185/70R13 vs 175/70R14: Tyre Size Comparison, Diameter Difference & Speedometer Effect

185/70R13 and 175/70R14 look like near neighbours on paper, but the numbers tell a more precise story. 175/70R14 is the taller tyre: its diameter is 600.6 mm against 589.2 mm, +11.4 mm (+1.9%) 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.

The short version: a +1.9% 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 10 mm and in rim size (13 inches against 14 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.

185/70R13 vs 175/70R14 side by side

Measurement 185/70R13 175/70R14 Difference
Overall diameter 589.2 mm (23.2 in) 600.6 mm (23.6 in) +11.4 mm (+1.9%)
Sidewall height 129.5 mm 122.5 mm -7.0 mm
Circumference 1851.0 mm 1886.8 mm +35.8 mm
Revolutions per mile 869.4 852.9 -16.5
Section width 185 mm 175 mm -10 mm
Rim diameter 13 in 14 in +1 in

Diameter difference between 185/70R13 and 175/70R14: +11.4 mm (+1.9%)

A +1.9% 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 -16.5, 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: 185/70R13 to 175/70R14 and back

The maths is a single ratio. 175/70R14 over 185/70R13 is 1.019, so every indicated speed after the swap is multiplied by that number to get the true speed: 30 becomes 30.6, 50 becomes 51.0 and 70 becomes 71.4 mph. In the reverse direction the ratio is 0.981, so 70 mph indicated is 68.7 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 175/70R14 (from 185/70R13) Actual speed fitting 185/70R13 (from 175/70R14)
30 mph 30.6 mph 29.4 mph
50 mph 51.0 mph 49.1 mph
70 mph 71.4 mph 68.7 mph

Sidewall height and ride comfort: 185/70R13 vs 175/70R14

Look at the sidewalls next. 129.5 mm on 185/70R13, 122.5 mm on 175/70R14: a difference of -7.0 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 70%) is simply the sidewall expressed as a share of the width.

Width, grip and fuel use: 185/70R13 vs 175/70R14

The width gap is 10 mm (185 mm against 175 mm). Going narrower usually trims rolling resistance and can improve behaviour in standing water, and it costs a little in dry grip. Remember that width also decides whether the tyre fits the rim: a 175 mm tyre needs a rim width within the range its maker approves, and a rim that suited 185/70R13 may not suit 175/70R14. That is a separate check from the diameter comparison, and it belongs on the list before anything is fitted.

Revolutions per mile and gearing: 185/70R13 vs 175/70R14

The revs-per-mile figures (869.4 for 185/70R13, 852.9 for 175/70R14) describe the gearing effect. 175/70R14 turns -16.5 fewer times per mile, so at any given road speed the engine turns slightly slower with 175/70R14 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 1.9% either way, so the effect on everyday driving is subtle rather than transformative.

Will 175/70R14 clear after 185/70R13? What to check before fitting

Do not skip the physical check. The diameter difference is +1.9% and the width difference is -10 mm, and both of those affect whether 175/70R14 fits where 185/70R13 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 175 mm tyre.

Which size suits which priority: 185/70R13 or 175/70R14

The decision comes down to priorities. Comfort follows the sidewall (-7.0 mm here). Grip follows the width (-10 mm here). Instrument accuracy follows the diameter (+1.9% here). Gearing and cruising engine speed follow the revs-per-mile change (-16.5). None of those is large in this comparison, which is why the practical checks — clearance, load index, speed rating, rim width — deserve as much attention as the numbers. Get those right and the size choice becomes a matter of preference rather than a risk.

Related 185/70R13 vs 175/70R14 comparisons and tools

Frequently asked questions

What is the diameter difference between 185/70R13 and 175/70R14?

175/70R14 is +11.4 mm (+1.9%) larger in diameter than 185/70R13: 600.6 mm against 589.2 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 175/70R14 instead of 185/70R13?

The error follows the diameter ratio, +1.9%. At an indicated 30 mph the true speed would be 30.6 mph, at 50 mph indicated it would be 51.0 mph, and at 70 mph indicated it would be 71.4 mph. The full table in both directions is in the speedometer section above.

Is 175/70R14 wider than 185/70R13, and does that matter?

No — 175/70R14 is 175 mm wide against 185 mm for 185/70R13, 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 175/70R14 fit my car if 185/70R13 fits?

Not necessarily. 175/70R14 is taller by 11.4 mm and 10 mm narrower, and it needs a 14-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, 185/70R13 or 175/70R14?

Neither is better in general — it depends on what you want. 175/70R14 has a -7.0 mm sidewall difference, a -10 mm width difference and a +1.9% 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.