265/45R20 Tire Size: Dimensions, Alternatives & Speedometer Effect

The 265/45R20 is a medium-profile tire size: 265 mm wide with a 45% aspect ratio on a 20-inch wheel. Its overall diameter is 746.5 mm (29.4 in), its sidewall stands 119.2 mm tall, it rolls 2345.2 mm per revolution and it turns 686.2 times per mile. That last number is the one the speedometer cares about most, because the car counts wheel revolutions to work out speed. Below you will find the full measurement table, a plain-English decoding of the size code, the three closest alternative sizes from the chart, and a worked speedometer example for each of them.

This is a measurement page, not a recommendation. Whether a 265/45R20 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 (746.5 mm diameter, 119.2 mm sidewall, 2345.2 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 265/45R20 mean?

There are only three numbers to decode. Width: 265 mm. Aspect ratio: 45%, which fixes the sidewall at 265 x 45/100 = 119.2 mm. Rim: 20 inches, or 508.0 mm. The diameter is the rim diameter plus the sidewall top and bottom, giving 746.5 mm in total. Because the aspect ratio is a percentage of the width, a wider tire with the same aspect ratio is also a taller tire — a point that catches people out when they go wider without dropping the aspect ratio to compensate.

265/45R20 dimensions

Measurement Value
Overall diameter 746.5 mm (29.4 in)
Sidewall height 119.2 mm
Circumference 2345.2 mm
Revolutions per mile 686.2
Rim diameter 20 in

The diameter of 746.5 mm (29.4 in) is the headline number because everything else follows from it. The circumference, 2345.2 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 686.2 revolutions per mile — the figure the speedometer and odometer effectively count. A tire with fewer revs per mile covers more ground per turn, so the car travels faster than the speedometer thinks if you fit a taller tire without recalibrating. All of these are nominal new-tire 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 tire.

Closest alternative sizes to 265/45R20

The three sizes below are the closest matches to 265/45R20 in the chart, drawn from the same rim or one inch either side and ranked by diameter difference. The closest, 265/50R19, differs by +0.1% (+1.1 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 tire 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?
265/50R19 747.6 mm (29.4 in) 132.5 mm +0.1% Within the guideline
305/35R21 746.9 mm (29.4 in) 106.8 mm +0.1% Within the guideline
235/50R20 743.0 mm (29.3 in) 117.5 mm -0.5% Within the guideline

Speedometer effect of changing from 265/45R20

Take the closest alternative, 265/50R19, as a worked example. Its diameter is 747.6 mm against 746.5 mm here, a difference of +0.1%. Because the speedometer counts revolutions, the error follows the diameter ratio: actual speed equals indicated speed multiplied by 747.6 / 746.5. At an indicated 30 mph the car would really be doing 30.0 mph; at 50 mph indicated it would be 50.1 mph; at 70 mph indicated, 70.1 mph. That means the speedometer would read slightly low, so you would be travelling faster 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
265/50R19 +0.1% 30.0 mph 50.1 mph 70.1 mph
305/35R21 +0.1% 30.0 mph 50.0 mph 70.0 mph
235/50R20 -0.5% 29.9 mph 49.8 mph 69.7 mph

Wider, narrower or taller than 265/45R20: what actually changes

What actually changes if you move away from 265/45R20? 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 45% 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 265/45R20 fits your vehicle

Start with the placard on the driver's door jamb: it lists the size, load index and speed rating the manufacturer specified for your vehicle, and it outranks whatever is currently fitted. Check that the load index of any replacement is at least the placard figure — a lower load index is not acceptable even if the size matches. Check the speed rating the same way, and check the rim width: a 265 mm tire needs a rim within the width range the tire maker approves for it. Then check clearance with the steering at full lock in both directions and the suspension compressed, and check the tyre-pressure monitoring system still reads correctly after the change. If any of those checks is unclear, a tire professional can confirm fitment in minutes. The diameter being checked in this case is 746.5 mm for the 265/45R20.

Mistakes to avoid with 265/45R20

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 265/45R20 is 265 mm wide on a 20-inch rim with a 746.5 mm overall diameter, so those are the numbers the placard check has to match.

Related 265/45R20 sizes and tools

Frequently asked questions

What is the overall diameter of a 265/45R20 tire?

The overall diameter of a new 265/45R20 tire is 746.5 mm, which is 29.4 inches. That is the rim diameter (508.0 mm) plus twice the sidewall height (119.2 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 265/45R20 tire?

The sidewall is 119.2 mm tall, because the aspect ratio is 45% of the 265 mm width (265 x 45/100 = 119.2 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 tire gives over bumps.

How many revolutions per mile does a 265/45R20 tire do?

A 265/45R20 tire completes 686.2 revolutions per mile, based on its 2345.2 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 265/45R20?

The closest size in the chart is 265/50R19, which differs in diameter by +0.1% (+1.1 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 265/45R20 affect my speedometer?

Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 265/50R19 instead of 265/45R20 changes the diameter by +0.1%, so an indicated 70 mph would really be 70.1 mph. Keep the diameter within plus or minus 3% to keep that error small, and check the placard before fitting anything different.