245/40R20 Tyre Size: Dimensions, Alternatives & Speedometer Effect
The 245/40R20 is a low-profile tyre size: 245 mm wide with a 40% aspect ratio on a 20-inch wheel. Its overall diameter is 704.0 mm (27.7 in), its sidewall stands 98.0 mm tall, it rolls 2211.7 mm per revolution and it turns 727.7 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 245/40R20 is a mid-width size, a short, stiff sidewall that sharpens steering response but leaves less cushioning over potholes and kerbs, 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 245/40R20 mean?
Read the code left to right. 245 means the tyre is 245 mm wide across the tread. 40 is the aspect ratio, so the sidewall is 40% of the width — 245 x 0.40 = 98.0 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 + 98.0 mm + 98.0 mm = 704.0 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.
245/40R20 dimensions
| Measurement | Value |
|---|---|
| Overall diameter | 704.0 mm (27.7 in) |
| Sidewall height | 98.0 mm |
| Circumference | 2211.7 mm |
| Revolutions per mile | 727.7 |
| Rim diameter | 20 in |
The revs-per-mile figure, 727.7, 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 2211.7 mm circumference, this tyre turns 727.7 times per mile, and any replacement with a different circumference will turn a different number of times over the same mile. The diameter (704.0 mm, 27.7 in) and the sidewall (98.0 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 245/40R20
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. 245/45R19 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? |
|---|---|---|---|---|
| 245/45R19 | 703.1 mm (27.7 in) | 110.2 mm | -0.1% | Within the guideline |
| 245/35R21 | 704.9 mm (27.8 in) | 85.8 mm | +0.1% | Within the guideline |
| 275/40R19 | 702.6 mm (27.7 in) | 110.0 mm | -0.2% | Within the guideline |
Speedometer effect of changing from 245/40R20
The speedometer effect is a straight ratio. Fitting 245/45R19 (703.1 mm) in place of 245/40R20 (704.0 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 |
|---|---|---|---|---|
| 245/45R19 | -0.1% | 30.0 mph | 49.9 mph | 69.9 mph |
| 245/35R21 | +0.1% | 30.0 mph | 50.1 mph | 70.1 mph |
| 275/40R19 | -0.2% | 29.9 mph | 49.9 mph | 69.9 mph |
Wider, narrower or taller than 245/40R20: what actually changes
What actually changes if you move away from 245/40R20? 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 40% 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 245/40R20 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 245 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 245/40R20 measures 704.0 mm across, so that is the diameter the placard figure has to be compared with.
Mistakes to avoid with 245/40R20
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 245/40R20, that means checking the 245 mm width, the 40% aspect ratio and the 20-inch rim against the placard before you buy.
Related 245/40R20 sizes and tools
- 245/35R20 tyre size
- 245/45R20 tyre size
- Use the Tyre Size Calculator to compare 245/40R20 against any other size.
- Use the Plus Sizing Calculator to find a wider size that keeps the diameter close to 245/40R20.
Frequently asked questions
What is the overall diameter of a 245/40R20 tyre?
The overall diameter of a new 245/40R20 tyre is 704.0 mm, which is 27.7 inches. That is the rim diameter (508.0 mm) plus twice the sidewall height (98.0 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 245/40R20 tyre?
For 245/40r20 tyre size, the point is this: The sidewall is 98.0 mm tall, because the aspect ratio is 40% of the 245 mm width (245 x 40/100 = 98.0 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 245/40R20 tyre do?
A 245/40R20 tyre completes 727.7 revolutions per mile, based on its 2211.7 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 245/40R20?
The closest size in the chart is 245/45R19, 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 245/40R20 affect my speedometer?
Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 245/45R19 instead of 245/40R20 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.