185/70R13 vs 185/65R14: Tyre Size Comparison, Diameter Difference & Speedometer Effect

Comparing 185/70R13 with 185/65R14 comes down to four numbers for each size: diameter, sidewall, circumference and revs per mile. 185/65R14 is the taller tyre: its diameter is 596.1 mm against 589.2 mm, +6.9 mm (+1.2%) larger. The width changes by +0 mm (185 mm to 185 mm) and the sidewall by -9.3 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.

Two things decide whether this swap makes sense: the diameter difference and the physical fit. On diameter, 185/65R14 is +1.2% larger than 185/70R13, which is inside the plus-or-minus 3% guideline. On physical fit, the width change of +0 mm and the rim change from 13 to 14 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.

185/70R13 vs 185/65R14 side by side

Measurement 185/70R13 185/65R14 Difference
Overall diameter 589.2 mm (23.2 in) 596.1 mm (23.5 in) +6.9 mm (+1.2%)
Sidewall height 129.5 mm 120.2 mm -9.3 mm
Circumference 1851.0 mm 1872.7 mm +21.7 mm
Revolutions per mile 869.4 859.4 -10.0
Section width 185 mm 185 mm +0 mm
Rim diameter 13 in 14 in +1 in

Diameter difference between 185/70R13 and 185/65R14: +6.9 mm (+1.2%)

A +1.2% 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 +21.7 mm per revolution, and the revs per mile change by -10.0, 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 185/65R14 and back

The maths is a single ratio. 185/65R14 over 185/70R13 is 1.012, so every indicated speed after the swap is multiplied by that number to get the true speed: 30 becomes 30.4, 50 becomes 50.6 and 70 becomes 70.8 mph. In the reverse direction the ratio is 0.988, so 70 mph indicated is 69.2 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 185/65R14 (from 185/70R13) Actual speed fitting 185/70R13 (from 185/65R14)
30 mph 30.4 mph 29.7 mph
50 mph 50.6 mph 49.4 mph
70 mph 70.8 mph 69.2 mph

Sidewall height and ride comfort: 185/70R13 vs 185/65R14

Look at the sidewalls next. 129.5 mm on 185/70R13, 120.2 mm on 185/65R14: a difference of -9.3 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 65%) is simply the sidewall expressed as a share of the width.

Width, grip and fuel use: 185/70R13 vs 185/65R14

On width, the two sizes are 0 mm apart. The width is the same, so neither size has a contact-patch advantage over the other. Both are 185 mm wide, so the difference here is the aspect ratio (70% vs 65%) and the rim (13 in vs 14 in). 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: 185/70R13 vs 185/65R14

Gearing follows the circumference. 185/65R14 covers 1872.7 mm per revolution against 1851.0 mm for 185/70R13, so it needs -10.0 fewer revolutions per mile. At a fixed road speed the engine speed drops or rises by the same 1.2% 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 185/65R14 clear after 185/70R13? What to check before fitting

Clearance is the check the numbers cannot do for you. 185/65R14 is taller and 0 mm the same width than 185/70R13, 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 185/65R14 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 185/65R14 meet what your placard asks for, and that the 14-inch rim suits the car.

Which size suits which priority: 185/70R13 or 185/65R14

Weigh the four effects separately. Sidewall (-9.3 mm) for comfort. Width (+0 mm) for grip and rolling resistance. Diameter (+1.2%) 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 185/70R13 vs 185/65R14 comparisons and tools

Frequently asked questions

What is the diameter difference between 185/70R13 and 185/65R14?

185/65R14 is +6.9 mm (+1.2%) larger in diameter than 185/70R13: 596.1 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 185/65R14 instead of 185/70R13?

Taking 185/70r13 vs 185/65r14 as the case in point: The error follows the diameter ratio, +1.2%. At an indicated 30 mph the true speed would be 30.4 mph, at 50 mph indicated it would be 50.6 mph, and at 70 mph indicated it would be 70.8 mph. The full table in both directions is in the speedometer section above.

Is 185/65R14 wider than 185/70R13, and does that matter?

Neither — 185/65R14 is 185 mm wide against 185 mm for 185/70R13, a difference of +0 mm. With the same width, grip differences come from the specific tyre rather than the size. Width also affects clearance, so check the arches and suspension at full lock.

Will 185/65R14 fit my car if 185/70R13 fits?

Not necessarily. 185/65R14 is taller by 6.9 mm and 0 mm the same width, 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 185/65R14?

Neither is better in general — it depends on what you want. 185/65R14 has a -9.3 mm sidewall difference, a +0 mm width difference and a +1.2% 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.