225/40R20 Tire Size: Dimensions, Alternatives & Speedometer Effect

If you are looking up the 225/40R20 tire size, the figures that matter are the diameter (688.0 mm, or 27.1 inches), the sidewall (90.0 mm), the circumference (2161.4 mm) and the revolutions per mile (744.6). The size code itself tells you the ingredients — 225 mm of width, an aspect ratio of 40%, 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 tire you already own or one you are thinking of buying.

Before the numbers, one practical point. A tire 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 225/40R20 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 225/40R20 mean?

Read the code left to right. 225 means the tire is 225 mm wide across the tread. 40 is the aspect ratio, so the sidewall is 40% of the width — 225 x 0.40 = 90.0 mm of rubber between rim and tread. R20 means it mounts on a 20-inch rim (508.0 mm). The overall diameter is the rim plus two sidewalls: 508.0 mm + 90.0 mm + 90.0 mm = 688.0 mm. Once you can do that sum, you can decode any size code you meet, and you can see why two sizes with the same rim can still be very different heights.

225/40R20 dimensions

Measurement Value
Overall diameter 688.0 mm (27.1 in)
Sidewall height 90.0 mm
Circumference 2161.4 mm
Revolutions per mile 744.6
Rim diameter 20 in

The revs-per-mile figure, 744.6, 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 2161.4 mm circumference, this tire turns 744.6 times per mile, and any replacement with a different circumference will turn a different number of times over the same mile. The diameter (688.0 mm, 27.1 in) and the sidewall (90.0 mm) complete the picture. Remember these are nominal new-tire figures — real tyres vary slightly by brand, model, wear and inflation pressure, so two tires with the same size code can differ by a millimetre or two in practice.

Closest alternative sizes to 225/40R20

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. 255/40R19 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?
255/40R19 686.6 mm (27.0 in) 102.0 mm -0.2% Within the guideline
295/35R19 689.1 mm (27.1 in) 103.2 mm +0.2% Within the guideline
255/35R20 686.5 mm (27.0 in) 89.2 mm -0.2% Within the guideline

Speedometer effect of changing from 225/40R20

Here is what the diameter difference does on the road. The speedometer assumes the original circumference, so a -0.2% diameter change from fitting 255/40R19 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
255/40R19 -0.2% 29.9 mph 49.9 mph 69.9 mph
295/35R19 +0.2% 30.0 mph 50.1 mph 70.1 mph
255/35R20 -0.2% 29.9 mph 49.9 mph 69.8 mph

Wider, narrower or taller than 225/40R20: what actually changes

Three things move when the size moves. Diameter drives the speedometer and the gearing: taller means the engine revs slightly lower at cruising speed, shorter means it revs slightly higher. Width drives grip and rolling resistance: this tire is mid-width, a middle-of-the-road width that balances grip, fuel use and road noise for everyday driving. Sidewall height drives comfort and response: at 90.0 mm this is a low-profile tire, a short, stiff sidewall that sharpens steering response but leaves less cushioning over potholes and kerbs. Small changes in any one of these are usually barely noticeable; combined changes — much wider and much taller at once — are where the car's behaviour and the speedometer both start to shift in ways that need checking rather than assuming.

How to check 225/40R20 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 225 mm tire. Then confirm physical clearance at full steering lock and under load, and confirm the pressure-monitoring system still works. A tire professional can run through all of that quickly, and that confirmation is the step this page cannot replace. The 225/40R20 measures 688.0 mm across, so that is the diameter the placard figure has to be compared with.

Mistakes to avoid with 225/40R20

The most common mistake is reading the size off the tire 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 tire 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 225/40R20, that means checking the 225 mm width, the 40% aspect ratio and the 20-inch rim against the placard before you buy.

Related 225/40R20 sizes and tools

Frequently asked questions

What is the overall diameter of a 225/40R20 tire?

The overall diameter of a new 225/40R20 tire is 688.0 mm, which is 27.1 inches. That is the rim diameter (508.0 mm) plus twice the sidewall height (90.0 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 225/40R20 tire?

The sidewall is 90.0 mm tall, because the aspect ratio is 40% of the 225 mm width (225 x 40/100 = 90.0 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 tire gives over bumps.

How many revolutions per mile does a 225/40R20 tire do?

A 225/40R20 tire completes 744.6 revolutions per mile, based on its 2161.4 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 225/40R20?

The closest size in the chart is 255/40R19, 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 225/40R20 affect my speedometer?

Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 255/40R19 instead of 225/40R20 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.