285/60R20 Tyre Size: Dimensions, Alternatives & Speedometer Effect
The 285/60R20 is a tall-sidewall tyre size: 285 mm wide with a 60% aspect ratio on a 20-inch wheel. Its overall diameter is 850.0 mm (33.5 in), its sidewall stands 171.0 mm tall, it rolls 2670.4 mm per revolution and it turns 602.7 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.
Before the numbers, one practical point. A tyre 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 285/60R20 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 285/60R20 mean?
Read the code left to right. 285 means the tyre is 285 mm wide across the tread. 60 is the aspect ratio, so the sidewall is 60% of the width — 285 x 0.60 = 171.0 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 + 171.0 mm + 171.0 mm = 850.0 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.
285/60R20 dimensions
| Measurement | Value |
|---|---|
| Overall diameter | 850.0 mm (33.5 in) |
| Sidewall height | 171.0 mm |
| Circumference | 2670.4 mm |
| Revolutions per mile | 602.7 |
| Rim diameter | 20 in |
Start with the circumference: 2670.4 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 (602.7) 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 (171.0 mm) is the flexible part: it absorbs the first shock from bumps and potholes, and a tall-sidewall sidewall like this one means a tall, flexible sidewall that absorbs bumps well and suits rougher roads, at the cost of slightly softer handling. These are nominal new-tyre dimensions, and real tyres vary slightly by brand, model, wear and inflation pressure.
Closest alternative sizes to 285/60R20
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. 275/60R20 is the nearest at -1.4% (-12.0 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? |
|---|---|---|---|---|
| 275/60R20 | 838.0 mm (33.0 in) | 165.0 mm | -1.4% | Within the guideline |
| 285/55R20 | 821.5 mm (32.3 in) | 156.8 mm | -3.4% | Outside the guideline |
| 275/55R20 | 810.5 mm (31.9 in) | 151.2 mm | -4.6% | Outside the guideline |
Speedometer effect of changing from 285/60R20
The speedometer effect is a straight ratio. Fitting 275/60R20 (838.0 mm) in place of 285/60R20 (850.0 mm) changes the diameter by -1.4%, so every indicated speed is multiplied by 0.986 to get the real speed. At 30 mph on the dial the true speed is 29.6 mph, at 50 mph it is 49.3 mph, and at 70 mph it is 69.0 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 |
|---|---|---|---|---|
| 275/60R20 | -1.4% | 29.6 mph | 49.3 mph | 69.0 mph |
| 285/55R20 | -3.4% | 29.0 mph | 48.3 mph | 67.7 mph |
| 275/55R20 | -4.6% | 28.6 mph | 47.7 mph | 66.7 mph |
Wider, narrower or taller than 285/60R20: 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 171.0 mm this is a tall-sidewall tyre, a tall, flexible sidewall that absorbs bumps well and suits rougher roads, at the cost of slightly softer handling. 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 285/60R20 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 285 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 285/60R20, the 285 mm width and 171.0 mm sidewall are part of that same check.
Mistakes to avoid with 285/60R20
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 285/60R20 is 285 mm wide on a 20-inch rim with a 850.0 mm overall diameter, so those are the numbers the placard check has to match.
Related 285/60R20 sizes and tools
- 285/40R20 tyre size
- 285/45R20 tyre size
- Use the Tyre Size Calculator to compare 285/60R20 against any other size.
- Use the Plus Sizing Calculator to find a wider size that keeps the diameter close to 285/60R20.
Frequently asked questions
What is the overall diameter of a 285/60R20 tyre?
The overall diameter of a new 285/60R20 tyre is 850.0 mm, which is 33.5 inches. That is the rim diameter (508.0 mm) plus twice the sidewall height (171.0 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 285/60R20 tyre?
The sidewall is 171.0 mm tall, because the aspect ratio is 60% of the 285 mm width (285 x 60/100 = 171.0 mm). A tall-sidewall sidewall like this one means a tall, flexible sidewall that absorbs bumps well and suits rougher roads, at the cost of slightly softer handling. The sidewall height is the main thing that decides how much cushioning the tyre gives over bumps.
How many revolutions per mile does a 285/60R20 tyre do?
A 285/60R20 tyre completes 602.7 revolutions per mile, based on its 2670.4 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 285/60R20?
The closest size in the chart is 275/60R20, which differs in diameter by -1.4% (-12.0 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 285/60R20 affect my speedometer?
Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 275/60R20 instead of 285/60R20 changes the diameter by -1.4%, so an indicated 70 mph would really be 69.0 mph. Keep the diameter within plus or minus 3% to keep that error small, and check the placard before fitting anything different.