185/70R14 vs 175/70R15: Tyre Size Comparison, Diameter Difference & Speedometer Effect

If you are choosing between 185/70R14 and 175/70R15, the first question is how far apart they really are. 175/70R15 is the taller tyre: its diameter is 626.0 mm against 614.6 mm, +11.4 mm (+1.9%) larger. The sidewall goes from 129.5 mm to 122.5 mm (-7.0 mm), the circumference from 1930.8 mm to 1966.6 mm, and the revs per mile from 833.5 to 818.3. The tables below lay all of that out, then work through the speedometer effect in each direction and the clearance checks that matter before either size goes on the car.

Two things decide whether this swap makes sense: the diameter difference and the physical fit. On diameter, 175/70R15 is +1.9% larger than 185/70R14, which is inside the plus-or-minus 3% guideline. On physical fit, the width change of -10 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.

185/70R14 vs 175/70R15 side by side

Measurement 185/70R14 175/70R15 Difference
Overall diameter 614.6 mm (24.2 in) 626.0 mm (24.6 in) +11.4 mm (+1.9%)
Sidewall height 129.5 mm 122.5 mm -7.0 mm
Circumference 1930.8 mm 1966.6 mm +35.8 mm
Revolutions per mile 833.5 818.3 -15.2
Section width 185 mm 175 mm -10 mm
Rim diameter 14 in 15 in +1 in

Diameter difference between 185/70R14 and 175/70R15: +11.4 mm (+1.9%)

A +1.9% 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 +35.8 mm per revolution, and the revs per mile change by -15.2, 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: 185/70R14 to 175/70R15 and back

Here is what the speedometer does in practice. Fit 175/70R15 where 185/70R14 was, and the dial understates or overstates speed by 1.9%: indicated 30 mph is really 30.6 mph, indicated 50 mph is really 50.9 mph, and indicated 70 mph is really 71.3 mph. Swap back from 175/70R15 to 185/70R14 and the error flips sign — 70 mph indicated becomes 68.7 mph. The odometer drifts by the same percentage in each direction, so recorded mileage and any service intervals based on it shift too.

Indicated speed Actual speed fitting 175/70R15 (from 185/70R14) Actual speed fitting 185/70R14 (from 175/70R15)
30 mph 30.6 mph 29.5 mph
50 mph 50.9 mph 49.1 mph
70 mph 71.3 mph 68.7 mph

Sidewall height and ride comfort: 185/70R14 vs 175/70R15

Look at the sidewalls next. 129.5 mm on 185/70R14, 122.5 mm on 175/70R15: a difference of -7.0 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 (70% vs 70%) is simply the sidewall expressed as a share of the width.

Width, grip and fuel use: 185/70R14 vs 175/70R15

Width changes by -10 mm, from 185 mm to 175 mm. A narrower tyre puts a smaller contact patch on the road, which generally reduces rolling resistance and can help in standing water, but it gives up some dry grip compared with the wider size. Fuel use moves with rolling resistance, so a wider tyre can cost a small amount of fuel over time — the effect is real but modest for a change of this size, and it is not something this page can put a precise number on without knowing the exact tyre models involved. That is the point this page works through for 185/70r14 vs 175/70r15.

Revolutions per mile and gearing: 185/70R14 vs 175/70R15

Think of the gearing change as the diameter change in disguise. The revs-per-mile gap is -15.2, 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 175/70R15 clear after 185/70R14? What to check before fitting

Whether 175/70R15 physically fits is a question for your car, not for this table. The numbers say it is taller by 11.4 mm and 10 mm narrower, 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 175 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: 185/70R14 or 175/70R15

Weigh the four effects separately. Sidewall (-7.0 mm) for comfort. Width (-10 mm) for grip and rolling resistance. Diameter (+1.9%) 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 185/70R14 vs 175/70R15 comparisons and tools

Frequently asked questions

What is the diameter difference between 185/70R14 and 175/70R15?

175/70R15 is +11.4 mm (+1.9%) larger in diameter than 185/70R14: 626.0 mm against 614.6 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 175/70R15 instead of 185/70R14?

On this page, 185/70r14 vs 175/70r15 is 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 175/70R15 wider than 185/70R14, and does that matter?

No — 175/70R15 is 175 mm wide against 185 mm for 185/70R14, 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 175/70R15 fit my car if 185/70R14 fits?

Not necessarily. 175/70R15 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, 185/70R14 or 175/70R15?

Neither is better in general — it depends on what you want. 175/70R15 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.