Speedometer Error 155/70R13 to 175/60R13: Indicated vs Actual Speed at 30, 50 & 70 mph

Changing from 155/70R13 to 175/60R13 changes the speedometer by -1.28%, because the overall diameter moves from 547.2 mm to 540.2 mm. That is the whole calculation: the speedometer was calibrated for the rolling circumference of the original size, and the new size covers a different distance per revolution, so the dial reads wrong by exactly the diameter percentage. This page works the error out step by step, shows the indicated-versus-actual speed at 30, 50 and 70 mph, and explains what the change does to the odometer, the gearing and the legal position — with the disclaimer below applying throughout.

The direction matters as much as the size. Because 175/60R13 is smaller than 155/70R13, the car travels slower than the speedometer shows, so at an indicated 70 mph the actual speed is 69.1 mph. If the new size were smaller the opposite would be true. The diameter difference is 7.0 mm, which is 1.28% — inside the ±3% guideline commonly used for keeping speedometer error small, and the sections below show the full table, the formula, the odometer effect and when a recalibration is worth doing.

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.

speedometer error 155/70r13 to 175/60r13: How the speedometer error is calculated

The arithmetic is short. First find the percentage change: error % = (new diameter − old diameter) ÷ old diameter × 100. Then find the actual speed: actual speed = indicated speed × (new diameter ÷ old diameter). Both formulas use the nominal new-tyre diameters from the size codes, and the tables in this article use those figures throughout. A positive result means the replacement tyre is larger, so the true speed is higher than the indicated speed and the odometer under-records; a negative result means the replacement is smaller, so the true speed is lower than the indicated speed and the odometer over-records. The same percentage applies at every speed, so one calculation covers the whole speed range. That is the point this page works through for speedometer error 155/70r13 to 175/60r13.

speedometer error 155/70r13 to 175/60r13: What the speedometer actually measures

Speedometer error is a percentage, not a fixed number of miles per hour, and that is the key to understanding it. The dial shows the speed the car would be doing if the original tyres were fitted, and the actual speed is the indicated speed multiplied by the ratio of the new diameter to the old diameter. A small diameter change gives a small error — a 1% change is 0.7 mph at an indicated 70 mph — while a large change gives an error that matters for speed limits and for the odometer. Because the error is proportional, the same table works at any speed once the percentage is known, and the worked examples in this article show how to find that percentage from the two tyre sizes. That is the point this page works through for speedometer error 155/70r13 to 175/60r13.

Worked example: 155/70R13 to 175/60R13

155/70R13 measures 547.2 mm in overall diameter (21.54 in), with a 108.5 mm sidewall, a 1719.1 mm circumference and 936.2 revolutions per mile. 175/60R13 measures 540.2 mm (21.27 in), with a 105.0 mm sidewall, a 1697.1 mm circumference and 948.3 revolutions per mile. The diameter difference is -7.0 mm, and the error % is (540.2 − 547.2) ÷ 547.2 × 100 = -1.28%. The circumference difference is -22.0 mm and the revolutions-per-mile difference is +12.1, which is the same change seen from the other side: more revolutions per mile means less distance per revolution. All of these are nominal new-tyre figures; real tyres vary slightly by brand, model, wear and inflation pressure.

Indicated vs actual speed: 155/70R13 to 175/60R13

Error: -1.28%. Actual speed = indicated speed × (540.2 ÷ 547.2).

Indicated speed Actual speed Difference
30 mph 29.6 mph -0.4 mph
50 mph 49.4 mph -0.6 mph
70 mph 69.1 mph -0.9 mph

At an indicated 70 mph the actual speed is 69.1 mph, a difference of -0.9 mph, and the odometer is out by the same -1.28% — it records +1.28% more miles than the car actually covers. The diameter change of 1.28% is inside the ±3% guideline commonly used for keeping speedometer error small. If the change is outside that band, or if the dial now under-reads, a recalibration is worth considering, and the placard should be checked and fitment confirmed with a tyre professional before fitting a different tyre size.

speedometer error 155/70r13 to 175/60r13: Size comparison table

Measurement 155/70R13 175/60R13
Overall diameter 547.2 mm (21.54 in) 540.2 mm (21.27 in)
Sidewall height 108.5 mm 105.0 mm
Circumference 1719.1 mm 1697.1 mm
Revolutions per mile 936.2 948.3
Rim diameter 13 in 13 in

Common causes of a wrong speedometer for speedometer error 155/70r13 to 175/60r13

On this page, speedometer error 155/70r13 to 175/60r13 is the case in point: Several things can put a speedometer out, and they are worth separating because the fixes differ. The first is tyre diameter: any change from the size the vehicle was calibrated for produces a fixed percentage error, and it is the cause this article is about. The second is wear and pressure: a worn tyre is smaller than a new one and an under-inflated tyre has a smaller effective rolling radius, so both make the dial read slightly high. The third is a previous recalibration — a programmer setting or a speedometer gear fitted for an earlier tyre size that no longer matches what is on the car. The fourth is a fault: a failing wheel-speed sensor, a damaged reluctor ring or, on older cars, a worn cable, which usually shows as a needle that jumps, sticks or reads inconsistently rather than a steady percentage error. A steady percentage error points at calibration; an erratic needle points at a fault.

How to fix a speedometer error for speedometer error 155/70r13 to 175/60r13

For speedometer error 155/70r13 to 175/60r13, the point is this: There are four common ways to correct a speedometer after a tyre change, and the right one depends on the vehicle. A handheld programmer or diagnostic tool can change the programmed tyre size on many modern vehicles, which corrects both the speedometer and the odometer at once. A speedometer drive gear — a small gear in the gearbox — does the same job mechanically on older vehicles, and the replacement gear is chosen from the diameter ratio. An inline calibrator, wired into the speed sensor signal, can scale the signal on vehicles where neither of the above is possible. And a dealer recalibration uses manufacturer software to change the setting where it is locked to franchised tools. Before spending money on any of them, measure the error against GPS at a steady speed so the correction needed is known, and confirm the method suits the vehicle — the wrong tool for the vehicle wastes the money and leaves the dial wrong.

Common mistakes with speedometer error for speedometer error 155/70r13 to 175/60r13

A few errors recur whenever speedometers are discussed. One is assuming the factory calibration was exact — it rarely is, because manufacturers deliberately calibrate dials to read slightly high, and in the UK the law requires that a speedometer never shows less than the true speed. Another is using a phone GPS reading taken while accelerating or cornering, when GPS speed is least reliable; the comparison should be made at a steady speed on a straight, level road. A third is forgetting the odometer: the same percentage error applies to recorded mileage, so a car that has run larger tyres for years has covered more miles than the odometer shows, which matters for service intervals and resale. And the last is assuming the fix is always a recalibration — sometimes the right answer is to return to a tyre size within the ±3% guideline and check the placard first. That is the point this page works through for speedometer error 155/70r13 to 175/60r13.

speedometer error 155/70r13 to 175/60r13: Related pages

Frequently asked questions

What is the speedometer error from 155/70R13 to 175/60R13?

The error is -1.28%, because the diameter moves from 547.2 mm to 540.2 mm. At an indicated 70 mph the actual speed is 69.1 mph, and the odometer is out by the same -1.28%. The change is inside the ±3% guideline commonly used for keeping speedometer error small.

Do I need to recalibrate after changing to 175/60R13?

It depends on the size of the error and its direction. A 1.28% change is inside the ±3% guideline, so many owners would not recalibrate, though the dial now reads high and the difference is worth knowing about. The correct method depends on the vehicle, so check the owner's manual or ask a specialist.

Will changing to 175/60R13 affect my odometer?

Yes, by the same -1.28%. The odometer counts wheel revolutions just as the speedometer does, so it records +1.28% more miles than the car actually covers. Over 10,000 actual miles that is a difference of about 128 miles.

How do I check my speedometer against GPS?

On this page, speedometer error 155/70r13 to 175/60r13 is the case in point: Drive at a steady speed on a straight, level road and compare the dial with a GPS speed reading. GPS is least reliable at low speed, while accelerating or braking, and on hills or in tunnels, so take the reading at a steady cruise. A difference that is the same percentage at several speeds is a calibration error; a needle that jumps or sticks is a fault.

Does tyre wear affect the speedometer?

Yes, slightly. A worn tyre has a smaller diameter than a new one, so the speedometer reads slightly high as the tread wears — typically well under 1% over the life of the tyre. Under-inflation has the same direction of effect by reducing the effective rolling radius. Both are small compared with a size change, but they are real. That is the point this page works through for speedometer error 155/70r13 to 175/60r13.