175/75R14 vs 185/70R14: Tyre Size Comparison, Diameter Difference & Speedometer Effect

175/75R14 and 185/70R14 look like near neighbours on paper, but the numbers tell a more precise story. 185/70R14 is the shorter tyre: its diameter is 614.6 mm against 618.1 mm, -3.5 mm (-0.6%) smaller. Width moves +10 mm, sidewall height moves -1.7 mm, and the circumference changes by -11.0 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, 185/70R14 is -0.6% smaller than 175/75R14, 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 14 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.

175/75R14 vs 185/70R14 side by side

Measurement 175/75R14 185/70R14 Difference
Overall diameter 618.1 mm (24.3 in) 614.6 mm (24.2 in) -3.5 mm (-0.6%)
Sidewall height 131.2 mm 129.5 mm -1.7 mm
Circumference 1941.8 mm 1930.8 mm -11.0 mm
Revolutions per mile 828.8 833.5 +4.7
Section width 175 mm 185 mm +10 mm
Rim diameter 14 in 14 in +0 in

Diameter difference between 175/75R14 and 185/70R14: -3.5 mm (-0.6%)

The diameter gap of -3.5 mm (-0.6%) 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 -11.0 mm and the revs-per-mile difference of +4.7 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: 175/75R14 to 185/70R14 and back

Here is what the speedometer does in practice. Fit 185/70R14 where 175/75R14 was, and the dial understates or overstates speed by 0.6%: indicated 30 mph is really 29.8 mph, indicated 50 mph is really 49.7 mph, and indicated 70 mph is really 69.6 mph. Swap back from 185/70R14 to 175/75R14 and the error flips sign — 70 mph indicated becomes 70.4 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 185/70R14 (from 175/75R14) Actual speed fitting 175/75R14 (from 185/70R14)
30 mph 29.8 mph 30.2 mph
50 mph 49.7 mph 50.3 mph
70 mph 69.6 mph 70.4 mph

Sidewall height and ride comfort: 175/75R14 vs 185/70R14

Sidewall height moves by -1.7 mm between these two sizes (131.2 mm to 129.5 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: 175/75R14 vs 185/70R14

Width changes by +10 mm, from 175 mm to 185 mm. A wider tyre puts a broader contact patch on the road, which generally helps dry grip and braking, but it can also increase road noise and rolling resistance, and it needs more clearance around the suspension and arches. 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 175/75r14 vs 185/70r14 tyre size comparison.

Revolutions per mile and gearing: 175/75R14 vs 185/70R14

Think of the gearing change as the diameter change in disguise. The revs-per-mile gap is +4.7, which is 0.6% 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 0.6% and no more. Small percentages mean small effects, which is another reason the plus-or-minus 3% guideline is a useful yardstick.

Will 185/70R14 clear after 175/75R14? What to check before fitting

Do not skip the physical check. The diameter difference is -0.6% and the width difference is +10 mm, and both of those affect whether 185/70R14 fits where 175/75R14 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 185 mm tyre.

Which size suits which priority: 175/75R14 or 185/70R14

The decision comes down to priorities. Comfort follows the sidewall (-1.7 mm here). Grip follows the width (+10 mm here). Instrument accuracy follows the diameter (-0.6% here). Gearing and cruising engine speed follow the revs-per-mile change (+4.7). 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 175/75R14 vs 185/70R14 comparisons and tools

Frequently asked questions

What is the diameter difference between 175/75R14 and 185/70R14?

185/70R14 is -3.5 mm (-0.6%) smaller in diameter than 175/75R14: 614.6 mm against 618.1 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 185/70R14 instead of 175/75R14?

The error follows the diameter ratio, -0.6%. At an indicated 30 mph the true speed would be 29.8 mph, at 50 mph indicated it would be 49.7 mph, and at 70 mph indicated it would be 69.6 mph. The full table in both directions is in the speedometer section above. That is the point this page works through for 175/75r14 vs 185/70r14 tyre size comparison.

Is 185/70R14 wider than 175/75R14, and does that matter?

Yes — 185/70R14 is 185 mm wide against 175 mm for 175/75R14, 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 185/70R14 fit my car if 175/75R14 fits?

Not necessarily. 185/70R14 is shorter by 3.5 mm and 10 mm wider, and it needs a 14-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, 175/75R14 or 185/70R14?

Neither is better in general — it depends on what you want. 185/70R14 has a -1.7 mm sidewall difference, a +10 mm width difference and a -0.6% 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.