225/35R20 Tyre Size: Dimensions, Alternatives & Speedometer Effect

Measured from the ground to the top of the tread, a new 225/35R20 tyre stands 665.5 mm tall (26.2 inches). The sidewall — the rubber between the rim edge and the tread — is 78.8 mm high, the tyre covers 2090.7 mm of road per revolution, and it completes 769.8 revolutions in a mile. Those numbers come straight from the three figures in the size code: 225 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.

This is a measurement page, not a recommendation. Whether a 225/35R20 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 tyre (665.5 mm diameter, 78.8 mm sidewall, 2090.7 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 225/35R20 mean?

There are only three numbers to decode. Width: 225 mm. Aspect ratio: 35%, which fixes the sidewall at 225 x 35/100 = 78.8 mm. Rim: 20 inches, or 508.0 mm. The diameter is the rim diameter plus the sidewall top and bottom, giving 665.5 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.

225/35R20 dimensions

Measurement Value
Overall diameter 665.5 mm (26.2 in)
Sidewall height 78.8 mm
Circumference 2090.7 mm
Revolutions per mile 769.8
Rim diameter 20 in

The diameter of 665.5 mm (26.2 in) is the headline number because everything else follows from it. The circumference, 2090.7 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 769.8 revolutions per mile — the figure the speedometer and odometer effectively count. A tyre with fewer revs per mile covers more ground per turn, so the car travels faster than the speedometer thinks if you fit a taller tyre without recalibrating. All of these are nominal new-tyre 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 tyre.

Closest alternative sizes to 225/35R20

Diameter is the right way to compare candidate replacements, and the table ranks the nearest three. The best match is 305/30R19 at +0.0% (+0.1 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?
305/30R19 665.6 mm (26.2 in) 91.5 mm +0.0% Within the guideline
225/40R19 662.6 mm (26.1 in) 90.0 mm -0.4% Within the guideline
265/35R19 668.1 mm (26.3 in) 92.8 mm +0.4% Within the guideline

Speedometer effect of changing from 225/35R20

Take the closest alternative, 305/30R19, as a worked example. Its diameter is 665.6 mm against 665.5 mm here, a difference of +0.0%. Because the speedometer counts revolutions, the error follows the diameter ratio: actual speed equals indicated speed multiplied by 665.6 / 665.5. At an indicated 30 mph the car would really be doing 30.0 mph; at 50 mph indicated it would be 50.0 mph; at 70 mph indicated, 70.0 mph. That means the speedometer would read slightly high, so you would be travelling a touch slower than the dial suggests. Even a small percentage matters at motorway speed, which is why the plus-or-minus 3% guideline exists.

Replacement size Diameter change Actual speed at 30 mph indicated Actual speed at 50 mph indicated Actual speed at 70 mph indicated
305/30R19 +0.0% 30.0 mph 50.0 mph 70.0 mph
225/40R19 -0.4% 29.9 mph 49.8 mph 69.7 mph
265/35R19 +0.4% 30.1 mph 50.2 mph 70.3 mph

Wider, narrower or taller than 225/35R20: what actually changes

What actually changes if you move away from 225/35R20? Width first: a wider tyre 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 35% 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 tyre still clears the suspension and arches at full lock before committing.

How to check 225/35R20 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 225 mm tyre. 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 225/35R20, the 225 mm width and 78.8 mm sidewall are part of that same check.

Mistakes to avoid with 225/35R20

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 225/35R20 the figures to check are a 665.5 mm diameter, a 78.8 mm sidewall and 769.8 revs per mile.

Related 225/35R20 sizes and tools

Frequently asked questions

What is the overall diameter of a 225/35R20 tyre?

The overall diameter of a new 225/35R20 tyre is 665.5 mm, which is 26.2 inches. That is the rim diameter (508.0 mm) plus twice the sidewall height (78.8 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 225/35R20 tyre?

The sidewall is 78.8 mm tall, because the aspect ratio is 35% of the 225 mm width (225 x 35/100 = 78.8 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 tyre gives over bumps.

How many revolutions per mile does a 225/35R20 tyre do?

A 225/35R20 tyre completes 769.8 revolutions per mile, based on its 2090.7 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 225/35R20?

The closest size in the chart is 305/30R19, which differs in diameter by +0.0% (+0.1 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 225/35R20 affect my speedometer?

Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 305/30R19 instead of 225/35R20 changes the diameter by +0.0%, 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.