Speedometer Error 145/80R13 to 155/70R14: Indicated vs Actual Speed at 30, 50 & 70 mph

Changing from 145/80R13 to 155/70R14 changes the speedometer by +1.85%, because the overall diameter moves from 562.2 mm to 572.6 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 155/70R14 is larger than 145/80R13, the car travels faster than the speedometer shows, so at an indicated 70 mph the actual speed is 71.3 mph. If the new size were smaller the opposite would be true. The diameter difference is 10.4 mm, which is 1.85% — 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 145/80r13 to 155/70r14

On this page, speedometer error 145/80r13 to 155/70r14 is the case in point: 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 145/80r13 to 155/70r14

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 145/80r13 to 155/70r14.

Worked example: 145/80R13 to 155/70R14

145/80R13 measures 562.2 mm in overall diameter (22.13 in), with a 116.0 mm sidewall, a 1766.2 mm circumference and 911.2 revolutions per mile. 155/70R14 measures 572.6 mm (22.54 in), with a 108.5 mm sidewall, a 1798.9 mm circumference and 894.6 revolutions per mile. The diameter difference is +10.4 mm, and the error % is (572.6 − 562.2) ÷ 562.2 × 100 = +1.85%. The circumference difference is +32.7 mm and the revolutions-per-mile difference is -16.6, 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: 145/80R13 to 155/70R14

Error: +1.85%. Actual speed = indicated speed × (572.6 ÷ 562.2).

Indicated speed Actual speed Difference
30 mph 30.6 mph +0.6 mph
50 mph 50.9 mph +0.9 mph
70 mph 71.3 mph +1.3 mph

At an indicated 70 mph the actual speed is 71.3 mph, a difference of +1.3 mph, and the odometer is out by the same +1.85% — it records -1.85% fewer miles than the car actually covers. The diameter change of 1.85% 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 145/80r13 to 155/70r14

Measurement 145/80R13 155/70R14
Overall diameter 562.2 mm (22.13 in) 572.6 mm (22.54 in)
Sidewall height 116.0 mm 108.5 mm
Circumference 1766.2 mm 1798.9 mm
Revolutions per mile 911.2 894.6
Rim diameter 13 in 14 in

Common causes of a wrong speedometer for speedometer error 145/80r13 to 155/70r14

For speedometer error 145/80r13 to 155/70r14, the point is this: Tyre size is the most common cause of a speedometer error, but it is not the only one. Worn tyres have a smaller diameter than new ones, so a speedometer calibrated for new tyres reads slightly high as the tread wears — the effect is small, typically well under 1%, but it is real. Under-inflation reduces the effective rolling radius and has the same direction of effect, while over-inflation increases it. A different final-drive ratio does not change the speedometer reading itself, because the sensor measures wheel speed rather than engine speed, but a speedometer drive gear change or a recalibration done for a previous tyre size will leave the dial wrong for the tyres now fitted. And on older vehicles a worn speedometer cable or a failing sensor can make the needle wander or stick, which is a fault rather than a calibration error and needs repair rather than recalibration.

speedometer error 145/80r13 to 155/70r14: How to fix a speedometer error

Taking speedometer error 145/80r13 to 155/70r14 as the case in point: 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.

Common mistakes with speedometer error for speedometer error 145/80r13 to 155/70r14

Mistakes usually start with the wrong reference. The first is comparing the dial with another vehicle's dial — both can be out by the same amount. GPS speed, measured at a steady speed on a straight road, is the practical reference, though it has its own limits at low speed and on hills. The second mistake is treating the error as a fixed mph figure: a 3% error is 0.9 mph at 30 mph and 2.1 mph at 70 mph, so a single-speed check does not describe the whole picture. The third is forgetting that most speedometers already read slightly high from the factory, so the error after a tyre change is measured from that baseline, not from zero. And the fourth is the dangerous one: fitting larger tyres makes the dial read low, so the car is going faster than the driver believes, and driving to the indicated limit in that condition means exceeding it. That is the point this page works through for speedometer error 145/80r13 to 155/70r14.

speedometer error 145/80r13 to 155/70r14: Related pages

Frequently asked questions

What is the speedometer error from 145/80R13 to 155/70R14?

The error is +1.85%, because the diameter moves from 562.2 mm to 572.6 mm. At an indicated 70 mph the actual speed is 71.3 mph, and the odometer is out by the same +1.85%. The change is inside the ±3% guideline commonly used for keeping speedometer error small.

Do I need to recalibrate after changing to 155/70R14?

It depends on the size of the error and its direction. A 1.85% 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 155/70R14 affect my odometer?

Yes, by the same +1.85%. The odometer counts wheel revolutions just as the speedometer does, so it records -1.85% fewer miles than the car actually covers. Over 10,000 actual miles that is a difference of about 185 miles.

How do I check my speedometer against GPS?

Taking speedometer error 145/80r13 to 155/70r14 as 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?

Taking speedometer error 145/80r13 to 155/70r14 as the case in point: 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.