235/45R20 Tire Size: Dimensions, Alternatives & Speedometer Effect

The 235/45R20 is a medium-profile tire size: 235 mm wide with a 45% aspect ratio on a 20-inch wheel. Its overall diameter is 719.5 mm (28.3 in), its sidewall stands 105.8 mm tall, it rolls 2260.4 mm per revolution and it turns 712.0 times per mile. That last number is the one the speedometer cares about most, because the car counts wheel revolutions to work out speed. Below you will find the full measurement table, a plain-English decoding of the size code, the three closest alternative sizes from the chart, and a worked speedometer example for each of them.

This is a measurement page, not a recommendation. Whether a 235/45R20 belongs on your car depends on the placard figure, the load index and the speed rating your vehicle requires — none of which a diameter table can settle on its own. What the table can do is give you the exact size of this tire (719.5 mm diameter, 105.8 mm sidewall, 2260.4 mm circumference) so that any comparison you make is grounded in real numbers. Work through the alternative-size table and the speedometer section before deciding anything, and treat the plus-or-minus 3% guideline as what it is: a rule of thumb, not an approval.

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 235/45R20 mean?

There are only three numbers to decode. Width: 235 mm. Aspect ratio: 45%, which fixes the sidewall at 235 x 45/100 = 105.8 mm. Rim: 20 inches, or 508.0 mm. The diameter is the rim diameter plus the sidewall top and bottom, giving 719.5 mm in total. Because the aspect ratio is a percentage of the width, a wider tire with the same aspect ratio is also a taller tire — a point that catches people out when they go wider without dropping the aspect ratio to compensate.

235/45R20 dimensions

Measurement Value
Overall diameter 719.5 mm (28.3 in)
Sidewall height 105.8 mm
Circumference 2260.4 mm
Revolutions per mile 712.0
Rim diameter 20 in

Start with the circumference: 2260.4 mm per revolution. That is the distance your car covers every time this tire turns once, and it is what makes the revs-per-mile figure (712.0) what it is — a mile divided by the circumference. Fit a tire 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 (105.8 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-tire dimensions, and real tyres vary slightly by brand, model, wear and inflation pressure.

Closest alternative sizes to 235/45R20

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. 265/40R20 is the nearest at +0.1% (+0.5 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?
265/40R20 720.0 mm (28.3 in) 106.0 mm +0.1% Within the guideline
265/35R21 718.9 mm (28.3 in) 92.8 mm -0.1% Within the guideline
265/45R19 721.1 mm (28.4 in) 119.2 mm +0.2% Within the guideline

Speedometer effect of changing from 235/45R20

The speedometer effect is a straight ratio. Fitting 265/40R20 (720.0 mm) in place of 235/45R20 (719.5 mm) changes the diameter by +0.1%, so every indicated speed is multiplied by 1.001 to get the real speed. At 30 mph on the dial the true speed is 30.0 mph, at 50 mph it is 50.0 mph, and at 70 mph it is 70.0 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
265/40R20 +0.1% 30.0 mph 50.0 mph 70.0 mph
265/35R21 -0.1% 30.0 mph 50.0 mph 69.9 mph
265/45R19 +0.2% 30.1 mph 50.1 mph 70.2 mph

Wider, narrower or taller than 235/45R20: what actually changes

What actually changes if you move away from 235/45R20? Width first: a wider tire 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 45% 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 tire still clears the suspension and arches at full lock before committing.

How to check 235/45R20 fits your vehicle

The checks are the same whatever size you are considering. First, the door-jamb placard: size, load index and speed rating. Second, the rim: a 20-inch rim of the right width for a 235 mm tire. Third, clearance — arches, suspension components and steering lock, with the car loaded as it will actually be used. Fourth, the pressure-monitoring system, which may need a reset or new sensors after a change. Fifth, the spare: if the spare is a different size, know its speed and distance limits before you need it. None of these is a formality, and together they are what turns a calculation into a safe fitment. For the 235/45R20, the 235 mm width and 105.8 mm sidewall are part of that same check.

Mistakes to avoid with 235/45R20

People go wrong in a handful of predictable ways. They trust the fitted tire instead of the placard. They pick a wider tire 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 tire 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 235/45R20 the figures to check are a 719.5 mm diameter, a 105.8 mm sidewall and 712.0 revs per mile.

Related 235/45R20 sizes and tools

Frequently asked questions

What is the overall diameter of a 235/45R20 tire?

The overall diameter of a new 235/45R20 tire is 719.5 mm, which is 28.3 inches. That is the rim diameter (508.0 mm) plus twice the sidewall height (105.8 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 235/45R20 tire?

The sidewall is 105.8 mm tall, because the aspect ratio is 45% of the 235 mm width (235 x 45/100 = 105.8 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 tire gives over bumps.

How many revolutions per mile does a 235/45R20 tire do?

A 235/45R20 tire completes 712.0 revolutions per mile, based on its 2260.4 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 235/45R20?

The closest size in the chart is 265/40R20, which differs in diameter by +0.1% (+0.5 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 235/45R20 affect my speedometer?

Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 265/40R20 instead of 235/45R20 changes the diameter by +0.1%, so an indicated 70 mph would really be 70.0 mph. Keep the diameter within plus or minus 3% to keep that error small, and check the placard before fitting anything different.