175/70R14 vs 165/70R15: Tyre Size Comparison, Diameter Difference & Speedometer Effect

Comparing 175/70R14 with 165/70R15 comes down to four numbers for each size: diameter, sidewall, circumference and revs per mile. 165/70R15 is the taller tyre: its diameter is 612.0 mm against 600.6 mm, +11.4 mm (+1.9%) larger. The width changes by -10 mm (175 mm to 165 mm) and the sidewall by -7.0 mm. This page puts both sizes side by side, works out the diameter difference in millimetres and percent, shows the speedometer error in both directions at 30, 50 and 70 mph, and explains what the change means for ride, grip and clearance — so you can judge the swap with the actual figures in front of you.

The short version: a +1.9% 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 (14 inches against 15 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.

175/70R14 vs 165/70R15 side by side

Measurement 175/70R14 165/70R15 Difference
Overall diameter 600.6 mm (23.6 in) 612.0 mm (24.1 in) +11.4 mm (+1.9%)
Sidewall height 122.5 mm 115.5 mm -7.0 mm
Circumference 1886.8 mm 1922.7 mm +35.9 mm
Revolutions per mile 852.9 837.0 -15.9
Section width 175 mm 165 mm -10 mm
Rim diameter 14 in 15 in +1 in

Diameter difference between 175/70R14 and 165/70R15: +11.4 mm (+1.9%)

The diameter gap of +11.4 mm (+1.9%) 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 +35.9 mm and the revs-per-mile difference of -15.9 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: 175/70R14 to 165/70R15 and back

The maths is a single ratio. 165/70R15 over 175/70R14 is 1.019, so every indicated speed after the swap is multiplied by that number to get the true speed: 30 becomes 30.6, 50 becomes 50.9 and 70 becomes 71.3 mph. In the reverse direction the ratio is 0.981, so 70 mph indicated is 68.7 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 175/70R14) Actual speed fitting 175/70R14 (from 165/70R15)
30 mph 30.6 mph 29.4 mph
50 mph 50.9 mph 49.1 mph
70 mph 71.3 mph 68.7 mph

Sidewall height and ride comfort: 175/70R14 vs 165/70R15

The sidewall is where the ride difference lives. 175/70R14 has 122.5 mm of sidewall and 165/70R15 has 115.5 mm, a change of -7.0 mm. Less sidewall generally means sharper steering response and less cushioning, so more of the road surface reaches the cabin. The aspect ratios tell the same story in percentage terms: 70% for 175/70R14 against 70% for 165/70R15. 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: 175/70R14 vs 165/70R15

The width gap is 10 mm (175 mm against 165 mm). Going narrower usually trims rolling resistance and can improve behaviour in standing water, and it costs a little in dry grip. 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 175/70R14 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: 175/70R14 vs 165/70R15

Think of the gearing change as the diameter change in disguise. The revs-per-mile gap is -15.9, which is 1.9% 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 1.9% and no more. Small percentages mean small effects, which is another reason the plus-or-minus 3% guideline is a useful yardstick.

Will 165/70R15 clear after 175/70R14? What to check before fitting

Clearance is the check the numbers cannot do for you. 165/70R15 is taller and 10 mm narrower than 175/70R14, 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 165/70R15 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 165/70R15 meet what your placard asks for, and that the 15-inch rim suits the car.

Which size suits which priority: 175/70R14 or 165/70R15

The decision comes down to priorities. Comfort follows the sidewall (-7.0 mm here). Grip follows the width (-10 mm here). Instrument accuracy follows the diameter (+1.9% here). Gearing and cruising engine speed follow the revs-per-mile change (-15.9). 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 175/70R14 vs 165/70R15 comparisons and tools

Frequently asked questions

What is the diameter difference between 175/70R14 and 165/70R15?

165/70R15 is +11.4 mm (+1.9%) larger in diameter than 175/70R14: 612.0 mm against 600.6 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 175/70R14?

Taking 175/70r14 vs 165/70r15 as the case in point: The error follows the diameter ratio, +1.9%. At an indicated 30 mph the true speed would be 30.6 mph, at 50 mph indicated it would be 50.9 mph, and at 70 mph indicated it would be 71.3 mph. The full table in both directions is in the speedometer section above.

Is 165/70R15 wider than 175/70R14, and does that matter?

No — 165/70R15 is 165 mm wide against 175 mm for 175/70R14, a difference of -10 mm. A narrower tyre generally gives less rolling resistance and can behave better in standing water, with slightly less dry grip. Width also affects clearance, so check the arches and suspension at full lock.

Will 165/70R15 fit my car if 175/70R14 fits?

Not necessarily. 165/70R15 is taller by 11.4 mm and 10 mm narrower, 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, 175/70R14 or 165/70R15?

Neither is better in general — it depends on what you want. 165/70R15 has a -7.0 mm sidewall difference, a -10 mm width difference and a +1.9% 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.