185/70R15 vs 205/60R15: Tyre Size Comparison, Diameter Difference & Speedometer Effect

185/70R15 and 205/60R15 look like near neighbours on paper, but the numbers tell a more precise story. 205/60R15 is the shorter tyre: its diameter is 627.0 mm against 640.0 mm, -13.0 mm (-2.0%) smaller. Width moves +20 mm, sidewall height moves -6.5 mm, and the circumference changes by -40.8 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.

Taking 185/70r15 vs 205/60r15 tyre size comparison as the case in point: 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 -2.0% 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.

185/70R15 vs 205/60R15 side by side

Measurement 185/70R15 205/60R15 Difference
Overall diameter 640.0 mm (25.2 in) 627.0 mm (24.7 in) -13.0 mm (-2.0%)
Sidewall height 129.5 mm 123.0 mm -6.5 mm
Circumference 2010.6 mm 1969.8 mm -40.8 mm
Revolutions per mile 800.4 817.0 +16.6
Section width 185 mm 205 mm +20 mm
Rim diameter 15 in 15 in +0 in

Diameter difference between 185/70R15 and 205/60R15: -13.0 mm (-2.0%)

Put the percentage in context. The plus-or-minus 3% guideline exists because small diameter changes produce small speedometer errors, and this change is -2.0%, which is inside that band. In revs per mile the shift is +16.6 (800.4 to 817.0), and in circumference it is -40.8 mm per turn. Those are not abstract figures: every mile you drive, the car covers -40.8 mm more or less per wheel revolution than the instruments expect, and the gap accumulates exactly like interest.

Speedometer error: 185/70R15 to 205/60R15 and back

The maths is a single ratio. 205/60R15 over 185/70R15 is 0.980, so every indicated speed after the swap is multiplied by that number to get the true speed: 30 becomes 29.4, 50 becomes 49.0 and 70 becomes 68.6 mph. In the reverse direction the ratio is 1.021, so 70 mph indicated is 71.5 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 205/60R15 (from 185/70R15) Actual speed fitting 185/70R15 (from 205/60R15)
30 mph 29.4 mph 30.6 mph
50 mph 49.0 mph 51.0 mph
70 mph 68.6 mph 71.5 mph

Sidewall height and ride comfort: 185/70R15 vs 205/60R15

The sidewall is where the ride difference lives. 185/70R15 has 129.5 mm of sidewall and 205/60R15 has 123.0 mm, a change of -6.5 mm. Less sidewall generally means sharper steering response and less cushioning, so more of the road surface reaches the cabin. The aspect ratios tell the same story in percentage terms: 70% for 185/70R15 against 60% for 205/60R15. Sidewall height also affects vulnerability to kerb damage — a shorter sidewall leaves less rubber between the rim and the road.

Width, grip and fuel use: 185/70R15 vs 205/60R15

On this page, 185/70r15 vs 205/60r15 tyre size comparison is the case in point: Width changes by +20 mm, from 185 mm to 205 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: 185/70R15 vs 205/60R15

Gearing follows the circumference. 205/60R15 covers 1969.8 mm per revolution against 2010.6 mm for 185/70R15, so it needs +16.6 fewer revolutions per mile. At a fixed road speed the engine speed drops or rises by the same 2.0% 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 205/60R15 clear after 185/70R15? What to check before fitting

Clearance is the check the numbers cannot do for you. 205/60R15 is shorter and 20 mm wider than 185/70R15, so check the wheel arches, the suspension components and the steering lock: turn the wheel fully in both directions and check for contact, and check again with the suspension compressed and the car loaded as it will actually be used. A size that clears when the car is stationary and unloaded can still rub over a bump or at full lock. This page does not claim that 205/60R15 clears your car, and no diameter table can — only a physical check on your vehicle can confirm it. Also confirm the load index and speed rating of 205/60R15 meet what your placard asks for, and that the 15-inch rim suits the car.

Which size suits which priority: 185/70R15 or 205/60R15

Weigh the four effects separately. Sidewall (-6.5 mm) for comfort. Width (+20 mm) for grip and rolling resistance. Diameter (-2.0%) 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 185/70R15 vs 205/60R15 comparisons and tools

Frequently asked questions

What is the diameter difference between 185/70R15 and 205/60R15?

205/60R15 is -13.0 mm (-2.0%) smaller in diameter than 185/70R15: 627.0 mm against 640.0 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 205/60R15 instead of 185/70R15?

For 185/70r15 vs 205/60r15 tyre size comparison, the point is this: The error follows the diameter ratio, -2.0%. At an indicated 30 mph the true speed would be 29.4 mph, at 50 mph indicated it would be 49.0 mph, and at 70 mph indicated it would be 68.6 mph. The full table in both directions is in the speedometer section above.

Is 205/60R15 wider than 185/70R15, and does that matter?

Yes — 205/60R15 is 205 mm wide against 185 mm for 185/70R15, 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 205/60R15 fit my car if 185/70R15 fits?

Not necessarily. 205/60R15 is shorter by 13.0 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, 185/70R15 or 205/60R15?

Neither is better in general — it depends on what you want. 205/60R15 has a -6.5 mm sidewall difference, a +20 mm width difference and a -2.0% 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.