175/80R15 vs 185/70R16: Tyre Size Comparison, Diameter Difference & Speedometer Effect

Comparing 175/80R15 with 185/70R16 comes down to four numbers for each size: diameter, sidewall, circumference and revs per mile. 185/70R16 is the taller tyre: its diameter is 665.4 mm against 661.0 mm, +4.4 mm (+0.7%) larger. The width changes by +10 mm (175 mm to 185 mm) and the sidewall by -10.5 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.

Two things decide whether this swap makes sense: the diameter difference and the physical fit. On diameter, 185/70R16 is +0.7% larger than 175/80R15, which is inside the plus-or-minus 3% guideline. On physical fit, the width change of +10 mm and the rim change from 15 to 16 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.

175/80R15 vs 185/70R16 side by side

Measurement 175/80R15 185/70R16 Difference
Overall diameter 661.0 mm (26.0 in) 665.4 mm (26.2 in) +4.4 mm (+0.7%)
Sidewall height 140.0 mm 129.5 mm -10.5 mm
Circumference 2076.6 mm 2090.4 mm +13.8 mm
Revolutions per mile 775.0 769.9 -5.1
Section width 175 mm 185 mm +10 mm
Rim diameter 15 in 16 in +1 in

Diameter difference between 175/80R15 and 185/70R16: +4.4 mm (+0.7%)

A +0.7% 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 +13.8 mm per revolution, and the revs per mile change by -5.1, 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: 175/80R15 to 185/70R16 and back

Going from 175/80R15 to 185/70R16, the diameter grows by +0.7%, so the true speed is the indicated speed multiplied by 1.007. At 30 mph on the dial the car is really doing 30.2 mph; at 50 mph indicated it is 50.3 mph; at 70 mph indicated it is 70.5 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 185/70R16 back to 175/80R15, the error reverses exactly: the ratio becomes 0.993, so 70 mph indicated is really 69.5 mph.

Indicated speed Actual speed fitting 185/70R16 (from 175/80R15) Actual speed fitting 175/80R15 (from 185/70R16)
30 mph 30.2 mph 29.8 mph
50 mph 50.3 mph 49.7 mph
70 mph 70.5 mph 69.5 mph

Sidewall height and ride comfort: 175/80R15 vs 185/70R16

Look at the sidewalls next. 140.0 mm on 175/80R15, 129.5 mm on 185/70R16: a difference of -10.5 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 (80% vs 70%) is simply the sidewall expressed as a share of the width.

Width, grip and fuel use: 175/80R15 vs 185/70R16

For 175/80r15 vs 185/70r16, the point is this: 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.

Revolutions per mile and gearing: 175/80R15 vs 185/70R16

Think of the gearing change as the diameter change in disguise. The revs-per-mile gap is -5.1, which is 0.7% 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 0.7% and no more. Small percentages mean small effects, which is another reason the plus-or-minus 3% guideline is a useful yardstick.

Will 185/70R16 clear after 175/80R15? What to check before fitting

Do not skip the physical check. The diameter difference is +0.7% and the width difference is +10 mm, and both of those affect whether 185/70R16 fits where 175/80R15 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: 175/80R15 or 185/70R16

The decision comes down to priorities. Comfort follows the sidewall (-10.5 mm here). Grip follows the width (+10 mm here). Instrument accuracy follows the diameter (+0.7% here). Gearing and cruising engine speed follow the revs-per-mile change (-5.1). 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 175/80R15 vs 185/70R16 comparisons and tools

Frequently asked questions

What is the diameter difference between 175/80R15 and 185/70R16?

185/70R16 is +4.4 mm (+0.7%) larger in diameter than 175/80R15: 665.4 mm against 661.0 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 185/70R16 instead of 175/80R15?

The error follows the diameter ratio, +0.7%. At an indicated 30 mph the true speed would be 30.2 mph, at 50 mph indicated it would be 50.3 mph, and at 70 mph indicated it would be 70.5 mph. The full table in both directions is in the speedometer section above.

Is 185/70R16 wider than 175/80R15, and does that matter?

Yes — 185/70R16 is 185 mm wide against 175 mm for 175/80R15, 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 185/70R16 fit my car if 175/80R15 fits?

Not necessarily. 185/70R16 is taller by 4.4 mm and 10 mm wider, and it needs a 16-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, 175/80R15 or 185/70R16?

Neither is better in general — it depends on what you want. 185/70R16 has a -10.5 mm sidewall difference, a +10 mm width difference and a +0.7% 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.