Speedometer Error 225/45R17 to 235/40R18: Indicated vs Actual Speed at 30, 50 & 70 mph

Changing from 225/45R17 to 235/40R18 changes the speedometer by +1.72%, because the overall diameter moves from 634.3 mm to 645.2 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 235/40R18 is larger than 225/45R17, the car travels faster than the speedometer shows, so at an indicated 70 mph the actual speed is 71.2 mph. If the new size were smaller the opposite would be true. The diameter difference is 10.9 mm, which is 1.72% — 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 225/45r17 to 235/40r18: How the speedometer error is calculated

For speedometer error 225/45r17 to 235/40r18, the point is this: Work it out in two steps. Step one, the percentage: error % = (new diameter − old diameter) ÷ old diameter × 100, using the nominal new-tyre diameters computed from the size codes. Step two, the actual speed at any indicated speed: actual speed = indicated speed × (new diameter ÷ old diameter). A tyre that is 2% larger in diameter gives an actual speed 2% above the indicated speed — 71.4 mph actual at an indicated 70 mph — and a tyre 2% smaller gives 68.6 mph actual at the same indicated speed. The odometer follows the same percentage, so the mileage it records is also out by that amount. All the tables below are built from those two formulas with the diameters stated.

speedometer error 225/45r17 to 235/40r18: What the speedometer actually measures

For speedometer error 225/45r17 to 235/40r18, the point is this: 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.

Worked example: 225/45R17 to 235/40R18

225/45R17 measures 634.3 mm in overall diameter (24.97 in), with a 101.2 mm sidewall, a 1992.7 mm circumference and 807.6 revolutions per mile. 235/40R18 measures 645.2 mm (25.40 in), with a 94.0 mm sidewall, a 2027.0 mm circumference and 794.0 revolutions per mile. The diameter difference is +10.9 mm, and the error % is (645.2 − 634.3) ÷ 634.3 × 100 = +1.72%. The circumference difference is +34.3 mm and the revolutions-per-mile difference is -13.6, 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: 225/45R17 to 235/40R18

Error: +1.72%. Actual speed = indicated speed × (645.2 ÷ 634.3).

Indicated speed Actual speed Difference
30 mph 30.5 mph +0.5 mph
50 mph 50.9 mph +0.9 mph
70 mph 71.2 mph +1.2 mph

At an indicated 70 mph the actual speed is 71.2 mph, a difference of +1.2 mph, and the odometer is out by the same +1.72% — it records -1.72% fewer miles than the car actually covers. The diameter change of 1.72% 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 225/45r17 to 235/40r18

Measurement 225/45R17 235/40R18
Overall diameter 634.3 mm (24.97 in) 645.2 mm (25.40 in)
Sidewall height 101.2 mm 94.0 mm
Circumference 1992.7 mm 2027.0 mm
Revolutions per mile 807.6 794.0
Rim diameter 17 in 18 in

speedometer error 225/45r17 to 235/40r18: Common causes of a wrong speedometer

On this page, speedometer error 225/45r17 to 235/40r18 is the case in point: 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.

How to fix a speedometer error for speedometer error 225/45r17 to 235/40r18

Taking speedometer error 225/45r17 to 235/40r18 as the case in point: 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.

speedometer error 225/45r17 to 235/40r18: Common mistakes with speedometer error

For speedometer error 225/45r17 to 235/40r18, the point is this: 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 225/45r17 to 235/40r18: Related pages

Frequently asked questions

What is the speedometer error from 225/45R17 to 235/40R18?

The error is +1.72%, because the diameter moves from 634.3 mm to 645.2 mm. At an indicated 70 mph the actual speed is 71.2 mph, and the odometer is out by the same +1.72%. The change is inside the ±3% guideline commonly used for keeping speedometer error small.

Do I need to recalibrate after changing to 235/40R18?

It depends on the size of the error and its direction. A 1.72% 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 235/40R18 affect my odometer?

Yes, by the same +1.72%. The odometer counts wheel revolutions just as the speedometer does, so it records -1.72% fewer miles than the car actually covers. Over 10,000 actual miles that is a difference of about 172 miles.

How do I check my speedometer against GPS?

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. That is the point this page works through for speedometer error 225/45r17 to 235/40r18.

Does tyre wear affect the speedometer?

For speedometer error 225/45r17 to 235/40r18, the point is this: 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.