245/45R20 Tyre Size: Dimensions, Alternatives & Speedometer Effect

The 245/45R20 is a medium-profile tyre size: 245 mm wide with a 45% aspect ratio on a 20-inch wheel. Its overall diameter is 728.5 mm (28.7 in), its sidewall stands 110.2 mm tall, it rolls 2288.7 mm per revolution and it turns 703.2 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.

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 245/45R20 is a mid-width 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 245/45R20 mean?

There are only three numbers to decode. Width: 245 mm. Aspect ratio: 45%, which fixes the sidewall at 245 x 45/100 = 110.2 mm. Rim: 20 inches, or 508.0 mm. The diameter is the rim diameter plus the sidewall top and bottom, giving 728.5 mm in total. Because the aspect ratio is a percentage of the width, a wider tyre with the same aspect ratio is also a taller tyre — a point that catches people out when they go wider without dropping the aspect ratio to compensate.

245/45R20 dimensions

Measurement Value
Overall diameter 728.5 mm (28.7 in)
Sidewall height 110.2 mm
Circumference 2288.7 mm
Revolutions per mile 703.2
Rim diameter 20 in

The diameter of 728.5 mm (28.7 in) is the headline number because everything else follows from it. The circumference, 2288.7 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 703.2 revolutions per mile — the figure the speedometer and odometer effectively count. A tyre with fewer revs per mile covers more ground per turn, so the car travels faster than the speedometer thinks if you fit a taller tyre without recalibrating. All of these are nominal new-tyre 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 tyre.

Closest alternative sizes to 245/45R20

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 20 inches so the wheel itself stays realistic. 245/50R19 is the nearest at -0.1% (-0.9 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?
245/50R19 727.6 mm (28.6 in) 122.5 mm -0.1% Within the guideline
275/40R20 728.0 mm (28.7 in) 110.0 mm -0.1% Within the guideline
225/55R19 730.1 mm (28.7 in) 123.8 mm +0.2% Within the guideline

Speedometer effect of changing from 245/45R20

The speedometer effect is a straight ratio. Fitting 245/50R19 (727.6 mm) in place of 245/45R20 (728.5 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
245/50R19 -0.1% 30.0 mph 49.9 mph 69.9 mph
275/40R20 -0.1% 30.0 mph 50.0 mph 70.0 mph
225/55R19 +0.2% 30.1 mph 50.1 mph 70.2 mph

Wider, narrower or taller than 245/45R20: 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 tyre 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 110.2 mm this is a medium-profile tyre, 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 245/45R20 fits your vehicle

The checks are the same whatever size you are considering. First, the door-jamb placard: size, load index and speed rating. Second, the rim: a 20-inch rim of the right width for a 245 mm tyre. Third, clearance — arches, suspension components and steering lock, with the car loaded as it will actually be used. Fourth, the pressure-monitoring system, which may need a reset or new sensors after a change. Fifth, the spare: if the spare is a different size, know its speed and distance limits before you need it. None of these is a formality, and together they are what turns a calculation into a safe fitment. For the 245/45R20, the 245 mm width and 110.2 mm sidewall are part of that same check.

Mistakes to avoid with 245/45R20

The most common mistake is reading the size off the tyre already fitted and assuming it is the correct size — a previous owner may have changed it. The second is going wider without adjusting the aspect ratio, which quietly makes the tyre taller and throws the speedometer off. The third is ignoring the load index and speed rating, which matter as much as the dimensions. The fourth is fitting different sizes on the same axle, which the car was not designed for. And the fifth is forgetting that the spare, the pressure-monitoring system and the speedometer all assume the standard size — change the size and all three need a thought. For a 245/45R20, that means checking the 245 mm width, the 45% aspect ratio and the 20-inch rim against the placard before you buy.

Related 245/45R20 sizes and tools

Frequently asked questions

What is the overall diameter of a 245/45R20 tyre?

The overall diameter of a new 245/45R20 tyre is 728.5 mm, which is 28.7 inches. That is the rim diameter (508.0 mm) plus twice the sidewall height (110.2 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 245/45R20 tyre?

The sidewall is 110.2 mm tall, because the aspect ratio is 45% of the 245 mm width (245 x 45/100 = 110.2 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 245/45R20 tyre do?

A 245/45R20 tyre completes 703.2 revolutions per mile, based on its 2288.7 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 245/45R20?

The closest size in the chart is 245/50R19, 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 245/45R20 affect my speedometer?

Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 245/50R19 instead of 245/45R20 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.