235/50R20 Tyre Size: Dimensions, Alternatives & Speedometer Effect

A 235/50R20 tyre is 235 mm wide, has an aspect ratio of 50% and fits a 20-inch rim. Worked through, that gives an overall diameter of 743.0 mm (29.3 inches), a sidewall height of 117.5 mm, a circumference of 2334.2 mm and 689.5 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.

Before the numbers, one practical point. A tyre 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 235/50R20 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 235/50R20 mean?

There are only three numbers to decode. Width: 235 mm. Aspect ratio: 50%, which fixes the sidewall at 235 x 50/100 = 117.5 mm. Rim: 20 inches, or 508.0 mm. The diameter is the rim diameter plus the sidewall top and bottom, giving 743.0 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.

235/50R20 dimensions

Measurement Value
Overall diameter 743.0 mm (29.3 in)
Sidewall height 117.5 mm
Circumference 2334.2 mm
Revolutions per mile 689.5
Rim diameter 20 in

Start with the circumference: 2334.2 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 (689.5) 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 (117.5 mm) is the flexible part: it absorbs the first shock from bumps and potholes, and a medium-profile sidewall like this one means a balanced sidewall that splits the difference between handling response and ride comfort. These are nominal new-tyre dimensions, and real tyres vary slightly by brand, model, wear and inflation pressure.

Closest alternative sizes to 235/50R20

Diameter is the right way to compare candidate replacements, and the table ranks the nearest three. The best match is 235/55R19 at -0.3% (-1.9 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?
235/55R19 741.1 mm (29.2 in) 129.2 mm -0.3% Within the guideline
295/35R21 739.9 mm (29.1 in) 103.2 mm -0.4% Within the guideline
285/45R19 739.1 mm (29.1 in) 128.2 mm -0.5% Within the guideline

Speedometer effect of changing from 235/50R20

Here is what the diameter difference does on the road. The speedometer assumes the original circumference, so a -0.3% diameter change from fitting 235/55R19 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 69.8 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
235/55R19 -0.3% 29.9 mph 49.9 mph 69.8 mph
295/35R21 -0.4% 29.9 mph 49.8 mph 69.7 mph
285/45R19 -0.5% 29.8 mph 49.7 mph 69.6 mph

Wider, narrower or taller than 235/50R20: what actually changes

On this page, 235/50r20 tyre size is 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 235/50R20 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 235/50R20 measures 743.0 mm across, so that is the diameter the placard figure has to be compared with.

Mistakes to avoid with 235/50R20

Watch for these traps. Assuming the current tyre is the correct one, instead of checking the placard. Changing width without compensating the aspect ratio, so the diameter drifts. Buying on size alone and overlooking load index and speed rating. Mixing different sizes on the same axle. And skipping the clearance check at full lock, which is exactly where a wider tyre usually makes contact. If you avoid those five, and keep the diameter within plus or minus 3% of the placard size, you have dealt with the common errors before they happen. A 235/50R20 is 235 mm wide on a 20-inch rim with a 743.0 mm overall diameter, so those are the numbers the placard check has to match.

Related 235/50R20 sizes and tools

Frequently asked questions

What is the overall diameter of a 235/50R20 tyre?

The overall diameter of a new 235/50R20 tyre is 743.0 mm, which is 29.3 inches. That is the rim diameter (508.0 mm) plus twice the sidewall height (117.5 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 235/50R20 tyre?

The sidewall is 117.5 mm tall, because the aspect ratio is 50% of the 235 mm width (235 x 50/100 = 117.5 mm). A medium-profile sidewall like this one means a balanced sidewall that splits the difference between handling response and ride comfort. The sidewall height is the main thing that decides how much cushioning the tyre gives over bumps. That is the point this page works through for 235/50r20 tyre size.

How many revolutions per mile does a 235/50R20 tyre do?

A 235/50R20 tyre completes 689.5 revolutions per mile, based on its 2334.2 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 235/50R20?

The closest size in the chart is 235/55R19, 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 235/50R20 affect my speedometer?

Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 235/55R19 instead of 235/50R20 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.