165/70R15 vs 175/65R15: Tyre Size Comparison, Diameter Difference & Speedometer Effect

Comparing 165/70R15 with 175/65R15 comes down to four numbers for each size: diameter, sidewall, circumference and revs per mile. 175/65R15 is the shorter tyre: its diameter is 608.5 mm against 612.0 mm, -3.5 mm (-0.6%) smaller. The width changes by +10 mm (165 mm to 175 mm) and the sidewall by -1.7 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.

On this page, 165/70r15 vs 175/65r15 tyre size comparison is the case in point: The short version: a -0.6% 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 (15 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.

165/70R15 vs 175/65R15 side by side

Measurement 165/70R15 175/65R15 Difference
Overall diameter 612.0 mm (24.1 in) 608.5 mm (24.0 in) -3.5 mm (-0.6%)
Sidewall height 115.5 mm 113.8 mm -1.7 mm
Circumference 1922.7 mm 1911.7 mm -11.0 mm
Revolutions per mile 837.0 841.9 +4.9
Section width 165 mm 175 mm +10 mm
Rim diameter 15 in 15 in +0 in

Diameter difference between 165/70R15 and 175/65R15: -3.5 mm (-0.6%)

A -0.6% 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 -11.0 mm per revolution, and the revs per mile change by +4.9, 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: 165/70R15 to 175/65R15 and back

Here is what the speedometer does in practice. Fit 175/65R15 where 165/70R15 was, and the dial understates or overstates speed by 0.6%: indicated 30 mph is really 29.8 mph, indicated 50 mph is really 49.7 mph, and indicated 70 mph is really 69.6 mph. Swap back from 175/65R15 to 165/70R15 and the error flips sign — 70 mph indicated becomes 70.4 mph. The odometer drifts by the same percentage in each direction, so recorded mileage and any service intervals based on it shift too.

Indicated speed Actual speed fitting 175/65R15 (from 165/70R15) Actual speed fitting 165/70R15 (from 175/65R15)
30 mph 29.8 mph 30.2 mph
50 mph 49.7 mph 50.3 mph
70 mph 69.6 mph 70.4 mph

Sidewall height and ride comfort: 165/70R15 vs 175/65R15

Taking 165/70r15 vs 175/65r15 tyre size comparison as the case in point: Sidewall height moves by -1.7 mm between these two sizes (115.5 mm to 113.8 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 65%.

Width, grip and fuel use: 165/70R15 vs 175/65R15

On width, the two sizes are 10 mm apart. The wider tyre (175/65R15) offers more rubber on the road, which tends to mean more grip in the dry and slightly more resistance to roll. 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: 165/70R15 vs 175/65R15

Gearing follows the circumference. 175/65R15 covers 1911.7 mm per revolution against 1922.7 mm for 165/70R15, so it needs +4.9 fewer revolutions per mile. At a fixed road speed the engine speed drops or rises by the same 0.6% as the diameter change. With the shorter size the engine sits slightly higher in the rev range at cruising speed, which sharpens response a little at the cost of a small amount of fuel. Either way the effect is proportional to the diameter difference, not larger than it.

Will 175/65R15 clear after 165/70R15? What to check before fitting

Whether 175/65R15 physically fits is a question for your car, not for this table. The numbers say it is shorter by 3.5 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 175 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: 165/70R15 or 175/65R15

The decision comes down to priorities. Comfort follows the sidewall (-1.7 mm here). Grip follows the width (+10 mm here). Instrument accuracy follows the diameter (-0.6% here). Gearing and cruising engine speed follow the revs-per-mile change (+4.9). 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 165/70R15 vs 175/65R15 comparisons and tools

Frequently asked questions

What is the diameter difference between 165/70R15 and 175/65R15?

175/65R15 is -3.5 mm (-0.6%) smaller in diameter than 165/70R15: 608.5 mm against 612.0 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 175/65R15 instead of 165/70R15?

The error follows the diameter ratio, -0.6%. At an indicated 30 mph the true speed would be 29.8 mph, at 50 mph indicated it would be 49.7 mph, and at 70 mph indicated it would be 69.6 mph. The full table in both directions is in the speedometer section above. That is the point this page works through for 165/70r15 vs 175/65r15 tyre size comparison.

Is 175/65R15 wider than 165/70R15, and does that matter?

Yes — 175/65R15 is 175 mm wide against 165 mm for 165/70R15, 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 175/65R15 fit my car if 165/70R15 fits?

Not necessarily. 175/65R15 is shorter by 3.5 mm and 10 mm wider, 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, 165/70R15 or 175/65R15?

Neither is better in general — it depends on what you want. 175/65R15 has a -1.7 mm sidewall difference, a +10 mm width difference and a -0.6% 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.