285/40R20 Tire Size: Dimensions, Alternatives & Speedometer Effect

If you are looking up the 285/40R20 tire size, the figures that matter are the diameter (736.0 mm, or 29.0 inches), the sidewall (114.0 mm), the circumference (2312.2 mm) and the revolutions per mile (696.0). The size code itself tells you the ingredients — 285 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.

This is a measurement page, not a recommendation. Whether a 285/40R20 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 (736.0 mm diameter, 114.0 mm sidewall, 2312.2 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 285/40R20 mean?

Read the code left to right. 285 means the tire is 285 mm wide across the tread. 40 is the aspect ratio, so the sidewall is 40% of the width — 285 x 0.40 = 114.0 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 + 114.0 mm + 114.0 mm = 736.0 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.

285/40R20 dimensions

Measurement Value
Overall diameter 736.0 mm (29.0 in)
Sidewall height 114.0 mm
Circumference 2312.2 mm
Revolutions per mile 696.0
Rim diameter 20 in

The revs-per-mile figure, 696.0, 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 2312.2 mm circumference, this tire turns 696.0 times per mile, and any replacement with a different circumference will turn a different number of times over the same mile. The diameter (736.0 mm, 29.0 in) and the sidewall (114.0 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 285/40R20

Diameter is the right way to compare candidate replacements, and the table ranks the nearest three. The best match is 255/50R19 at +0.2% (+1.6 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?
255/50R19 737.6 mm (29.0 in) 127.5 mm +0.2% Within the guideline
255/45R20 737.5 mm (29.0 in) 114.8 mm +0.2% Within the guideline
255/40R21 737.4 mm (29.0 in) 102.0 mm +0.2% Within the guideline

Speedometer effect of changing from 285/40R20

The speedometer effect is a straight ratio. Fitting 255/50R19 (737.6 mm) in place of 285/40R20 (736.0 mm) changes the diameter by +0.2%, so every indicated speed is multiplied by 1.002 to get the real speed. At 30 mph on the dial the true speed is 30.1 mph, at 50 mph it is 50.1 mph, and at 70 mph it is 70.2 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
255/50R19 +0.2% 30.1 mph 50.1 mph 70.2 mph
255/45R20 +0.2% 30.1 mph 50.1 mph 70.1 mph
255/40R21 +0.2% 30.1 mph 50.1 mph 70.1 mph

Wider, narrower or taller than 285/40R20: what actually changes

What actually changes if you move away from 285/40R20? 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 40% 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 285/40R20 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 285 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 285/40R20 measures 736.0 mm across, so that is the diameter the placard figure has to be compared with.

Mistakes to avoid with 285/40R20

Watch for these traps. Assuming the current tire 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 tire 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 285/40R20 is 285 mm wide on a 20-inch rim with a 736.0 mm overall diameter, so those are the numbers the placard check has to match.

Related 285/40R20 sizes and tools

Frequently asked questions

What is the overall diameter of a 285/40R20 tire?

The overall diameter of a new 285/40R20 tire is 736.0 mm, which is 29.0 inches. That is the rim diameter (508.0 mm) plus twice the sidewall height (114.0 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 285/40R20 tire?

The sidewall is 114.0 mm tall, because the aspect ratio is 40% of the 285 mm width (285 x 40/100 = 114.0 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 285/40R20 tire do?

A 285/40R20 tire completes 696.0 revolutions per mile, based on its 2312.2 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 285/40R20?

The closest size in the chart is 255/50R19, which differs in diameter by +0.2% (+1.6 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 285/40R20 affect my speedometer?

Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 255/50R19 instead of 285/40R20 changes the diameter by +0.2%, so an indicated 70 mph would really be 70.2 mph. Keep the diameter within plus or minus 3% to keep that error small, and check the placard before fitting anything different.