165/75R14 vs 165/70R15: Tyre Size Comparison, Diameter Difference & Speedometer Effect

165/75R14 and 165/70R15 look like near neighbours on paper, but the numbers tell a more precise story. 165/70R15 is the taller tyre: its diameter is 612.0 mm against 603.1 mm, +8.9 mm (+1.5%) larger. Width moves +0 mm, sidewall height moves -8.3 mm, and the circumference changes by +28.0 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, 165/70R15 is +1.5% larger than 165/75R14, which is inside the plus-or-minus 3% guideline. On physical fit, the width change of +0 mm and the rim change from 14 to 15 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.

165/75R14 vs 165/70R15 side by side

Measurement 165/75R14 165/70R15 Difference
Overall diameter 603.1 mm (23.7 in) 612.0 mm (24.1 in) +8.9 mm (+1.5%)
Sidewall height 123.8 mm 115.5 mm -8.3 mm
Circumference 1894.7 mm 1922.7 mm +28.0 mm
Revolutions per mile 849.4 837.0 -12.4
Section width 165 mm 165 mm +0 mm
Rim diameter 14 in 15 in +1 in

Diameter difference between 165/75R14 and 165/70R15: +8.9 mm (+1.5%)

A +1.5% diameter change is small enough to sit inside the commonly used plus-or-minus 3% guideline, so the speedometer error it introduces stays modest. The circumference changes by +28.0 mm per revolution, and the revs per mile change by -12.4, which is the same story told from the car's point of view: the instruments count revolutions, so fewer revs per mile means the car travels further per revolution than the instruments assume. That drift also affects the odometer, so recorded mileage moves by the same percentage over time.

Speedometer error: 165/75R14 to 165/70R15 and back

The maths is a single ratio. 165/70R15 over 165/75R14 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 165/70R15 (from 165/75R14) Actual speed fitting 165/75R14 (from 165/70R15)
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: 165/75R14 vs 165/70R15

Sidewall height moves by -8.3 mm between these two sizes (123.8 mm to 115.5 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 75% to 70%.

Width, grip and fuel use: 165/75R14 vs 165/70R15

The width gap is 0 mm (165 mm against 165 mm). Because the width is identical, grip differences here depend on the specific tyre rather than the size code. Remember that width also decides whether the tyre fits the rim: a 165 mm tyre needs a rim width within the range its maker approves, and a rim that suited 165/75R14 may not suit 165/70R15. That is a separate check from the diameter comparison, and it belongs on the list before anything is fitted.

Revolutions per mile and gearing: 165/75R14 vs 165/70R15

The revs-per-mile figures (849.4 for 165/75R14, 837.0 for 165/70R15) describe the gearing effect. 165/70R15 turns -12.4 fewer times per mile, so at any given road speed the engine turns slightly slower with 165/70R15 fitted — the gearing is effectively longer. That can make cruising a touch more relaxed and can trim fuel use slightly, but it also means slightly less acceleration from the same throttle opening, because each gear is effectively taller. The change is 1.5% either way, so the effect on everyday driving is subtle rather than transformative.

Will 165/70R15 clear after 165/75R14? What to check before fitting

Whether 165/70R15 physically fits is a question for your car, not for this table. The numbers say it is taller by 8.9 mm and 0 mm the same width, 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 165 mm tyre. A tyre 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: 165/75R14 or 165/70R15

The decision comes down to priorities. Comfort follows the sidewall (-8.3 mm here). Grip follows the width (+0 mm here). Instrument accuracy follows the diameter (+1.5% here). Gearing and cruising engine speed follow the revs-per-mile change (-12.4). 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 165/75R14 vs 165/70R15 comparisons and tools

Frequently asked questions

What is the diameter difference between 165/75R14 and 165/70R15?

165/70R15 is +8.9 mm (+1.5%) larger in diameter than 165/75R14: 612.0 mm against 603.1 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 165/70R15 instead of 165/75R14?

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. That is the point this page works through for 165/75r14 vs 165/70r15.

Is 165/70R15 wider than 165/75R14, and does that matter?

Neither — 165/70R15 is 165 mm wide against 165 mm for 165/75R14, a difference of +0 mm. With the same width, grip differences come from the specific tyre rather than the size. Width also affects clearance, so check the arches and suspension at full lock.

Will 165/70R15 fit my car if 165/75R14 fits?

Not necessarily. 165/70R15 is taller by 8.9 mm and 0 mm the same width, and it needs a 15-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, 165/75R14 or 165/70R15?

Neither is better in general — it depends on what you want. 165/70R15 has a -8.3 mm sidewall difference, a +0 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.