165/70R15 vs 185/60R15: Tyre Size Comparison, Diameter Difference & Speedometer Effect

If you are choosing between 165/70R15 and 185/60R15, the first question is how far apart they really are. 185/60R15 is the shorter tyre: its diameter is 603.0 mm against 612.0 mm, -9.0 mm (-1.5%) smaller. The sidewall goes from 115.5 mm to 111.0 mm (-4.5 mm), the circumference from 1922.7 mm to 1894.4 mm, and the revs per mile from 837.0 to 849.5. 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.

Before the tables, keep the context straight. These are nominal new-tyre dimensions, computed from the size codes: real tyres vary slightly by brand, model, wear and inflation pressure. A -1.5% difference is within the plus-or-minus 3% guideline that is commonly used for speedometer accuracy, and the speedometer tables below show exactly what that percentage means at real road speeds. The rest of the page covers sidewall, grip, gearing and the clearance checks that decide whether the larger size can physically fit.

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.

165/70R15 vs 185/60R15 side by side

Measurement 165/70R15 185/60R15 Difference
Overall diameter 612.0 mm (24.1 in) 603.0 mm (23.7 in) -9.0 mm (-1.5%)
Sidewall height 115.5 mm 111.0 mm -4.5 mm
Circumference 1922.7 mm 1894.4 mm -28.3 mm
Revolutions per mile 837.0 849.5 +12.5
Section width 165 mm 185 mm +20 mm
Rim diameter 15 in 15 in +0 in

Diameter difference between 165/70R15 and 185/60R15: -9.0 mm (-1.5%)

A -1.5% 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 -28.3 mm per revolution, and the revs per mile change by +12.5, 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: 165/70R15 to 185/60R15 and back

The maths is a single ratio. 185/60R15 over 165/70R15 is 0.985, so every indicated speed after the swap is multiplied by that number to get the true speed: 30 becomes 29.6, 50 becomes 49.3 and 70 becomes 69.0 mph. In the reverse direction the ratio is 1.015, so 70 mph indicated is 71.0 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 185/60R15 (from 165/70R15) Actual speed fitting 165/70R15 (from 185/60R15)
30 mph 29.6 mph 30.4 mph
50 mph 49.3 mph 50.7 mph
70 mph 69.0 mph 71.0 mph

Sidewall height and ride comfort: 165/70R15 vs 185/60R15

Look at the sidewalls next. 115.5 mm on 165/70R15, 111.0 mm on 185/60R15: a difference of -4.5 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 (70% vs 60%) is simply the sidewall expressed as a share of the width.

Width, grip and fuel use: 165/70R15 vs 185/60R15

On width, the two sizes are 20 mm apart. The wider tyre (185/60R15) offers more rubber on the road, which tends to mean more grip in the dry and slightly more resistance to roll. Do not expect a dramatic change in fuel use from a width change of this size; the bigger influences on fuel use are the tyre compound, the pressure and the way the car is driven.

Revolutions per mile and gearing: 165/70R15 vs 185/60R15

Gearing follows the circumference. 185/60R15 covers 1894.4 mm per revolution against 1922.7 mm for 165/70R15, so it needs +12.5 fewer revolutions per mile. At a fixed road speed the engine speed drops or rises by the same 1.5% as the diameter change. With the shorter size the engine sits slightly higher in the rev range at cruising speed, which sharpens response a little at the cost of a small amount of fuel. Either way the effect is proportional to the diameter difference, not larger than it.

Will 185/60R15 clear after 165/70R15? What to check before fitting

Do not skip the physical check. The diameter difference is -1.5% and the width difference is +20 mm, and both of those affect whether 185/60R15 fits where 165/70R15 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: 165/70R15 or 185/60R15

Choose on the basis of what matters to you, not on the headline alone. If ride comfort matters most, the sidewall change of -4.5 mm is the figure to weigh. If grip matters most, the width change of +20 mm is the one to weigh. If keeping the instruments accurate matters most, the diameter change of -1.5% is the one to weigh — and if it is outside the plus-or-minus 3% guideline, ask about recalibration. And if the answer to the clearance check is uncertain, that outranks all three: a tyre that rubs is not a viable option at any percentage.

Related 165/70R15 vs 185/60R15 comparisons and tools

Frequently asked questions

What is the diameter difference between 165/70R15 and 185/60R15?

185/60R15 is -9.0 mm (-1.5%) smaller in diameter than 165/70R15: 603.0 mm against 612.0 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 185/60R15 instead of 165/70R15?

Taking 165/70r15 vs 185/60r15 as the case in point: The error follows the diameter ratio, -1.5%. At an indicated 30 mph the true speed would be 29.6 mph, at 50 mph indicated it would be 49.3 mph, and at 70 mph indicated it would be 69.0 mph. The full table in both directions is in the speedometer section above.

Is 185/60R15 wider than 165/70R15, and does that matter?

Yes — 185/60R15 is 185 mm wide against 165 mm for 165/70R15, a difference of +20 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 185/60R15 fit my car if 165/70R15 fits?

Not necessarily. 185/60R15 is shorter by 9.0 mm and 20 mm wider, 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, 165/70R15 or 185/60R15?

Neither is better in general — it depends on what you want. 185/60R15 has a -4.5 mm sidewall difference, a +20 mm width difference and a -1.5% 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.