275/45R20 Tyre Size: Dimensions, Alternatives & Speedometer Effect
The 275/45R20 is a medium-profile tyre size: 275 mm wide with a 45% aspect ratio on a 20-inch wheel. Its overall diameter is 755.5 mm (29.7 in), its sidewall stands 123.8 mm tall, it rolls 2373.5 mm per revolution and it turns 678.1 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 275/45R20 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 275/45R20 mean?
Read the code left to right. 275 means the tyre is 275 mm wide across the tread. 45 is the aspect ratio, so the sidewall is 45% of the width — 275 x 0.45 = 123.8 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 + 123.8 mm + 123.8 mm = 755.5 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.
275/45R20 dimensions
| Measurement | Value |
|---|---|
| Overall diameter | 755.5 mm (29.7 in) |
| Sidewall height | 123.8 mm |
| Circumference | 2373.5 mm |
| Revolutions per mile | 678.1 |
| Rim diameter | 20 in |
Start with the circumference: 2373.5 mm per revolution. That is the distance your car covers every time this tyre turns once, and it is what makes the revs-per-mile figure (678.1) what it is — a mile divided by the circumference. Fit a tyre with a larger circumference and each revolution carries you further, so at the same wheel speed the car is moving faster than the instruments report. The sidewall (123.8 mm) is the flexible part: it absorbs the first shock from bumps and potholes, and a medium-profile sidewall like this one means a balanced sidewall that splits the difference between handling response and ride comfort. These are nominal new-tyre dimensions, and real tyres vary slightly by brand, model, wear and inflation pressure.
Closest alternative sizes to 275/45R20
The three sizes below are the closest matches to 275/45R20 in the chart, drawn from the same rim or one inch either side and ranked by diameter difference. The closest, 245/50R20, differs by -0.3% (-2.5 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? |
|---|---|---|---|---|
| 245/50R20 | 753.0 mm (29.6 in) | 122.5 mm | -0.3% | Within the guideline |
| 275/40R21 | 753.4 mm (29.7 in) | 110.0 mm | -0.3% | Within the guideline |
| 245/55R19 | 752.1 mm (29.6 in) | 134.8 mm | -0.5% | Within the guideline |
Speedometer effect of changing from 275/45R20
The speedometer effect is a straight ratio. Fitting 245/50R20 (753.0 mm) in place of 275/45R20 (755.5 mm) changes the diameter by -0.3%, so every indicated speed is multiplied by 0.997 to get the real speed. At 30 mph on the dial the true speed is 29.9 mph, at 50 mph it is 49.8 mph, and at 70 mph it is 69.8 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 |
|---|---|---|---|---|
| 245/50R20 | -0.3% | 29.9 mph | 49.8 mph | 69.8 mph |
| 275/40R21 | -0.3% | 29.9 mph | 49.9 mph | 69.8 mph |
| 245/55R19 | -0.5% | 29.9 mph | 49.8 mph | 69.7 mph |
Wider, narrower or taller than 275/45R20: 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 wide, a broad contact patch, which generally means more grip in the dry and more resistance to aquaplaning when the tread is in good condition. Sidewall height drives comfort and response: at 123.8 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 275/45R20 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 275 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 275/45R20 measures 755.5 mm across, so that is the diameter the placard figure has to be compared with.
Mistakes to avoid with 275/45R20
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 275/45R20 is 275 mm wide on a 20-inch rim with a 755.5 mm overall diameter, so those are the numbers the placard check has to match.
Related 275/45R20 sizes and tools
- 275/40R20 tyre size
- 275/50R20 tyre size
- Use the Tyre Size Calculator to compare 275/45R20 against any other size.
- Use the Plus Sizing Calculator to find a wider size that keeps the diameter close to 275/45R20.
Frequently asked questions
What is the overall diameter of a 275/45R20 tyre?
The overall diameter of a new 275/45R20 tyre is 755.5 mm, which is 29.7 inches. That is the rim diameter (508.0 mm) plus twice the sidewall height (123.8 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 275/45R20 tyre?
The sidewall is 123.8 mm tall, because the aspect ratio is 45% of the 275 mm width (275 x 45/100 = 123.8 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 275/45R20 tyre do?
A 275/45R20 tyre completes 678.1 revolutions per mile, based on its 2373.5 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 275/45R20?
The closest size in the chart is 245/50R20, which differs in diameter by -0.3% (-2.5 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 275/45R20 affect my speedometer?
Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 245/50R20 instead of 275/45R20 changes the diameter by -0.3%, so an indicated 70 mph would really be 69.8 mph. Keep the diameter within plus or minus 3% to keep that error small, and check the placard before fitting anything different.