245/40R18 vs 255/35R19: Tyre Size Comparison, Diameter Difference & Speedometer Effect

If you are choosing between 245/40R18 and 255/35R19, the first question is how far apart they really are. 255/35R19 is the taller tyre: its diameter is 661.1 mm against 653.2 mm, +7.9 mm (+1.2%) larger. The sidewall goes from 98.0 mm to 89.2 mm (-8.8 mm), the circumference from 2052.1 mm to 2076.9 mm, and the revs per mile from 784.2 to 774.9. The tables below lay all of that out, then work through the speedometer effect in each direction and the clearance checks that matter before either size goes on the car.

For 245/40r18 vs 255/35r19, the point is this: Before the tables, keep the context straight. These are nominal new-tyre dimensions, computed from the size codes: real tyres vary slightly by brand, model, wear and inflation pressure. A +1.2% difference is within the plus-or-minus 3% guideline that is commonly used for speedometer accuracy, and the speedometer tables below show exactly what that percentage means at real road speeds. The rest of the page covers sidewall, grip, gearing and the clearance checks that decide whether the larger size can physically fit.

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.

245/40R18 vs 255/35R19 side by side

Measurement 245/40R18 255/35R19 Difference
Overall diameter 653.2 mm (25.7 in) 661.1 mm (26.0 in) +7.9 mm (+1.2%)
Sidewall height 98.0 mm 89.2 mm -8.8 mm
Circumference 2052.1 mm 2076.9 mm +24.8 mm
Revolutions per mile 784.2 774.9 -9.3
Section width 245 mm 255 mm +10 mm
Rim diameter 18 in 19 in +1 in

Diameter difference between 245/40R18 and 255/35R19: +7.9 mm (+1.2%)

Put the percentage in context. The plus-or-minus 3% guideline exists because small diameter changes produce small speedometer errors, and this change is +1.2%, which is inside that band. In revs per mile the shift is -9.3 (784.2 to 774.9), and in circumference it is +24.8 mm per turn. Those are not abstract figures: every mile you drive, the car covers +24.8 mm more or less per wheel revolution than the instruments expect, and the gap accumulates exactly like interest.

Speedometer error: 245/40R18 to 255/35R19 and back

The maths is a single ratio. 255/35R19 over 245/40R18 is 1.012, so every indicated speed after the swap is multiplied by that number to get the true speed: 30 becomes 30.4, 50 becomes 50.6 and 70 becomes 70.8 mph. In the reverse direction the ratio is 0.988, so 70 mph indicated is 69.2 mph in reality. The table below shows both directions at all three speeds, so you can check whichever direction applies to your car without doing the arithmetic yourself.

Indicated speed Actual speed fitting 255/35R19 (from 245/40R18) Actual speed fitting 245/40R18 (from 255/35R19)
30 mph 30.4 mph 29.6 mph
50 mph 50.6 mph 49.4 mph
70 mph 70.8 mph 69.2 mph

Sidewall height and ride comfort: 245/40R18 vs 255/35R19

Sidewall height moves by -8.8 mm between these two sizes (98.0 mm to 89.2 mm), and that is enough to feel. A taller sidewall flexes more, which softens sharp impacts and gives the tyre more give over potholes; a shorter sidewall flexes less, which sharpens the steering response and leaves the rim closer to the road surface on a hard hit. Which of those you get depends on the direction of the change — here the sidewall shrinks — and on the aspect ratio shift from 40% to 35%.

Width, grip and fuel use: 245/40R18 vs 255/35R19

Width changes by +10 mm, from 245 mm to 255 mm. A wider tyre puts a broader contact patch on the road, which generally helps dry grip and braking, but it can also increase road noise and rolling resistance, and it needs more clearance around the suspension and arches. Fuel use moves with rolling resistance, so a wider tyre can cost a small amount of fuel over time — the effect is real but modest for a change of this size, and it is not something this page can put a precise number on without knowing the exact tyre models involved.

Revolutions per mile and gearing: 245/40R18 vs 255/35R19

Think of the gearing change as the diameter change in disguise. The revs-per-mile gap is -9.3, which is 1.2% of the original figure — the same percentage as the diameter gap. That is because revs per mile is just a mile divided by the circumference, and the circumference is pi times the diameter. So everything the gearing does — engine speed at a given road speed, acceleration in each gear, cruising refinement — shifts by 1.2% and no more. Small percentages mean small effects, which is another reason the plus-or-minus 3% guideline is a useful yardstick.

Will 255/35R19 clear after 245/40R18? What to check before fitting

Clearance is the check the numbers cannot do for you. 255/35R19 is taller and 10 mm wider than 245/40R18, so check the wheel arches, the suspension components and the steering lock: turn the wheel fully in both directions and check for contact, and check again with the suspension compressed and the car loaded as it will actually be used. A size that clears when the car is stationary and unloaded can still rub over a bump or at full lock. This page does not claim that 255/35R19 clears your car, and no diameter table can — only a physical check on your vehicle can confirm it. Also confirm the load index and speed rating of 255/35R19 meet what your placard asks for, and that the 19-inch rim suits the car.

Which size suits which priority: 245/40R18 or 255/35R19

The decision comes down to priorities. Comfort follows the sidewall (-8.8 mm here). Grip follows the width (+10 mm here). Instrument accuracy follows the diameter (+1.2% here). Gearing and cruising engine speed follow the revs-per-mile change (-9.3). None of those is large in this comparison, which is why the practical checks — clearance, load index, speed rating, rim width — deserve as much attention as the numbers. Get those right and the size choice becomes a matter of preference rather than a risk.

Related 245/40R18 vs 255/35R19 comparisons and tools

Frequently asked questions

What is the diameter difference between 245/40R18 and 255/35R19?

255/35R19 is +7.9 mm (+1.2%) larger in diameter than 245/40R18: 661.1 mm against 653.2 mm. That is inside the commonly used plus-or-minus 3% guideline for keeping speedometer error small.

How much will my speedometer be out if I fit 255/35R19 instead of 245/40R18?

For 245/40r18 vs 255/35r19, the point is this: The error follows the diameter ratio, +1.2%. At an indicated 30 mph the true speed would be 30.4 mph, at 50 mph indicated it would be 50.6 mph, and at 70 mph indicated it would be 70.8 mph. The full table in both directions is in the speedometer section above.

Is 255/35R19 wider than 245/40R18, and does that matter?

Yes — 255/35R19 is 255 mm wide against 245 mm for 245/40R18, a difference of +10 mm. A wider tyre generally gives more dry grip and can increase rolling resistance and road noise slightly. Width also affects clearance, so check the arches and suspension at full lock.

Will 255/35R19 fit my car if 245/40R18 fits?

Not necessarily. 255/35R19 is taller by 7.9 mm and 10 mm wider, and it needs a 19-inch rim. Check the wheel arches, suspension and steering lock at full lock in both directions, with the car loaded, and confirm the load index and speed rating against your door-jamb placard. A tyre professional can confirm fitment on your vehicle.

Which size is better, 245/40R18 or 255/35R19?

Neither is better in general — it depends on what you want. 255/35R19 has a -8.8 mm sidewall difference, a +10 mm width difference and a +1.2% diameter difference, so the trade-offs are comfort, grip and instrument accuracy respectively. If the diameter is within the plus-or-minus 3% guideline and the clearance check passes, the choice is a matter of preference; if not, resolve those two points first.