165/80R15 vs 185/70R15: Tyre Size Comparison, Diameter Difference & Speedometer Effect

Comparing 165/80R15 with 185/70R15 comes down to four numbers for each size: diameter, sidewall, circumference and revs per mile. 185/70R15 is the shorter tyre: its diameter is 640.0 mm against 645.0 mm, -5.0 mm (-0.8%) smaller. The width changes by +20 mm (165 mm to 185 mm) and the sidewall by -2.5 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.

The short version: a -0.8% 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 20 mm and in rim size (15 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.

165/80R15 vs 185/70R15 side by side

Measurement 165/80R15 185/70R15 Difference
Overall diameter 645.0 mm (25.4 in) 640.0 mm (25.2 in) -5.0 mm (-0.8%)
Sidewall height 132.0 mm 129.5 mm -2.5 mm
Circumference 2026.3 mm 2010.6 mm -15.7 mm
Revolutions per mile 794.2 800.4 +6.2
Section width 165 mm 185 mm +20 mm
Rim diameter 15 in 15 in +0 in

Diameter difference between 165/80R15 and 185/70R15: -5.0 mm (-0.8%)

The diameter gap of -5.0 mm (-0.8%) 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 -15.7 mm and the revs-per-mile difference of +6.2 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: 165/80R15 to 185/70R15 and back

The maths is a single ratio. 185/70R15 over 165/80R15 is 0.992, so every indicated speed after the swap is multiplied by that number to get the true speed: 30 becomes 29.8, 50 becomes 49.6 and 70 becomes 69.5 mph. In the reverse direction the ratio is 1.008, so 70 mph indicated is 70.5 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 185/70R15 (from 165/80R15) Actual speed fitting 165/80R15 (from 185/70R15)
30 mph 29.8 mph 30.2 mph
50 mph 49.6 mph 50.4 mph
70 mph 69.5 mph 70.5 mph

Sidewall height and ride comfort: 165/80R15 vs 185/70R15

Look at the sidewalls next. 132.0 mm on 165/80R15, 129.5 mm on 185/70R15: a difference of -2.5 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: 165/80R15 vs 185/70R15

On width, the two sizes are 20 mm apart. The wider tyre (185/70R15) 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: 165/80R15 vs 185/70R15

Think of the gearing change as the diameter change in disguise. The revs-per-mile gap is +6.2, which is 0.8% 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 0.8% and no more. Small percentages mean small effects, which is another reason the plus-or-minus 3% guideline is a useful yardstick.

Will 185/70R15 clear after 165/80R15? What to check before fitting

Do not skip the physical check. The diameter difference is -0.8% and the width difference is +20 mm, and both of those affect whether 185/70R15 fits where 165/80R15 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: 165/80R15 or 185/70R15

Weigh the four effects separately. Sidewall (-2.5 mm) for comfort. Width (+20 mm) for grip and rolling resistance. Diameter (-0.8%) 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 165/80R15 vs 185/70R15 comparisons and tools

Frequently asked questions

What is the diameter difference between 165/80R15 and 185/70R15?

185/70R15 is -5.0 mm (-0.8%) smaller in diameter than 165/80R15: 640.0 mm against 645.0 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 185/70R15 instead of 165/80R15?

For 165/80r15 vs 185/70r15, the point is this: The error follows the diameter ratio, -0.8%. At an indicated 30 mph the true speed would be 29.8 mph, at 50 mph indicated it would be 49.6 mph, and at 70 mph indicated it would be 69.5 mph. The full table in both directions is in the speedometer section above.

Is 185/70R15 wider than 165/80R15, and does that matter?

Yes — 185/70R15 is 185 mm wide against 165 mm for 165/80R15, 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 185/70R15 fit my car if 165/80R15 fits?

Not necessarily. 185/70R15 is shorter by 5.0 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, 165/80R15 or 185/70R15?

Neither is better in general — it depends on what you want. 185/70R15 has a -2.5 mm sidewall difference, a +20 mm width difference and a -0.8% 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.