285/45R20 Tyre Size: Dimensions, Alternatives & Speedometer Effect

If you are looking up the 285/45R20 tyre size, the figures that matter are the diameter (764.5 mm, or 30.1 inches), the sidewall (128.2 mm), the circumference (2401.7 mm) and the revolutions per mile (670.1). The size code itself tells you the ingredients — 285 mm of width, an aspect ratio of 45%, 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 tyre you already own or one you are thinking of buying.

People usually land on this page for one of three reasons: checking a tyre already on the car, shopping for a replacement, or wondering whether a slightly different size will work. In all three cases the approach is the same — start with the exact measurements, then compare. A 285/45R20 is a wide size, a balanced sidewall that splits the difference between handling response and ride comfort, and the tables below let you hold it up against the nearest alternatives on diameter, sidewall and revs per mile. The disclaimer box just below is part of that comparison: these are calculations, and the final fitment check belongs to your placard and a tyre professional.

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 285/45R20 mean?

Read the code left to right. 285 means the tyre is 285 mm wide across the tread. 45 is the aspect ratio, so the sidewall is 45% of the width — 285 x 0.45 = 128.2 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 + 128.2 mm + 128.2 mm = 764.5 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.

285/45R20 dimensions

Measurement Value
Overall diameter 764.5 mm (30.1 in)
Sidewall height 128.2 mm
Circumference 2401.7 mm
Revolutions per mile 670.1
Rim diameter 20 in

The revs-per-mile figure, 670.1, 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 2401.7 mm circumference, this tyre turns 670.1 times per mile, and any replacement with a different circumference will turn a different number of times over the same mile. The diameter (764.5 mm, 30.1 in) and the sidewall (128.2 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 285/45R20

The three sizes below are the closest matches to 285/45R20 in the chart, drawn from the same rim or one inch either side and ranked by diameter difference. The closest, 255/55R19, differs by -0.2% (-1.4 mm). The commonly used guideline is to stay within plus or minus 3% of the original diameter, which keeps the speedometer error small and the gearing close to standard. That guideline is about the speedometer and the car's calibration, not about whether the tyre physically clears the arches — a size can be within 3% on diameter and still rub if it is much wider, so width and clearance need their own check.

Alternative size Diameter Sidewall Diameter difference Within ±3% guideline?
255/55R19 763.1 mm (30.0 in) 140.2 mm -0.2% Within the guideline
255/50R20 763.0 mm (30.0 in) 127.5 mm -0.2% Within the guideline
235/55R20 766.5 mm (30.2 in) 129.2 mm +0.3% Within the guideline

Speedometer effect of changing from 285/45R20

Here is what the diameter difference does on the road. The speedometer assumes the original circumference, so a -0.2% diameter change from fitting 255/55R19 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
255/55R19 -0.2% 29.9 mph 49.9 mph 69.9 mph
255/50R20 -0.2% 29.9 mph 49.9 mph 69.9 mph
235/55R20 +0.3% 30.1 mph 50.1 mph 70.2 mph

Wider, narrower or taller than 285/45R20: what actually changes

Going wider than 285 mm while keeping the same aspect ratio also makes the tyre taller, because the sidewall is a percentage of the width — that is the single most common surprise in a size change. Going wider and dropping the aspect ratio keeps the diameter closer to standard, which is the idea behind plus-sizing. A taller tyre raises the car slightly and lengthens the gearing, so the engine turns a little slower at a given road speed; a shorter tyre does the opposite. Wider rubber generally brings more dry grip and more road noise, and it can increase fuel use slightly through higher rolling resistance. None of these effects is dramatic for a small change, but they all point the same way: change one dimension at a time and keep the diameter close.

How to check 285/45R20 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 285/45R20 measures 764.5 mm across, so that is the diameter the placard figure has to be compared with.

Mistakes to avoid with 285/45R20

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 285/45R20 is 285 mm wide on a 20-inch rim with a 764.5 mm overall diameter, so those are the numbers the placard check has to match.

Related 285/45R20 sizes and tools

Frequently asked questions

What is the overall diameter of a 285/45R20 tyre?

The overall diameter of a new 285/45R20 tyre is 764.5 mm, which is 30.1 inches. That is the rim diameter (508.0 mm) plus twice the sidewall height (128.2 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 285/45R20 tyre?

For 285/45r20 tyre size, the point is this: The sidewall is 128.2 mm tall, because the aspect ratio is 45% of the 285 mm width (285 x 45/100 = 128.2 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 tyre gives over bumps.

How many revolutions per mile does a 285/45R20 tyre do?

A 285/45R20 tyre completes 670.1 revolutions per mile, based on its 2401.7 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 285/45R20?

The closest size in the chart is 255/55R19, 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 285/45R20 affect my speedometer?

Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 255/55R19 instead of 285/45R20 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.