195/70R14 vs 185/70R15: Tyre Size Comparison, Diameter Difference & Speedometer Effect

195/70R14 and 185/70R15 look like near neighbours on paper, but the numbers tell a more precise story. 185/70R15 is the taller tyre: its diameter is 640.0 mm against 628.6 mm, +11.4 mm (+1.8%) larger. Width moves -10 mm, sidewall height moves -7.0 mm, and the circumference changes by +35.8 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 195/70r14 vs 185/70r15 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 +1.8% 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.

195/70R14 vs 185/70R15 side by side

Measurement 195/70R14 185/70R15 Difference
Overall diameter 628.6 mm (24.7 in) 640.0 mm (25.2 in) +11.4 mm (+1.8%)
Sidewall height 136.5 mm 129.5 mm -7.0 mm
Circumference 1974.8 mm 2010.6 mm +35.8 mm
Revolutions per mile 814.9 800.4 -14.5
Section width 195 mm 185 mm -10 mm
Rim diameter 14 in 15 in +1 in

Diameter difference between 195/70R14 and 185/70R15: +11.4 mm (+1.8%)

A +1.8% 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 +35.8 mm per revolution, and the revs per mile change by -14.5, 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: 195/70R14 to 185/70R15 and back

Going from 195/70R14 to 185/70R15, the diameter grows by +1.8%, so the true speed is the indicated speed multiplied by 1.018. At 30 mph on the dial the car is really doing 30.5 mph; at 50 mph indicated it is 50.9 mph; at 70 mph indicated it is 71.3 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 185/70R15 back to 195/70R14, the error reverses exactly: the ratio becomes 0.982, so 70 mph indicated is really 68.8 mph.

Indicated speed Actual speed fitting 185/70R15 (from 195/70R14) Actual speed fitting 195/70R14 (from 185/70R15)
30 mph 30.5 mph 29.5 mph
50 mph 50.9 mph 49.1 mph
70 mph 71.3 mph 68.8 mph

Sidewall height and ride comfort: 195/70R14 vs 185/70R15

For 195/70r14 vs 185/70r15 tyre size comparison, the point is this: Sidewall height moves by -7.0 mm between these two sizes (136.5 mm to 129.5 mm), and that is enough to feel. A taller sidewall flexes more, which softens sharp impacts and gives the tyre more give over potholes; a shorter sidewall flexes less, which sharpens the steering response and leaves the rim closer to the road surface on a hard hit. Which of those you get depends on the direction of the change — here the sidewall shrinks — and on the aspect ratio shift from 70% to 70%.

Width, grip and fuel use: 195/70R14 vs 185/70R15

On this page, 195/70r14 vs 185/70r15 tyre size comparison is the case in point: Width changes by -10 mm, from 195 mm to 185 mm. A narrower tyre puts a smaller contact patch on the road, which generally reduces rolling resistance and can help in standing water, but it gives up some dry grip compared with the wider size. 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: 195/70R14 vs 185/70R15

Gearing follows the circumference. 185/70R15 covers 2010.6 mm per revolution against 1974.8 mm for 195/70R14, so it needs -14.5 fewer revolutions per mile. At a fixed road speed the engine speed drops or rises by the same 1.8% as the diameter change. With the taller size the engine sits slightly lower in the rev range at cruising speed, which some drivers like and which can help fuel use a little. Either way the effect is proportional to the diameter difference, not larger than it.

Will 185/70R15 clear after 195/70R14? What to check before fitting

Do not skip the physical check. The diameter difference is +1.8% and the width difference is -10 mm, and both of those affect whether 185/70R15 fits where 195/70R14 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: 195/70R14 or 185/70R15

Choose on the basis of what matters to you, not on the headline alone. If ride comfort matters most, the sidewall change of -7.0 mm is the figure to weigh. If grip matters most, the width change of -10 mm is the one to weigh. If keeping the instruments accurate matters most, the diameter change of +1.8% 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 195/70R14 vs 185/70R15 comparisons and tools

Frequently asked questions

What is the diameter difference between 195/70R14 and 185/70R15?

185/70R15 is +11.4 mm (+1.8%) larger in diameter than 195/70R14: 640.0 mm against 628.6 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 195/70R14?

On this page, 195/70r14 vs 185/70r15 tyre size comparison is the case in point: The error follows the diameter ratio, +1.8%. At an indicated 30 mph the true speed would be 30.5 mph, at 50 mph indicated it would be 50.9 mph, and at 70 mph indicated it would be 71.3 mph. The full table in both directions is in the speedometer section above.

Is 185/70R15 wider than 195/70R14, and does that matter?

No — 185/70R15 is 185 mm wide against 195 mm for 195/70R14, a difference of -10 mm. A narrower tyre generally gives less rolling resistance and can behave better in standing water, with slightly less dry grip. Width also affects clearance, so check the arches and suspension at full lock.

Will 185/70R15 fit my car if 195/70R14 fits?

Not necessarily. 185/70R15 is taller by 11.4 mm and 10 mm narrower, 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, 195/70R14 or 185/70R15?

Neither is better in general — it depends on what you want. 185/70R15 has a -7.0 mm sidewall difference, a -10 mm width difference and a +1.8% 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.