225/45R17 vs 235/40R18: Tyre Size Comparison, Diameter Difference & Speedometer Effect

225/45R17 and 235/40R18 look like near neighbours on paper, but the numbers tell a more precise story. 235/40R18 is the taller tyre: its diameter is 645.2 mm against 634.3 mm, +10.9 mm (+1.7%) larger. Width moves +10 mm, sidewall height moves -7.2 mm, and the circumference changes by +34.3 mm, which is what the speedometer feels. This comparison works through the side-by-side measurements, the speedometer error at 30, 50 and 70 mph in both directions, and the practical effects on ride comfort, grip and wheel-arch clearance.

Two things decide whether this swap makes sense: the diameter difference and the physical fit. On diameter, 235/40R18 is +1.7% larger than 225/45R17, which is inside the plus-or-minus 3% guideline. On physical fit, the width change of +10 mm and the rim change from 17 to 18 inches both need checking against the arches, suspension and steering lock. The sections below take each of those in turn, with the numbers worked out rather than asserted.

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.

225/45R17 vs 235/40R18 side by side

Measurement 225/45R17 235/40R18 Difference
Overall diameter 634.3 mm (25.0 in) 645.2 mm (25.4 in) +10.9 mm (+1.7%)
Sidewall height 101.2 mm 94.0 mm -7.2 mm
Circumference 1992.7 mm 2027.0 mm +34.3 mm
Revolutions per mile 807.6 794.0 -13.6
Section width 225 mm 235 mm +10 mm
Rim diameter 17 in 18 in +1 in

Diameter difference between 225/45R17 and 235/40R18: +10.9 mm (+1.7%)

The diameter gap of +10.9 mm (+1.7%) is the number to carry away from this section. It is inside the plus-or-minus 3% guideline, and it shows up directly in the speedometer tables below. The circumference difference of +34.3 mm and the revs-per-mile difference of -13.6 are the same change expressed differently — the first is what the road sees, the second is what the car's sensors count. Neither is a safety judgement on its own; both are inputs to the fitment checks further down the page.

Speedometer error: 225/45R17 to 235/40R18 and back

The maths is a single ratio. 235/40R18 over 225/45R17 is 1.017, so every indicated speed after the swap is multiplied by that number to get the true speed: 30 becomes 30.5, 50 becomes 50.9 and 70 becomes 71.2 mph. In the reverse direction the ratio is 0.983, so 70 mph indicated is 68.8 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 235/40R18 (from 225/45R17) Actual speed fitting 225/45R17 (from 235/40R18)
30 mph 30.5 mph 29.5 mph
50 mph 50.9 mph 49.2 mph
70 mph 71.2 mph 68.8 mph

Sidewall height and ride comfort: 225/45R17 vs 235/40R18

Look at the sidewalls next. 101.2 mm on 225/45R17, 94.0 mm on 235/40R18: a difference of -7.2 mm. That is the flexible part of the tyre, and it is the first thing that absorbs a bump. A shorter sidewall transmits more of the road to the driver, which feels more direct but less forgiving on rough surfaces. Aspect ratio (45% vs 40%) is simply the sidewall expressed as a share of the width.

Width, grip and fuel use: 225/45R17 vs 235/40R18

On width, the two sizes are 10 mm apart. The wider tyre (235/40R18) offers more rubber on the road, which tends to mean more grip in the dry and slightly more resistance to roll. Do not expect a dramatic change in fuel use from a width change of this size; the bigger influences on fuel use are the tyre compound, the pressure and the way the car is driven.

Revolutions per mile and gearing: 225/45R17 vs 235/40R18

Gearing follows the circumference. 235/40R18 covers 2027.0 mm per revolution against 1992.7 mm for 225/45R17, so it needs -13.6 fewer revolutions per mile. At a fixed road speed the engine speed drops or rises by the same 1.7% as the diameter change. With the taller size the engine sits slightly lower in the rev range at cruising speed, which some drivers like and which can help fuel use a little. Either way the effect is proportional to the diameter difference, not larger than it.

Will 235/40R18 clear after 225/45R17? What to check before fitting

Clearance is the check the numbers cannot do for you. 235/40R18 is taller and 10 mm wider than 225/45R17, 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 235/40R18 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 235/40R18 meet what your placard asks for, and that the 18-inch rim suits the car.

Which size suits which priority: 225/45R17 or 235/40R18

Choose on the basis of what matters to you, not on the headline alone. If ride comfort matters most, the sidewall change of -7.2 mm is the figure to weigh. If grip matters most, the width change of +10 mm is the one to weigh. If keeping the instruments accurate matters most, the diameter change of +1.7% is the one to weigh — and if it is outside the plus-or-minus 3% guideline, ask about recalibration. And if the answer to the clearance check is uncertain, that outranks all three: a tyre that rubs is not a viable option at any percentage.

Related 225/45R17 vs 235/40R18 comparisons and tools

Frequently asked questions

What is the diameter difference between 225/45R17 and 235/40R18?

235/40R18 is +10.9 mm (+1.7%) larger in diameter than 225/45R17: 645.2 mm against 634.3 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 235/40R18 instead of 225/45R17?

Taking 225/45r17 vs 235/40r18 tyre size comparison as the case in point: The error follows the diameter ratio, +1.7%. At an indicated 30 mph the true speed would be 30.5 mph, at 50 mph indicated it would be 50.9 mph, and at 70 mph indicated it would be 71.2 mph. The full table in both directions is in the speedometer section above.

Is 235/40R18 wider than 225/45R17, and does that matter?

Yes — 235/40R18 is 235 mm wide against 225 mm for 225/45R17, 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 235/40R18 fit my car if 225/45R17 fits?

Not necessarily. 235/40R18 is taller by 10.9 mm and 10 mm wider, and it needs a 18-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, 225/45R17 or 235/40R18?

Neither is better in general — it depends on what you want. 235/40R18 has a -7.2 mm sidewall difference, a +10 mm width difference and a +1.7% 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.