285/40R19 Tyre Size: Dimensions, Alternatives & Speedometer Effect

If you are looking up the 285/40R19 tyre size, the figures that matter are the diameter (710.6 mm, or 28.0 inches), the sidewall (114.0 mm), the circumference (2232.4 mm) and the revolutions per mile (720.9). The size code itself tells you the ingredients — 285 mm of width, an aspect ratio of 40%, a 19-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 tyre you already own or one you are thinking of buying.

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 285/40R19 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 285/40R19 mean?

The code breaks into three parts. The first number, 285, is the section width in millimetres — the width of the tyre from sidewall to sidewall when mounted on its measuring rim. The second number, 40, is the aspect ratio: the sidewall height expressed as a percentage of the width. Multiply them (285 x 40% = 114.0 mm) and you get the sidewall height. The final number, 19, is the rim diameter in inches — 19 x 25.4 = 482.6 mm. Add the rim diameter to twice the sidewall (482.6 + 2 x 114.0 = 710.6 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.

285/40R19 dimensions

Measurement Value
Overall diameter 710.6 mm (28.0 in)
Sidewall height 114.0 mm
Circumference 2232.4 mm
Revolutions per mile 720.9
Rim diameter 19 in

Start with the circumference: 2232.4 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 (720.9) 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 (114.0 mm) is the flexible part: it absorbs the first shock from bumps and potholes, and a low-profile sidewall like this one means a short, stiff sidewall that sharpens steering response but leaves less cushioning over potholes and kerbs. These are nominal new-tyre dimensions, and real tyres vary slightly by brand, model, wear and inflation pressure.

Closest alternative sizes to 285/40R19

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 19 inches so the wheel itself stays realistic. 255/50R18 is the nearest at +0.2% (+1.6 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?
255/50R18 712.2 mm (28.0 in) 127.5 mm +0.2% Within the guideline
255/45R19 712.1 mm (28.0 in) 114.8 mm +0.2% Within the guideline
255/40R20 712.0 mm (28.0 in) 102.0 mm +0.2% Within the guideline

Speedometer effect of changing from 285/40R19

Take the closest alternative, 255/50R18, as a worked example. Its diameter is 712.2 mm against 710.6 mm here, a difference of +0.2%. Because the speedometer counts revolutions, the error follows the diameter ratio: actual speed equals indicated speed multiplied by 712.2 / 710.6. At an indicated 30 mph the car would really be doing 30.1 mph; at 50 mph indicated it would be 50.1 mph; at 70 mph indicated, 70.2 mph. That means the speedometer would read slightly low, so you would be travelling faster than the dial suggests. Even a small percentage matters at motorway speed, which is why the plus-or-minus 3% guideline exists.

Replacement size Diameter change Actual speed at 30 mph indicated Actual speed at 50 mph indicated Actual speed at 70 mph indicated
255/50R18 +0.2% 30.1 mph 50.1 mph 70.2 mph
255/45R19 +0.2% 30.1 mph 50.1 mph 70.1 mph
255/40R20 +0.2% 30.1 mph 50.1 mph 70.1 mph

Wider, narrower or taller than 285/40R19: what actually changes

What actually changes if you move away from 285/40R19? 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 285/40R19 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 285 mm tyre needs a rim within the width range the tyre 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 tyre professional can confirm fitment in minutes. The diameter being checked in this case is 710.6 mm for the 285/40R19.

Mistakes to avoid with 285/40R19

People go wrong in a handful of predictable ways. They trust the fitted tyre instead of the placard. They pick a wider tyre 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 tyre 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 285/40R19 the figures to check are a 710.6 mm diameter, a 114.0 mm sidewall and 720.9 revs per mile.

Related 285/40R19 sizes and tools

Frequently asked questions

What is the overall diameter of a 285/40R19 tyre?

The overall diameter of a new 285/40R19 tyre is 710.6 mm, which is 28.0 inches. That is the rim diameter (482.6 mm) plus twice the sidewall height (114.0 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 285/40R19 tyre?

On this page, 285/40r19 tyre size is the case in point: The sidewall is 114.0 mm tall, because the aspect ratio is 40% of the 285 mm width (285 x 40/100 = 114.0 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 285/40R19 tyre do?

A 285/40R19 tyre completes 720.9 revolutions per mile, based on its 2232.4 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 285/40R19?

The closest size in the chart is 255/50R18, which differs in diameter by +0.2% (+1.6 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 285/40R19 affect my speedometer?

Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 255/50R18 instead of 285/40R19 changes the diameter by +0.2%, so an indicated 70 mph would really be 70.2 mph. Keep the diameter within plus or minus 3% to keep that error small, and check the placard before fitting anything different.