235/65R17 vs 245/65R17: Tyre Size Comparison, Diameter Difference & Speedometer Effect

235/65R17 and 245/65R17 look like near neighbours on paper, but the numbers tell a more precise story. 245/65R17 is the taller tyre: its diameter is 750.3 mm against 737.3 mm, +13.0 mm (+1.8%) larger. Width moves +10 mm, sidewall height moves +6.4 mm, and the circumference changes by +40.8 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.

The short version: a +1.8% diameter difference is inside the commonly used plus-or-minus 3% guideline, which is the usual yardstick for keeping speedometer error small. But diameter is only the first check. The two sizes also differ in width by 10 mm and in rim size (17 inches against 17 inches), so a swap means different wheels as well unless the rims already match. Read the whole comparison before deciding — the clearance and load-index sections matter as much as the headline percentage.

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.

235/65R17 vs 245/65R17 side by side

Measurement 235/65R17 245/65R17 Difference
Overall diameter 737.3 mm (29.0 in) 750.3 mm (29.5 in) +13.0 mm (+1.8%)
Sidewall height 152.8 mm 159.2 mm +6.4 mm
Circumference 2316.3 mm 2357.1 mm +40.8 mm
Revolutions per mile 694.8 682.8 -12.0
Section width 235 mm 245 mm +10 mm
Rim diameter 17 in 17 in +0 in

Diameter difference between 235/65R17 and 245/65R17: +13.0 mm (+1.8%)

The diameter gap of +13.0 mm (+1.8%) 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 +40.8 mm and the revs-per-mile difference of -12.0 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: 235/65R17 to 245/65R17 and back

Going from 235/65R17 to 245/65R17, the diameter grows by +1.8%, so the true speed is the indicated speed multiplied by 1.018. At 30 mph on the dial the car is really doing 30.5 mph; at 50 mph indicated it is 50.9 mph; at 70 mph indicated it is 71.2 mph. The speedometer therefore reads low after the change, which matters most at motorway speed, where even a small percentage is a real difference in miles per hour. Going the other way, from 245/65R17 back to 235/65R17, the error reverses exactly: the ratio becomes 0.983, so 70 mph indicated is really 68.8 mph.

Indicated speed Actual speed fitting 245/65R17 (from 235/65R17) Actual speed fitting 235/65R17 (from 245/65R17)
30 mph 30.5 mph 29.5 mph
50 mph 50.9 mph 49.1 mph
70 mph 71.2 mph 68.8 mph

Sidewall height and ride comfort: 235/65R17 vs 245/65R17

The sidewall is where the ride difference lives. 235/65R17 has 152.8 mm of sidewall and 245/65R17 has 159.2 mm, a change of +6.4 mm. More sidewall generally means more cushioning over bumps and potholes, and a slightly softer, less immediate steering feel. The aspect ratios tell the same story in percentage terms: 65% for 235/65R17 against 65% for 245/65R17. Sidewall height also affects vulnerability to kerb damage — a shorter sidewall leaves less rubber between the rim and the road.

Width, grip and fuel use: 235/65R17 vs 245/65R17

On width, the two sizes are 10 mm apart. The wider tyre (245/65R17) 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: 235/65R17 vs 245/65R17

Think of the gearing change as the diameter change in disguise. The revs-per-mile gap is -12.0, which is 1.8% 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 1.8% and no more. Small percentages mean small effects, which is another reason the plus-or-minus 3% guideline is a useful yardstick.

Will 245/65R17 clear after 235/65R17? What to check before fitting

Do not skip the physical check. The diameter difference is +1.8% and the width difference is +10 mm, and both of those affect whether 245/65R17 fits where 235/65R17 did. Check the gap between the tyre and the arch, the gap to the suspension arms and struts, and the clearance at full steering lock in both directions. Repeat the check with weight in the car, because suspension travel changes the geometry. If the tyre touches anything at any point in that check, it does not fit, whatever the diameter percentage says. The load index and speed rating need confirming against the placard too, and the rim width must suit a 245 mm tyre.

Which size suits which priority: 235/65R17 or 245/65R17

Choose on the basis of what matters to you, not on the headline alone. If ride comfort matters most, the sidewall change of +6.4 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 +1.8% 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 235/65R17 vs 245/65R17 comparisons and tools

Frequently asked questions

What is the diameter difference between 235/65R17 and 245/65R17?

245/65R17 is +13.0 mm (+1.8%) larger in diameter than 235/65R17: 750.3 mm against 737.3 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 245/65R17 instead of 235/65R17?

For 235/65r17 vs 245/65r17, the point is this: The error follows the diameter ratio, +1.8%. 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 245/65R17 wider than 235/65R17, and does that matter?

Yes — 245/65R17 is 245 mm wide against 235 mm for 235/65R17, 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 245/65R17 fit my car if 235/65R17 fits?

Not necessarily. 245/65R17 is taller by 13.0 mm and 10 mm wider, and it needs a 17-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, 235/65R17 or 245/65R17?

Neither is better in general — it depends on what you want. 245/65R17 has a +6.4 mm sidewall difference, a +10 mm width difference and a +1.8% 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.