245/45R19 Tire Size: Dimensions, Alternatives & Speedometer Effect

If you are looking up the 245/45R19 tire size, the figures that matter are the diameter (703.1 mm, or 27.7 inches), the sidewall (110.2 mm), the circumference (2208.9 mm) and the revolutions per mile (728.6). The size code itself tells you the ingredients — 245 mm of width, an aspect ratio of 45%, 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 tire you already own or one you are thinking of buying.

This is a measurement page, not a recommendation. Whether a 245/45R19 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 tire (703.1 mm diameter, 110.2 mm sidewall, 2208.9 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 245/45R19 mean?

There are only three numbers to decode. Width: 245 mm. Aspect ratio: 45%, which fixes the sidewall at 245 x 45/100 = 110.2 mm. Rim: 19 inches, or 482.6 mm. The diameter is the rim diameter plus the sidewall top and bottom, giving 703.1 mm in total. Because the aspect ratio is a percentage of the width, a wider tire with the same aspect ratio is also a taller tire — a point that catches people out when they go wider without dropping the aspect ratio to compensate.

245/45R19 dimensions

Measurement Value
Overall diameter 703.1 mm (27.7 in)
Sidewall height 110.2 mm
Circumference 2208.9 mm
Revolutions per mile 728.6
Rim diameter 19 in

The diameter of 703.1 mm (27.7 in) is the headline number because everything else follows from it. The circumference, 2208.9 mm, is pi times the diameter, and it is the distance the car travels each time the wheel turns once. Divide a mile (63,360 inches) by the circumference in inches and you get 728.6 revolutions per mile — the figure the speedometer and odometer effectively count. A tire with fewer revs per mile covers more ground per turn, so the car travels faster than the speedometer thinks if you fit a taller tire without recalibrating. All of these are nominal new-tire figures: real tyres vary slightly by brand, model, wear and inflation pressure, so treat the decimals as precise maths on a nominal size, not a measurement of your individual tire.

Closest alternative sizes to 245/45R19

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. 245/50R18 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?
245/50R18 702.2 mm (27.6 in) 122.5 mm -0.1% Within the guideline
275/40R19 702.6 mm (27.7 in) 110.0 mm -0.1% Within the guideline
245/40R20 704.0 mm (27.7 in) 98.0 mm +0.1% Within the guideline

Speedometer effect of changing from 245/45R19

Take the closest alternative, 245/50R18, as a worked example. Its diameter is 702.2 mm against 703.1 mm here, a difference of -0.1%. Because the speedometer counts revolutions, the error follows the diameter ratio: actual speed equals indicated speed multiplied by 702.2 / 703.1. At an indicated 30 mph the car would really be doing 30.0 mph; at 50 mph indicated it would be 49.9 mph; at 70 mph indicated, 69.9 mph. That means the speedometer would read slightly high, so you would be travelling a touch slower 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
245/50R18 -0.1% 30.0 mph 49.9 mph 69.9 mph
275/40R19 -0.1% 30.0 mph 50.0 mph 70.0 mph
245/40R20 +0.1% 30.0 mph 50.1 mph 70.1 mph

Wider, narrower or taller than 245/45R19: what actually changes

What actually changes if you move away from 245/45R19? Width first: a wider tire 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 45% 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 tire still clears the suspension and arches at full lock before committing.

How to check 245/45R19 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 19-inch rim of the right width for a 245 mm tire. 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 245/45R19, the 245 mm width and 110.2 mm sidewall are part of that same check.

Mistakes to avoid with 245/45R19

Watch for these traps. Assuming the current tire 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 tire 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 245/45R19 is 245 mm wide on a 19-inch rim with a 703.1 mm overall diameter, so those are the numbers the placard check has to match.

Related 245/45R19 sizes and tools

Frequently asked questions

What is the overall diameter of a 245/45R19 tire?

The overall diameter of a new 245/45R19 tire is 703.1 mm, which is 27.7 inches. That is the rim diameter (482.6 mm) plus twice the sidewall height (110.2 mm). It is a nominal figure, so an individual tire can vary slightly by brand, model, wear and inflation pressure.

How tall is the sidewall on a 245/45R19 tire?

The sidewall is 110.2 mm tall, because the aspect ratio is 45% of the 245 mm width (245 x 45/100 = 110.2 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 tire gives over bumps.

How many revolutions per mile does a 245/45R19 tire do?

A 245/45R19 tire completes 728.6 revolutions per mile, based on its 2208.9 mm circumference. That figure matters because the speedometer and odometer count revolutions: fit a tire with a different circumference and the readings drift by the same percentage as the diameter change.

What is the closest alternative size to 245/45R19?

The closest size in the chart is 245/50R18, 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 245/45R19 affect my speedometer?

Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 245/50R18 instead of 245/45R19 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.