185/80R15 vs 205/70R15: Tyre Size Comparison, Diameter Difference & Speedometer Effect

185/80R15 and 205/70R15 look like near neighbours on paper, but the numbers tell a more precise story. 205/70R15 is the shorter tyre: its diameter is 668.0 mm against 677.0 mm, -9.0 mm (-1.3%) smaller. Width moves +20 mm, sidewall height moves -4.5 mm, and the circumference changes by -28.3 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, 205/70R15 is -1.3% smaller than 185/80R15, which is inside the plus-or-minus 3% guideline. On physical fit, the width change of +20 mm and the rim change from 15 to 15 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.

185/80R15 vs 205/70R15 side by side

Measurement 185/80R15 205/70R15 Difference
Overall diameter 677.0 mm (26.7 in) 668.0 mm (26.3 in) -9.0 mm (-1.3%)
Sidewall height 148.0 mm 143.5 mm -4.5 mm
Circumference 2126.9 mm 2098.6 mm -28.3 mm
Revolutions per mile 756.7 766.9 +10.2
Section width 185 mm 205 mm +20 mm
Rim diameter 15 in 15 in +0 in

Diameter difference between 185/80R15 and 205/70R15: -9.0 mm (-1.3%)

The diameter gap of -9.0 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 -28.3 mm and the revs-per-mile difference of +10.2 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: 185/80R15 to 205/70R15 and back

Going from 185/80R15 to 205/70R15, the diameter grows by -1.3%, so the true speed is the indicated speed multiplied by 0.987. At 30 mph on the dial the car is really doing 29.6 mph; at 50 mph indicated it is 49.3 mph; at 70 mph indicated it is 69.1 mph. The speedometer therefore reads high 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 205/70R15 back to 185/80R15, the error reverses exactly: the ratio becomes 1.013, so 70 mph indicated is really 70.9 mph.

Indicated speed Actual speed fitting 205/70R15 (from 185/80R15) Actual speed fitting 185/80R15 (from 205/70R15)
30 mph 29.6 mph 30.4 mph
50 mph 49.3 mph 50.7 mph
70 mph 69.1 mph 70.9 mph

Sidewall height and ride comfort: 185/80R15 vs 205/70R15

Look at the sidewalls next. 148.0 mm on 185/80R15, 143.5 mm on 205/70R15: 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 (80% vs 70%) is simply the sidewall expressed as a share of the width.

Width, grip and fuel use: 185/80R15 vs 205/70R15

On width, the two sizes are 20 mm apart. The wider tyre (205/70R15) 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: 185/80R15 vs 205/70R15

The revs-per-mile figures (756.7 for 185/80R15, 766.9 for 205/70R15) describe the gearing effect. 205/70R15 turns +10.2 fewer times per mile, so at any given road speed the engine turns slightly slower with 205/70R15 fitted — the gearing is effectively longer. That can make cruising a touch more relaxed and can trim fuel use slightly, but it also means slightly less acceleration from the same throttle opening, because each gear is effectively taller. The change is 1.3% either way, so the effect on everyday driving is subtle rather than transformative.

Will 205/70R15 clear after 185/80R15? What to check before fitting

Whether 205/70R15 physically fits is a question for your car, not for this table. The numbers say it is shorter by 9.0 mm and 20 mm wider, 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 205 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: 185/80R15 or 205/70R15

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 -1.3% 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 185/80R15 vs 205/70R15 comparisons and tools

Frequently asked questions

What is the diameter difference between 185/80R15 and 205/70R15?

205/70R15 is -9.0 mm (-1.3%) smaller in diameter than 185/80R15: 668.0 mm against 677.0 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 205/70R15 instead of 185/80R15?

The error follows the diameter ratio, -1.3%. 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 69.1 mph. The full table in both directions is in the speedometer section above.

Is 205/70R15 wider than 185/80R15, and does that matter?

Yes — 205/70R15 is 205 mm wide against 185 mm for 185/80R15, 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 205/70R15 fit my car if 185/80R15 fits?

Not necessarily. 205/70R15 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, 185/80R15 or 205/70R15?

Neither is better in general — it depends on what you want. 205/70R15 has a -4.5 mm sidewall difference, a +20 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.