +0.9% Within the guideline

Speedometer effect of changing from 235/40R20

Here is what the diameter difference does on the road. The speedometer assumes the original circumference, so a -0.2% diameter change from fitting 265/40R19 shifts every reading by the same percentage. Indicated 30 mph becomes 29.9 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
265/40R19 -0.2% 29.9 mph 49.9 mph 69.9 mph
235/45R19 -0.3% 29.9 mph 49.9 mph 69.8 mph
275/40R19 +0.9% 30.3 mph 50.5 mph 70.7 mph

Wider, narrower or taller than 235/40R20: what actually changes

For 235/40r20 tyre size, the point is this: 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 235/40R20 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 235/40R20 measures 696.0 mm across, so that is the diameter the placard figure has to be compared with.

Mistakes to avoid with 235/40R20

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 235/40R20 the figures to check are a 696.0 mm diameter, a 94.0 mm sidewall and 736.0 revs per mile.

Related 235/40R20 sizes and tools

Frequently asked questions

What is the overall diameter of a 235/40R20 tyre?

The overall diameter of a new 235/40R20 tyre is 696.0 mm, which is 27.4 inches. That is the rim diameter (508.0 mm) plus twice the sidewall height (94.0 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 235/40R20 tyre?

The sidewall is 94.0 mm tall, because the aspect ratio is 40% of the 235 mm width (235 x 40/100 = 94.0 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 235/40R20 tyre do?

A 235/40R20 tyre completes 736.0 revolutions per mile, based on its 2186.5 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 235/40R20?

The closest size in the chart is 265/40R19, which differs in diameter by -0.2% (-1.4 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 235/40R20 affect my speedometer?

Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 265/40R19 instead of 235/40R20 changes the diameter by -0.2%, 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.