155/80R15 vs 175/70R15: Tyre Size Comparison, Diameter Difference & Speedometer Effect

Comparing 155/80R15 with 175/70R15 comes down to four numbers for each size: diameter, sidewall, circumference and revs per mile. 175/70R15 is the shorter tyre: its diameter is 626.0 mm against 629.0 mm, -3.0 mm (-0.5%) smaller. The width changes by +20 mm (155 mm to 175 mm) and the sidewall by -1.5 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, 175/70R15 is -0.5% smaller than 155/80R15, which is inside the plus-or-minus 3% guideline. On physical fit, the width change of +20 mm and the rim change from 15 to 15 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.

155/80R15 vs 175/70R15 side by side

Measurement 155/80R15 175/70R15 Difference
Overall diameter 629.0 mm (24.8 in) 626.0 mm (24.6 in) -3.0 mm (-0.5%)
Sidewall height 124.0 mm 122.5 mm -1.5 mm
Circumference 1976.1 mm 1966.6 mm -9.5 mm
Revolutions per mile 814.4 818.3 +3.9
Section width 155 mm 175 mm +20 mm
Rim diameter 15 in 15 in +0 in

Diameter difference between 155/80R15 and 175/70R15: -3.0 mm (-0.5%)

A -0.5% 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 -9.5 mm per revolution, and the revs per mile change by +3.9, 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: 155/80R15 to 175/70R15 and back

Here is what the speedometer does in practice. Fit 175/70R15 where 155/80R15 was, and the dial understates or overstates speed by 0.5%: indicated 30 mph is really 29.9 mph, indicated 50 mph is really 49.8 mph, and indicated 70 mph is really 69.7 mph. Swap back from 175/70R15 to 155/80R15 and the error flips sign — 70 mph indicated becomes 70.3 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 175/70R15 (from 155/80R15) Actual speed fitting 155/80R15 (from 175/70R15)
30 mph 29.9 mph 30.1 mph
50 mph 49.8 mph 50.2 mph
70 mph 69.7 mph 70.3 mph

Sidewall height and ride comfort: 155/80R15 vs 175/70R15

For 155/80r15 vs 175/70r15, the point is this: Sidewall height moves by -1.5 mm between these two sizes (124.0 mm to 122.5 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 80% to 70%.

Width, grip and fuel use: 155/80R15 vs 175/70R15

On this page, 155/80r15 vs 175/70r15 is the case in point: Width changes by +20 mm, from 155 mm to 175 mm. A wider tyre puts a broader contact patch on the road, which generally helps dry grip and braking, but it can also increase road noise and rolling resistance, and it needs more clearance around the suspension and arches. Fuel use moves with rolling resistance, so a wider tyre can cost a small amount of fuel over time — the effect is real but modest for a change of this size, and it is not something this page can put a precise number on without knowing the exact tyre models involved.

Revolutions per mile and gearing: 155/80R15 vs 175/70R15

The revs-per-mile figures (814.4 for 155/80R15, 818.3 for 175/70R15) describe the gearing effect. 175/70R15 turns +3.9 fewer times per mile, so at any given road speed the engine turns slightly slower with 175/70R15 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 0.5% either way, so the effect on everyday driving is subtle rather than transformative.

Will 175/70R15 clear after 155/80R15? What to check before fitting

Clearance is the check the numbers cannot do for you. 175/70R15 is shorter and 20 mm wider than 155/80R15, 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 175/70R15 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 175/70R15 meet what your placard asks for, and that the 15-inch rim suits the car.

Which size suits which priority: 155/80R15 or 175/70R15

Choose on the basis of what matters to you, not on the headline alone. If ride comfort matters most, the sidewall change of -1.5 mm is the figure to weigh. If grip matters most, the width change of +20 mm is the one to weigh. If keeping the instruments accurate matters most, the diameter change of -0.5% 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. That is the point this page works through for 155/80r15 vs 175/70r15.

Related 155/80R15 vs 175/70R15 comparisons and tools

Frequently asked questions

What is the diameter difference between 155/80R15 and 175/70R15?

175/70R15 is -3.0 mm (-0.5%) smaller in diameter than 155/80R15: 626.0 mm against 629.0 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 175/70R15 instead of 155/80R15?

The error follows the diameter ratio, -0.5%. At an indicated 30 mph the true speed would be 29.9 mph, at 50 mph indicated it would be 49.8 mph, and at 70 mph indicated it would be 69.7 mph. The full table in both directions is in the speedometer section above. That is the point this page works through for 155/80r15 vs 175/70r15.

Is 175/70R15 wider than 155/80R15, and does that matter?

Yes — 175/70R15 is 175 mm wide against 155 mm for 155/80R15, a difference of +20 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 175/70R15 fit my car if 155/80R15 fits?

Not necessarily. 175/70R15 is shorter by 3.0 mm and 20 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, 155/80R15 or 175/70R15?

Neither is better in general — it depends on what you want. 175/70R15 has a -1.5 mm sidewall difference, a +20 mm width difference and a -0.5% 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.