Speedometer Error 155/80R13 to 165/70R14: Indicated vs Actual Speed at 30, 50 & 70 mph

Changing from 155/80R13 to 165/70R14 changes the speedometer by +1.45%, because the overall diameter moves from 578.2 mm to 586.6 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 165/70R14 is larger than 155/80R13, the car travels faster than the speedometer shows, so at an indicated 70 mph the actual speed is 71.0 mph. If the new size were smaller the opposite would be true. The diameter difference is 8.4 mm, which is 1.45% — 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 155/80r13 to 165/70r14

Taking speedometer error 155/80r13 to 165/70r14 as the case in point: Two formulas cover everything on this page. Error % = (new diameter − old diameter) ÷ old diameter × 100, and actual speed = indicated speed × (new diameter ÷ old diameter). The diameters are the nominal new-tyre figures computed from the size codes — for example 205/55R16 measures 631.9 mm and 225/45R17 measures 634.3 mm — and real tyres vary slightly by brand, model, wear and inflation pressure, so the error figures are accurate to within a small margin rather than exact to the tenth of a percent. A positive error % means the new tyre is larger and the car travels faster than the dial shows; a negative error % means the new tyre is smaller and the car travels slower than the dial shows. The odometer error is the same percentage in the same direction.

What the speedometer actually measures for speedometer error 155/80r13 to 165/70r14

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. That is the point this page works through for speedometer error 155/80r13 to 165/70r14.

Worked example: 155/80R13 to 165/70R14

155/80R13 measures 578.2 mm in overall diameter (22.76 in), with a 124.0 mm sidewall, a 1816.5 mm circumference and 886.0 revolutions per mile. 165/70R14 measures 586.6 mm (23.09 in), with a 115.5 mm sidewall, a 1842.9 mm circumference and 873.3 revolutions per mile. The diameter difference is +8.4 mm, and the error % is (586.6 − 578.2) ÷ 578.2 × 100 = +1.45%. The circumference difference is +26.4 mm and the revolutions-per-mile difference is -12.7, which is the same change seen from the other side: fewer revolutions per mile means more 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: 155/80R13 to 165/70R14

Error: +1.45%. Actual speed = indicated speed × (586.6 ÷ 578.2).

Indicated speed Actual speed Difference
30 mph 30.4 mph +0.4 mph
50 mph 50.7 mph +0.7 mph
70 mph 71.0 mph +1.0 mph

At an indicated 70 mph the actual speed is 71.0 mph, a difference of +1.0 mph, and the odometer is out by the same +1.45% — it records -1.45% fewer miles than the car actually covers. The diameter change of 1.45% 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.

Size comparison table for speedometer error 155/80r13 to 165/70r14

Measurement 155/80R13 165/70R14
Overall diameter 578.2 mm (22.76 in) 586.6 mm (23.09 in)
Sidewall height 124.0 mm 115.5 mm
Circumference 1816.5 mm 1842.9 mm
Revolutions per mile 886.0 873.3
Rim diameter 13 in 14 in

speedometer error 155/80r13 to 165/70r14: Common causes of a wrong speedometer

Taking speedometer error 155/80r13 to 165/70r14 as the case in point: 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 155/80r13 to 165/70r14: How to fix a speedometer error

The fix is chosen by vehicle, not by preference. Modern vehicles with electronic speedometers are usually corrected by reprogramming the tyre-size setting with a diagnostic tool or a handheld programmer, which adjusts the speedometer and the odometer together. Older vehicles with a cable-driven speedometer need a different speedometer drive gear, and the new gear is selected using the diameter ratio — new teeth = old teeth × (old diameter ÷ new diameter) — so the gear compensates exactly for the size change. Where neither is available, an inline signal calibrator fitted to the speed sensor wiring can scale the reading. A dealer may be needed where the setting is locked behind manufacturer software. In every case, measure the error against GPS before and after the fix, at a steady speed on a straight road, to confirm the dial is right. That is the point this page works through for speedometer error 155/80r13 to 165/70r14.

Common mistakes with speedometer error for speedometer error 155/80r13 to 165/70r14

Taking speedometer error 155/80r13 to 165/70r14 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.

Related pages for speedometer error 155/80r13 to 165/70r14

Frequently asked questions

What is the speedometer error from 155/80R13 to 165/70R14?

The error is +1.45%, because the diameter moves from 578.2 mm to 586.6 mm. At an indicated 70 mph the actual speed is 71.0 mph, and the odometer is out by the same +1.45%. The change is inside the ±3% guideline commonly used for keeping speedometer error small.

Do I need to recalibrate after changing to 165/70R14?

It depends on the size of the error and its direction. A 1.45% change is inside the ±3% guideline, so many owners would not recalibrate, though the dial now reads low 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 165/70R14 affect my odometer?

Yes, by the same +1.45%. The odometer counts wheel revolutions just as the speedometer does, so it records -1.45% fewer miles than the car actually covers. Over 10,000 actual miles that is a difference of about 145 miles.

How do I check my speedometer against GPS?

For speedometer error 155/80r13 to 165/70r14, the point is this: 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 155/80r13 to 165/70r14.