Speedometer Error 155/70R13 to 165/65R13: Indicated vs Actual Speed at 30, 50 & 70 mph

Changing from 155/70R13 to 165/65R13 changes the speedometer by -0.46%, because the overall diameter moves from 547.2 mm to 544.7 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 165/65R13 is smaller than 155/70R13, the car travels slower than the speedometer shows, so at an indicated 70 mph the actual speed is 69.7 mph. If the new size were smaller the opposite would be true. The diameter difference is 2.5 mm, which is 0.46% — 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.

How the speedometer error is calculated for speedometer error 155/70r13 to 165/65r13

For speedometer error 155/70r13 to 165/65r13, the point is this: 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.

What the speedometer actually measures for speedometer error 155/70r13 to 165/65r13

For speedometer error 155/70r13 to 165/65r13, the point is this: The speedometer does not measure speed directly. It counts wheel revolutions — from a sensor on the gearbox output, a wheel-speed sensor or the ABS system, depending on the vehicle — and multiplies by the rolling circumference the manufacturer programmed in. That circumference belongs to the original tyre size, so fitting a different diameter changes the distance covered per revolution while the speedometer keeps using the old figure. The result is a fixed percentage error: if the new tyre is 2% larger in diameter, the actual speed is 2% higher than the indicated speed at every speed, and the odometer under-records by the same 2%. The error does not grow or shrink with speed; it is the same percentage at 30 mph and at 70 mph, which is what the tables below show.

Worked example: 155/70R13 to 165/65R13

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. 165/65R13 measures 544.7 mm (21.44 in), with a 107.2 mm sidewall, a 1711.2 mm circumference and 940.5 revolutions per mile. The diameter difference is -2.5 mm, and the error % is (544.7 − 547.2) ÷ 547.2 × 100 = -0.46%. The circumference difference is -7.9 mm and the revolutions-per-mile difference is +4.3, 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 165/65R13

Error: -0.46%. Actual speed = indicated speed × (544.7 ÷ 547.2).

Indicated speed Actual speed Difference
30 mph 29.9 mph -0.1 mph
50 mph 49.8 mph -0.2 mph
70 mph 69.7 mph -0.3 mph

At an indicated 70 mph the actual speed is 69.7 mph, a difference of -0.3 mph, and the odometer is out by the same -0.46% — it records +0.46% more miles than the car actually covers. The diameter change of 0.46% 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 165/65r13: Size comparison table

Measurement 155/70R13 165/65R13
Overall diameter 547.2 mm (21.54 in) 544.7 mm (21.44 in)
Sidewall height 108.5 mm 107.2 mm
Circumference 1719.1 mm 1711.2 mm
Revolutions per mile 936.2 940.5
Rim diameter 13 in 13 in

Common causes of a wrong speedometer for speedometer error 155/70r13 to 165/65r13

For speedometer error 155/70r13 to 165/65r13, the point is this: The causes divide into calibration and faults, and the distinction decides the fix. Calibration causes are tyre diameter changes, wear and inflation pressure: all of them change the distance covered per wheel revolution, and all of them produce a steady percentage error that the formulas on this page can predict. Fault causes are a failing wheel-speed sensor, a damaged reluctor ring, a worn speedometer cable on older vehicles, or a fault in the instrument cluster itself — these usually make the reading jump, stick or drop to zero rather than sit a steady percentage out. If the error is the same percentage at every speed, it is calibration; if it changes with speed or the needle behaves erratically, it is a fault and the vehicle needs diagnosis rather than a recalibration.

speedometer error 155/70r13 to 165/65r13: How to fix a speedometer error

On this page, speedometer error 155/70r13 to 165/65r13 is the case in point: Fixing a speedometer error means telling the vehicle the new tyre diameter, and the method depends on the vehicle. On many modern cars a handheld programmer or a diagnostic tool can rewrite the tyre-size setting in the body or engine control module, and the speedometer and odometer then read correctly for the new size. On older vehicles with a mechanical speedometer, the fix is a different speedometer drive gear in the gearbox, chosen so the gear ratio compensates for the diameter change. Some vehicles need a dealer visit because the setting is locked behind manufacturer software, and some need an inline speedometer calibrator fitted in the sensor wiring. Whichever method is used, the correct one depends on the vehicle, so the owner's manual or a specialist should be consulted before buying a tool — and the error should be measured against GPS before and after, at a steady speed, to confirm the fix worked.

speedometer error 155/70r13 to 165/65r13: Common mistakes with speedometer error

Taking speedometer error 155/70r13 to 165/65r13 as the case in point: The most common mistake is assuming the speedometer was accurate before the tyre change. Most speedometers are deliberately calibrated to read slightly high — in the UK the law requires that a speedometer never under-reads, and it may over-read by up to 10% plus 4 km/h — so a car that read 2% high on the original tyres and now reads 2% low on larger tyres has actually changed by 4% from what the driver was used to. The second mistake is checking the speedometer against another car's speedometer instead of against GPS: two dials can share the same error. The third is testing at one speed only and assuming the error is a fixed number of mph — it is a percentage, so the mph error grows with speed. And the fourth is treating a calculation as a reason to ignore the limit: if the dial reads low, the car is travelling faster than the driver thinks, and that is the direction that risks a speeding offence.

speedometer error 155/70r13 to 165/65r13: Related pages

Frequently asked questions

What is the speedometer error from 155/70R13 to 165/65R13?

The error is -0.46%, because the diameter moves from 547.2 mm to 544.7 mm. At an indicated 70 mph the actual speed is 69.7 mph, and the odometer is out by the same -0.46%. The change is inside the ±3% guideline commonly used for keeping speedometer error small.

Do I need to recalibrate after changing to 165/65R13?

It depends on the size of the error and its direction. A 0.46% 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 165/65R13 affect my odometer?

Yes, by the same -0.46%. The odometer counts wheel revolutions just as the speedometer does, so it records +0.46% more miles than the car actually covers. Over 10,000 actual miles that is a difference of about 46 miles.

How do I check my speedometer against GPS?

On this page, speedometer error 155/70r13 to 165/65r13 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 165/65r13.