285/35R21 Tyre Size: Dimensions, Alternatives & Speedometer Effect

Measured from the ground to the top of the tread, a new 285/35R21 tyre stands 732.9 mm tall (28.9 inches). The sidewall — the rubber between the rim edge and the tread — is 99.8 mm high, the tyre covers 2302.5 mm of road per revolution, and it completes 699.0 revolutions in a mile. Those numbers come straight from the three figures in the size code: 285 mm width, 35% aspect ratio and a 21-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 285/35R21 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 (732.9 mm diameter, 99.8 mm sidewall, 2302.5 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 285/35R21 mean?

The code breaks into three parts. The first number, 285, is the section width in millimetres — the width of the tyre from sidewall to sidewall when mounted on its measuring rim. The second number, 35, is the aspect ratio: the sidewall height expressed as a percentage of the width. Multiply them (285 x 35% = 99.8 mm) and you get the sidewall height. The final number, 21, is the rim diameter in inches — 21 x 25.4 = 533.4 mm. Add the rim diameter to twice the sidewall (533.4 + 2 x 99.8 = 732.9 mm) and you have the overall diameter. That is the whole formula, and it is why every size in the chart can be checked rather than trusted blindly.

285/35R21 dimensions

Measurement Value
Overall diameter 732.9 mm (28.9 in)
Sidewall height 99.8 mm
Circumference 2302.5 mm
Revolutions per mile 699.0
Rim diameter 21 in

The revs-per-mile figure, 699.0, 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 2302.5 mm circumference, this tyre turns 699.0 times per mile, and any replacement with a different circumference will turn a different number of times over the same mile. The diameter (732.9 mm, 28.9 in) and the sidewall (99.8 mm) complete the picture. Remember these are nominal new-tyre figures — real tyres vary slightly by brand, model, wear and inflation pressure, so two tyres with the same size code can differ by a millimetre or two in practice.

Closest alternative sizes to 285/35R21

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 21 inches so the wheel itself stays realistic. 285/40R20 is the nearest at +0.4% (+3.1 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?
285/40R20 736.0 mm (29.0 in) 114.0 mm +0.4% Within the guideline
245/45R20 728.5 mm (28.7 in) 110.2 mm -0.6% Within the guideline
255/45R20 737.5 mm (29.0 in) 114.8 mm +0.6% Within the guideline

Speedometer effect of changing from 285/35R21

Here is what the diameter difference does on the road. The speedometer assumes the original circumference, so a +0.4% diameter change from fitting 285/40R20 shifts every reading by the same percentage. Indicated 30 mph becomes 30.1 mph in reality, indicated 50 mph becomes 50.2 mph, and indicated 70 mph becomes 70.3 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
285/40R20 +0.4% 30.1 mph 50.2 mph 70.3 mph
245/45R20 -0.6% 29.8 mph 49.7 mph 69.6 mph
255/45R20 +0.6% 30.2 mph 50.3 mph 70.4 mph

Wider, narrower or taller than 285/35R21: what actually changes

What actually changes if you move away from 285/35R21? 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 285/35R21 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 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 285/35R21 measures 732.9 mm across, so that is the diameter the placard figure has to be compared with.

Mistakes to avoid with 285/35R21

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 285/35R21 is 285 mm wide on a 21-inch rim with a 732.9 mm overall diameter, so those are the numbers the placard check has to match.

Related 285/35R21 sizes and tools

Frequently asked questions

What is the overall diameter of a 285/35R21 tyre?

The overall diameter of a new 285/35R21 tyre is 732.9 mm, which is 28.9 inches. That is the rim diameter (533.4 mm) plus twice the sidewall height (99.8 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 285/35R21 tyre?

The sidewall is 99.8 mm tall, because the aspect ratio is 35% of the 285 mm width (285 x 35/100 = 99.8 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 285/35R21 tyre do?

A 285/35R21 tyre completes 699.0 revolutions per mile, based on its 2302.5 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 285/35R21?

The closest size in the chart is 285/40R20, which differs in diameter by +0.4% (+3.1 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 285/35R21 affect my speedometer?

Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 285/40R20 instead of 285/35R21 changes the diameter by +0.4%, so an indicated 70 mph would really be 70.3 mph. Keep the diameter within plus or minus 3% to keep that error small, and check the placard before fitting anything different.