Rolling circumference & speedometer error

The first field is the size currently on the vehicle. That alone gives you four figures: overall diameter, free circumference, an estimated rolling circumference, and revolutions per kilometre.

The second field is optional and allows you to compare a proposed replacement size. Put in the size you are thinking of moving to and it returns the difference in overall diameter as a percentage, then converts that into speedometer error at a reference speed of 56 km/h — roughly 35 mph. Anything beyond ±3% is flagged, because that is the tolerance most operators and most calibration work treat as the practical limit.

Both metric and imperial sizes work, so you can compare 315/80R22.5 against 315/70R22.5, or 12.4R28 against 320/85R28.

Start with 315/80R22.5 on its own:

• Overall diameter 1076 mm
• Free circumference 3379 mm
• Rolling circumference, estimated: 3261 mm
• Revolutions per kilometre: 307

Now add 315/70R22.5 as the second size — a common move when a fleet wants a lower ride height. Overall diameter drops to 1013 mm, a difference of −5.9%. A speedometer calibrated for the 80-series would read 56 km/h while the vehicle was actually doing 52.7 km/h. The calculator flags that as outside the ±3% window, and it is right to: the speedometer would over-read by nearly six per cent, every recorded distance would be six per cent long, and the tachograph would need recalibrating before the change.

Compare 295/80R22.5 instead and the picture changes. Diameter 1044 mm, a difference of −3.0%, reported as inside tolerance. Worth knowing why: the percentage is displayed rounded to one decimal place but the ±3% test runs on the unrounded value, which here is 2.98%. A result that reads exactly −3.0% can fall either side of the line, so treat that boundary as a prompt to check the manufacturer’s real figures rather than as a verdict.

Two approximations stack up here, and both matter.

The diameter is nominal. It is calculated from the size marking — rim diameter plus twice the sidewall height — not taken from a manufacturer’s data sheet. Imperial sizes have no aspect ratio on the sidewall, so 92% is assumed for those, which makes an imperial comparison rougher still.

The rolling circumference is a rule of thumb. The calculator takes 96.5% of the free circumference. A loaded tyre deflects, so the distance it actually covers in one revolution is less than π times its unloaded diameter. That 3.5% deduction is a reasonable average, but the real figure moves with load, inflation pressure, construction, tread pattern, tread depth and speed. A lightly loaded tyre at high pressure sits closer to its free circumference; a heavily loaded one on a soft agricultural pressure sits further from it.

Tread wear alone is enough to swamp the precision on offer. A truck steer tyre worn from around 15 mm of tread to the 3 mm removal point loses roughly 12 mm of radius, which is about 24 mm of diameter and a little over 2% of circumference — most of the ±3% window, from wear on one tyre.

Manufacturers publish a measured rolling circumference for every pattern, determined under a standard load and pressure. Use the manufacturer’s published figure when confirming a fitment.

It is not a tachograph calculation. The w factor for a vehicle is determined on the vehicle, with the tyres that are actually fitted, by an approved calibration centre. Changing tyre size means recalibration. Nothing on this page substitutes for that, and the estimate here is not accurate enough to try.

It is not precise enough for lead-lag on a four-wheel-drive tractor. Mechanical front wheel drive relies on the front tyres turning slightly faster than the rears — the manufacturer specifies a narrow window for the ratio of front to rear rolling circumference. Getting that wrong scrubs the front tyres or winds up the transmission. That job needs the published rolling circumference index figures for both fitments, not a 96.5% estimate.

The result does not confirm that a size change is suitable. Diameter is one variable among several. A different size will also change load index, speed symbol, section width, required rim width, arch and mudguard clearance, and on twins the gap between the pair. Check all of them.

Finally, the speedometer comparison assumes the instrument was accurate for the original size in the first place, and that error scales exactly with the diameter ratio. On a vehicle that has already had a size change at some point in its life, that is not a safe assumption.