265/70R17 vs 275/65R18: Tire Size Comparison, Diameter Difference & Speedometer Effect

265/70R17 and 275/65R18 look like near neighbours on paper, but the numbers tell a more precise story. 275/65R18 is the taller tire: its diameter is 814.7 mm against 802.8 mm, +11.9 mm (+1.5%) larger. Width moves +10 mm, sidewall height moves -6.7 mm, and the circumference changes by +37.4 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.

Before the tables, keep the context straight. These are nominal new-tire dimensions, computed from the size codes: real tyres vary slightly by brand, model, wear and inflation pressure. A +1.5% 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.

265/70R17 vs 275/65R18 side by side

Measurement 265/70R17 275/65R18 Difference
Overall diameter 802.8 mm (31.6 in) 814.7 mm (32.1 in) +11.9 mm (+1.5%)
Sidewall height 185.5 mm 178.8 mm -6.7 mm
Circumference 2522.1 mm 2559.5 mm +37.4 mm
Revolutions per mile 638.1 628.8 -9.3
Section width 265 mm 275 mm +10 mm
Rim diameter 17 in 18 in +1 in

Diameter difference between 265/70R17 and 275/65R18: +11.9 mm (+1.5%)

Put the percentage in context. The plus-or-minus 3% guideline exists because small diameter changes produce small speedometer errors, and this change is +1.5%, which is inside that band. In revs per mile the shift is -9.3 (638.1 to 628.8), and in circumference it is +37.4 mm per turn. Those are not abstract figures: every mile you drive, the car covers +37.4 mm more or less per wheel revolution than the instruments expect, and the gap accumulates exactly like interest.

Speedometer error: 265/70R17 to 275/65R18 and back

The maths is a single ratio. 275/65R18 over 265/70R17 is 1.015, so every indicated speed after the swap is multiplied by that number to get the true speed: 30 becomes 30.4, 50 becomes 50.7 and 70 becomes 71.0 mph. In the reverse direction the ratio is 0.985, so 70 mph indicated is 69.0 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 275/65R18 (from 265/70R17) Actual speed fitting 265/70R17 (from 275/65R18)
30 mph 30.4 mph 29.6 mph
50 mph 50.7 mph 49.3 mph
70 mph 71.0 mph 69.0 mph

Sidewall height and ride comfort: 265/70R17 vs 275/65R18

Sidewall height moves by -6.7 mm between these two sizes (185.5 mm to 178.8 mm), and that is enough to feel. A taller sidewall flexes more, which softens sharp impacts and gives the tire 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 70% to 65%.

Width, grip and fuel use: 265/70R17 vs 275/65R18

The width gap is 10 mm (265 mm against 275 mm). Going wider usually buys dry grip and a more planted feel, and it costs a little in rolling resistance and road noise. Remember that width also decides whether the tire fits the rim: a 275 mm tire needs a rim width within the range its maker approves, and a rim that suited 265/70R17 may not suit 275/65R18. That is a separate check from the diameter comparison, and it belongs on the list before anything is fitted.

Revolutions per mile and gearing: 265/70R17 vs 275/65R18

Think of the gearing change as the diameter change in disguise. The revs-per-mile gap is -9.3, which is 1.5% 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 1.5% and no more. Small percentages mean small effects, which is another reason the plus-or-minus 3% guideline is a useful yardstick.

Will 275/65R18 clear after 265/70R17? What to check before fitting

Whether 275/65R18 physically fits is a question for your car, not for this table. The numbers say it is taller by 11.9 mm and 10 mm wider, and those are the dimensions that decide clearance — but the actual gaps depend on your arches, suspension and steering geometry. Check at full lock both ways, check with the suspension compressed, and check with the car loaded. Check the placard for the load index and speed rating, and check the rim width for a 275 mm tire. A tire professional can confirm all of it in one visit, and that confirmation is the step that turns a comparison into a fitment.

Which size suits which priority: 265/70R17 or 275/65R18

Weigh the four effects separately. Sidewall (-6.7 mm) for comfort. Width (+10 mm) for grip and rolling resistance. Diameter (+1.5%) for the speedometer, odometer and gearing. And clearance for whether the swap is physically possible at all. If the diameter is inside the plus-or-minus 3% guideline and the clearance check passes, the remaining differences are small enough to be a matter of taste; if either of those fails, the comparison is academic until it is resolved.

Related 265/70R17 vs 275/65R18 comparisons and tools

Frequently asked questions

What is the diameter difference between 265/70R17 and 275/65R18?

275/65R18 is +11.9 mm (+1.5%) larger in diameter than 265/70R17: 814.7 mm against 802.8 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 275/65R18 instead of 265/70R17?

For 265/70r17 vs 275/65r18 tire size comparison, the point is this: The error follows the diameter ratio, +1.5%. At an indicated 30 mph the true speed would be 30.4 mph, at 50 mph indicated it would be 50.7 mph, and at 70 mph indicated it would be 71.0 mph. The full table in both directions is in the speedometer section above.

Is 275/65R18 wider than 265/70R17, and does that matter?

Yes — 275/65R18 is 275 mm wide against 265 mm for 265/70R17, a difference of +10 mm. A wider tire 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 275/65R18 fit my car if 265/70R17 fits?

Not necessarily. 275/65R18 is taller by 11.9 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 tire professional can confirm fitment on your vehicle.

Which size is better, 265/70R17 or 275/65R18?

Neither is better in general — it depends on what you want. 275/65R18 has a -6.7 mm sidewall difference, a +10 mm width difference and a +1.5% 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.