245/50R20 Tire Size: Dimensions, Alternatives & Speedometer Effect

If you are looking up the 245/50R20 tire size, the figures that matter are the diameter (753.0 mm, or 29.6 inches), the sidewall (122.5 mm), the circumference (2365.6 mm) and the revolutions per mile (680.3). The size code itself tells you the ingredients — 245 mm of width, an aspect ratio of 50%, 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 245/50R20 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 245/50R20 mean?

The code breaks into three parts. The first number, 245, is the section width in millimetres — the width of the tire from sidewall to sidewall when mounted on its measuring rim. The second number, 50, is the aspect ratio: the sidewall height expressed as a percentage of the width. Multiply them (245 x 50% = 122.5 mm) and you get the sidewall height. The final number, 20, is the rim diameter in inches — 20 x 25.4 = 508.0 mm. Add the rim diameter to twice the sidewall (508.0 + 2 x 122.5 = 753.0 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.

245/50R20 dimensions

Measurement Value
Overall diameter 753.0 mm (29.6 in)
Sidewall height 122.5 mm
Circumference 2365.6 mm
Revolutions per mile 680.3
Rim diameter 20 in

The diameter of 753.0 mm (29.6 in) is the headline number because everything else follows from it. The circumference, 2365.6 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 680.3 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 245/50R20

Diameter is the right way to compare candidate replacements, and the table ranks the nearest three. The best match is 245/55R19 at -0.1% (-0.9 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?
245/55R19 752.1 mm (29.6 in) 134.8 mm -0.1% Within the guideline
275/40R21 753.4 mm (29.7 in) 110.0 mm +0.1% Within the guideline
275/45R20 755.5 mm (29.7 in) 123.8 mm +0.3% Within the guideline

Speedometer effect of changing from 245/50R20

The speedometer effect is a straight ratio. Fitting 245/55R19 (752.1 mm) in place of 245/50R20 (753.0 mm) changes the diameter by -0.1%, so every indicated speed is multiplied by 0.999 to get the real speed. At 30 mph on the dial the true speed is 30.0 mph, at 50 mph it is 49.9 mph, and at 70 mph it is 69.9 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
245/55R19 -0.1% 30.0 mph 49.9 mph 69.9 mph
275/40R21 +0.1% 30.0 mph 50.0 mph 70.0 mph
275/45R20 +0.3% 30.1 mph 50.2 mph 70.2 mph

Wider, narrower or taller than 245/50R20: 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 122.5 mm this is a medium-profile tire, a balanced sidewall that splits the difference between handling response and ride comfort. 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 245/50R20 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 245 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 753.0 mm for the 245/50R20.

Mistakes to avoid with 245/50R20

People go wrong in a handful of predictable ways. They trust the fitted tire instead of the placard. They pick a wider tire and forget the aspect ratio, so the diameter creeps up. They match the size code but buy a lower load index or speed rating. They mix sizes across an axle. They forget to check clearance at full steering lock, where a tire that clears going straight can still rub. Each of these is easy to avoid once you know to look for it, and the tables on this page give you the numbers to check against. On a 245/50R20 the figures to check are a 753.0 mm diameter, a 122.5 mm sidewall and 680.3 revs per mile.

Related 245/50R20 sizes and tools

Frequently asked questions

What is the overall diameter of a 245/50R20 tire?

The overall diameter of a new 245/50R20 tire is 753.0 mm, which is 29.6 inches. That is the rim diameter (508.0 mm) plus twice the sidewall height (122.5 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 245/50R20 tire?

The sidewall is 122.5 mm tall, because the aspect ratio is 50% of the 245 mm width (245 x 50/100 = 122.5 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 245/50R20 tire do?

A 245/50R20 tire completes 680.3 revolutions per mile, based on its 2365.6 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 245/50R20?

The closest size in the chart is 245/55R19, which differs in diameter by -0.1% (-0.9 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 245/50R20 affect my speedometer?

Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 245/55R19 instead of 245/50R20 changes the diameter by -0.1%, 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.