175/75R14 vs 175/70R15: Tyre Size Comparison, Diameter Difference & Speedometer Effect

175/75R14 and 175/70R15 look like near neighbours on paper, but the numbers tell a more precise story. 175/70R15 is the taller tyre: its diameter is 626.0 mm against 618.1 mm, +7.9 mm (+1.3%) larger. Width moves +0 mm, sidewall height moves -8.7 mm, and the circumference changes by +24.8 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.

The short version: a +1.3% 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 0 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.

175/75R14 vs 175/70R15 side by side

Measurement 175/75R14 175/70R15 Difference
Overall diameter 618.1 mm (24.3 in) 626.0 mm (24.6 in) +7.9 mm (+1.3%)
Sidewall height 131.2 mm 122.5 mm -8.7 mm
Circumference 1941.8 mm 1966.6 mm +24.8 mm
Revolutions per mile 828.8 818.3 -10.5
Section width 175 mm 175 mm +0 mm
Rim diameter 14 in 15 in +1 in

Diameter difference between 175/75R14 and 175/70R15: +7.9 mm (+1.3%)

The diameter gap of +7.9 mm (+1.3%) 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 +24.8 mm and the revs-per-mile difference of -10.5 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 175/70R15 and back

Going from 175/75R14 to 175/70R15, the diameter grows by +1.3%, so the true speed is the indicated speed multiplied by 1.013. At 30 mph on the dial the car is really doing 30.4 mph; at 50 mph indicated it is 50.6 mph; at 70 mph indicated it is 70.9 mph. The speedometer therefore reads low after the change, which matters most at motorway speed, where even a small percentage is a real difference in miles per hour. Going the other way, from 175/70R15 back to 175/75R14, the error reverses exactly: the ratio becomes 0.987, so 70 mph indicated is really 69.1 mph.

Indicated speed Actual speed fitting 175/70R15 (from 175/75R14) Actual speed fitting 175/75R14 (from 175/70R15)
30 mph 30.4 mph 29.6 mph
50 mph 50.6 mph 49.4 mph
70 mph 70.9 mph 69.1 mph

Sidewall height and ride comfort: 175/75R14 vs 175/70R15

Sidewall height moves by -8.7 mm between these two sizes (131.2 mm to 122.5 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 175/70R15

Width changes by +0 mm, from 175 mm to 175 mm. With the same width, the contact patch is essentially unchanged, so the grip difference between these two sizes comes from the construction and compound rather than the dimensions. 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 175/70r15.

Revolutions per mile and gearing: 175/75R14 vs 175/70R15

Think of the gearing change as the diameter change in disguise. The revs-per-mile gap is -10.5, which is 1.3% 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 1.3% 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 175/75R14? 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 7.9 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 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: 175/75R14 or 175/70R15

Weigh the four effects separately. Sidewall (-8.7 mm) for comfort. Width (+0 mm) for grip and rolling resistance. Diameter (+1.3%) 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 175/75R14 vs 175/70R15 comparisons and tools

Frequently asked questions

What is the diameter difference between 175/75R14 and 175/70R15?

175/70R15 is +7.9 mm (+1.3%) larger in diameter than 175/75R14: 626.0 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 175/70R15 instead of 175/75R14?

The error follows the diameter ratio, +1.3%. At an indicated 30 mph the true speed would be 30.4 mph, at 50 mph indicated it would be 50.6 mph, and at 70 mph indicated it would be 70.9 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 175/70r15.

Is 175/70R15 wider than 175/75R14, and does that matter?

Neither — 175/70R15 is 175 mm wide against 175 mm for 175/75R14, 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 175/70R15 fit my car if 175/75R14 fits?

Not necessarily. 175/70R15 is taller by 7.9 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, 175/75R14 or 175/70R15?

Neither is better in general — it depends on what you want. 175/70R15 has a -8.7 mm sidewall difference, a +0 mm width difference and a +1.3% 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.