245/40R19 Tyre Size: Dimensions, Alternatives & Speedometer Effect

The 245/40R19 is a low-profile tyre size: 245 mm wide with a 40% aspect ratio on a 19-inch wheel. Its overall diameter is 678.6 mm (26.7 in), its sidewall stands 98.0 mm tall, it rolls 2131.9 mm per revolution and it turns 754.9 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.

This is a measurement page, not a recommendation. Whether a 245/40R19 belongs on your car depends on the placard figure, the load index and the speed rating your vehicle requires — none of which a diameter table can settle on its own. What the table can do is give you the exact size of this tyre (678.6 mm diameter, 98.0 mm sidewall, 2131.9 mm circumference) so that any comparison you make is grounded in real numbers. Work through the alternative-size table and the speedometer section before deciding anything, and treat the plus-or-minus 3% guideline as what it is: a rule of thumb, not an approval.

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 245/40R19 mean?

There are only three numbers to decode. Width: 245 mm. Aspect ratio: 40%, which fixes the sidewall at 245 x 40/100 = 98.0 mm. Rim: 19 inches, or 482.6 mm. The diameter is the rim diameter plus the sidewall top and bottom, giving 678.6 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.

245/40R19 dimensions

Measurement Value
Overall diameter 678.6 mm (26.7 in)
Sidewall height 98.0 mm
Circumference 2131.9 mm
Revolutions per mile 754.9
Rim diameter 19 in

The revs-per-mile figure, 754.9, 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 2131.9 mm circumference, this tyre turns 754.9 times per mile, and any replacement with a different circumference will turn a different number of times over the same mile. The diameter (678.6 mm, 26.7 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 245/40R19

The three sizes below are the closest matches to 245/40R19 in the chart, drawn from the same rim or one inch either side and ranked by diameter difference. The closest, 245/45R18, differs by -0.1% (-0.9 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?
245/45R18 677.7 mm (26.7 in) 110.2 mm -0.1% Within the guideline
245/35R20 679.5 mm (26.8 in) 85.8 mm +0.1% Within the guideline
275/40R18 677.2 mm (26.7 in) 110.0 mm -0.2% Within the guideline

Speedometer effect of changing from 245/40R19

The speedometer effect is a straight ratio. Fitting 245/45R18 (677.7 mm) in place of 245/40R19 (678.6 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
245/45R18 -0.1% 30.0 mph 49.9 mph 69.9 mph
245/35R20 +0.1% 30.0 mph 50.1 mph 70.1 mph
275/40R18 -0.2% 29.9 mph 49.9 mph 69.9 mph

Wider, narrower or taller than 245/40R19: what actually changes

Taking 245/40r19 tyre size as the case in point: Going wider than 245 mm while keeping the same aspect ratio also makes the tyre taller, because the sidewall is a percentage of the width — that is the single most common surprise in a size change. Going wider and dropping the aspect ratio keeps the diameter closer to standard, which is the idea behind plus-sizing. A taller tyre raises the car slightly and lengthens the gearing, so the engine turns a little slower at a given road speed; a shorter tyre does the opposite. Wider rubber generally brings more dry grip and more road noise, and it can increase fuel use slightly through higher rolling resistance. None of these effects is dramatic for a small change, but they all point the same way: change one dimension at a time and keep the diameter close.

How to check 245/40R19 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 245 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 678.6 mm for the 245/40R19.

Mistakes to avoid with 245/40R19

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 245/40R19 the figures to check are a 678.6 mm diameter, a 98.0 mm sidewall and 754.9 revs per mile.

Related 245/40R19 sizes and tools

Frequently asked questions

What is the overall diameter of a 245/40R19 tyre?

The overall diameter of a new 245/40R19 tyre is 678.6 mm, which is 26.7 inches. That is the rim diameter (482.6 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 245/40R19 tyre?

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. That is the point this page works through for 245/40r19 tyre size.

How many revolutions per mile does a 245/40R19 tyre do?

A 245/40R19 tyre completes 754.9 revolutions per mile, based on its 2131.9 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 245/40R19?

The closest size in the chart is 245/45R18, 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 245/40R19 affect my speedometer?

Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 245/45R18 instead of 245/40R19 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.