175/65R15 vs 185/55R16: Tyre Size Comparison, Diameter Difference & Speedometer Effect

If you are choosing between 175/65R15 and 185/55R16, the first question is how far apart they really are. 185/55R16 is the taller tyre: its diameter is 609.9 mm against 608.5 mm, +1.4 mm (+0.2%) larger. The sidewall goes from 113.8 mm to 101.8 mm (-12.0 mm), the circumference from 1911.7 mm to 1916.1 mm, and the revs per mile from 841.9 to 839.9. 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.

Two things decide whether this swap makes sense: the diameter difference and the physical fit. On diameter, 185/55R16 is +0.2% larger than 175/65R15, which is inside the plus-or-minus 3% guideline. On physical fit, the width change of +10 mm and the rim change from 15 to 16 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.

175/65R15 vs 185/55R16 side by side

Measurement 175/65R15 185/55R16 Difference
Overall diameter 608.5 mm (24.0 in) 609.9 mm (24.0 in) +1.4 mm (+0.2%)
Sidewall height 113.8 mm 101.8 mm -12.0 mm
Circumference 1911.7 mm 1916.1 mm +4.4 mm
Revolutions per mile 841.9 839.9 -2.0
Section width 175 mm 185 mm +10 mm
Rim diameter 15 in 16 in +1 in

Diameter difference between 175/65R15 and 185/55R16: +1.4 mm (+0.2%)

The diameter gap of +1.4 mm (+0.2%) is the number to carry away from this section. It is inside the plus-or-minus 3% guideline, and it shows up directly in the speedometer tables below. The circumference difference of +4.4 mm and the revs-per-mile difference of -2.0 are the same change expressed differently — the first is what the road sees, the second is what the car's sensors count. Neither is a safety judgement on its own; both are inputs to the fitment checks further down the page.

Speedometer error: 175/65R15 to 185/55R16 and back

Going from 175/65R15 to 185/55R16, the diameter grows by +0.2%, so the true speed is the indicated speed multiplied by 1.002. At 30 mph on the dial the car is really doing 30.1 mph; at 50 mph indicated it is 50.1 mph; at 70 mph indicated it is 70.2 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 185/55R16 back to 175/65R15, the error reverses exactly: the ratio becomes 0.998, so 70 mph indicated is really 69.8 mph.

Indicated speed Actual speed fitting 185/55R16 (from 175/65R15) Actual speed fitting 175/65R15 (from 185/55R16)
30 mph 30.1 mph 29.9 mph
50 mph 50.1 mph 49.9 mph
70 mph 70.2 mph 69.8 mph

Sidewall height and ride comfort: 175/65R15 vs 185/55R16

Look at the sidewalls next. 113.8 mm on 175/65R15, 101.8 mm on 185/55R16: a difference of -12.0 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 (65% vs 55%) is simply the sidewall expressed as a share of the width.

Width, grip and fuel use: 175/65R15 vs 185/55R16

The width gap is 10 mm (175 mm against 185 mm). Going wider usually buys dry grip and a more planted feel, and it costs a little in rolling resistance and road noise. Remember that width also decides whether the tyre fits the rim: a 185 mm tyre needs a rim width within the range its maker approves, and a rim that suited 175/65R15 may not suit 185/55R16. That is a separate check from the diameter comparison, and it belongs on the list before anything is fitted.

Revolutions per mile and gearing: 175/65R15 vs 185/55R16

Gearing follows the circumference. 185/55R16 covers 1916.1 mm per revolution against 1911.7 mm for 175/65R15, so it needs -2.0 fewer revolutions per mile. At a fixed road speed the engine speed drops or rises by the same 0.2% 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 185/55R16 clear after 175/65R15? What to check before fitting

Whether 185/55R16 physically fits is a question for your car, not for this table. The numbers say it is taller by 1.4 mm and 10 mm wider, 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: 175/65R15 or 185/55R16

Choose on the basis of what matters to you, not on the headline alone. If ride comfort matters most, the sidewall change of -12.0 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 +0.2% 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 175/65R15 vs 185/55R16 comparisons and tools

Frequently asked questions

What is the diameter difference between 175/65R15 and 185/55R16?

185/55R16 is +1.4 mm (+0.2%) larger in diameter than 175/65R15: 609.9 mm against 608.5 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 185/55R16 instead of 175/65R15?

On this page, 175/65r15 vs 185/55r16 is the case in point: The error follows the diameter ratio, +0.2%. At an indicated 30 mph the true speed would be 30.1 mph, at 50 mph indicated it would be 50.1 mph, and at 70 mph indicated it would be 70.2 mph. The full table in both directions is in the speedometer section above.

Is 185/55R16 wider than 175/65R15, and does that matter?

Yes — 185/55R16 is 185 mm wide against 175 mm for 175/65R15, a difference of +10 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 185/55R16 fit my car if 175/65R15 fits?

Not necessarily. 185/55R16 is taller by 1.4 mm and 10 mm wider, and it needs a 16-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, 175/65R15 or 185/55R16?

Neither is better in general — it depends on what you want. 185/55R16 has a -12.0 mm sidewall difference, a +10 mm width difference and a +0.2% 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.