185/75R14 vs 185/70R15: Tyre Size Comparison, Diameter Difference & Speedometer Effect

If you are choosing between 185/75R14 and 185/70R15, the first question is how far apart they really are. 185/70R15 is the taller tyre: its diameter is 640.0 mm against 633.1 mm, +6.9 mm (+1.1%) larger. The sidewall goes from 138.8 mm to 129.5 mm (-9.3 mm), the circumference from 1988.9 mm to 2010.6 mm, and the revs per mile from 809.1 to 800.4. The tables below lay all of that out, then work through the speedometer effect in each direction and the clearance checks that matter before either size goes on the car.

The short version: a +1.1% 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 0 mm and in rim size (14 inches against 15 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.

185/75R14 vs 185/70R15 side by side

Measurement 185/75R14 185/70R15 Difference
Overall diameter 633.1 mm (24.9 in) 640.0 mm (25.2 in) +6.9 mm (+1.1%)
Sidewall height 138.8 mm 129.5 mm -9.3 mm
Circumference 1988.9 mm 2010.6 mm +21.7 mm
Revolutions per mile 809.1 800.4 -8.7
Section width 185 mm 185 mm +0 mm
Rim diameter 14 in 15 in +1 in

Diameter difference between 185/75R14 and 185/70R15: +6.9 mm (+1.1%)

Put the percentage in context. The plus-or-minus 3% guideline exists because small diameter changes produce small speedometer errors, and this change is +1.1%, which is inside that band. In revs per mile the shift is -8.7 (809.1 to 800.4), and in circumference it is +21.7 mm per turn. Those are not abstract figures: every mile you drive, the car covers +21.7 mm more or less per wheel revolution than the instruments expect, and the gap accumulates exactly like interest.

Speedometer error: 185/75R14 to 185/70R15 and back

The maths is a single ratio. 185/70R15 over 185/75R14 is 1.011, so every indicated speed after the swap is multiplied by that number to get the true speed: 30 becomes 30.3, 50 becomes 50.5 and 70 becomes 70.8 mph. In the reverse direction the ratio is 0.989, 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 185/70R15 (from 185/75R14) Actual speed fitting 185/75R14 (from 185/70R15)
30 mph 30.3 mph 29.7 mph
50 mph 50.5 mph 49.5 mph
70 mph 70.8 mph 69.2 mph

Sidewall height and ride comfort: 185/75R14 vs 185/70R15

Look at the sidewalls next. 138.8 mm on 185/75R14, 129.5 mm on 185/70R15: 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 (75% vs 70%) is simply the sidewall expressed as a share of the width.

Width, grip and fuel use: 185/75R14 vs 185/70R15

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 (75% vs 70%) and the rim (14 in vs 15 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: 185/75R14 vs 185/70R15

The revs-per-mile figures (809.1 for 185/75R14, 800.4 for 185/70R15) describe the gearing effect. 185/70R15 turns -8.7 fewer times per mile, so at any given road speed the engine turns slightly slower with 185/70R15 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 1.1% either way, so the effect on everyday driving is subtle rather than transformative.

Will 185/70R15 clear after 185/75R14? What to check before fitting

Whether 185/70R15 physically fits is a question for your car, not for this table. The numbers say it is taller by 6.9 mm and 0 mm the same width, 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 185 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: 185/75R14 or 185/70R15

For 185/75r14 vs 185/70r15, the point is this: Weigh the four effects separately. Sidewall (-9.3 mm) for comfort. Width (+0 mm) for grip and rolling resistance. Diameter (+1.1%) 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 185/75R14 vs 185/70R15 comparisons and tools

Frequently asked questions

What is the diameter difference between 185/75R14 and 185/70R15?

185/70R15 is +6.9 mm (+1.1%) larger in diameter than 185/75R14: 640.0 mm against 633.1 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 185/75R14?

On this page, 185/75r14 vs 185/70r15 is the case in point: The error follows the diameter ratio, +1.1%. At an indicated 30 mph the true speed would be 30.3 mph, at 50 mph indicated it would be 50.5 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 185/70R15 wider than 185/75R14, and does that matter?

Neither — 185/70R15 is 185 mm wide against 185 mm for 185/75R14, 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 185/70R15 fit my car if 185/75R14 fits?

Not necessarily. 185/70R15 is taller by 6.9 mm and 0 mm the same width, 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, 185/75R14 or 185/70R15?

Neither is better in general — it depends on what you want. 185/70R15 has a -9.3 mm sidewall difference, a +0 mm width difference and a +1.1% 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.