Tractor tyre rolling circumference
Rolling circumference is the distance a tyre travels in one full revolution under load. It is the key figure when front and rear tyres must match on a four-wheel-drive tractor, because it determines the front axle lead. It also affects the speedometer and slip calculations.
Why it is smaller than the circumference
Under load the tyre deflects where it meets the ground, so the distance from hub to ground, the rolling radius, is less than half the overall diameter. Examples from Michelin's databook:
| Tyre | Overall diameter | π × diameter | Rolling circumference | Ratio |
|---|---|---|---|---|
| 650/65R42 Multibib | 1,931 mm | 6,066 mm | 5,722 mm | 94 % |
| 540/65R30 Multibib | 1,472 mm | 4,624 mm | 4,353 mm | 94 % |
Calculating lead with π × diameter gives values about 6 % too large, and the error is not necessarily equal front and rear.
SRI – static rolling radius
Many databooks give SRI (static rolling radius, sometimes speed radius index), a radius in millimetres:
Rolling circumference ≈ 2 × π × SRI
Conversely, 5,722 mm corresponds to a radius of 5,722 ÷ (2 × π) ≈ 911 mm. The catalogue on Däcktryck.se shows SRI where the manufacturer gives it.
Where the values on Däcktryck.se come from
| Source shown | Meaning |
|---|---|
| Manufacturer | Taken directly from the manufacturer's databook |
| STRO/ETRTO | Calculated from SRI using the standards organisation's values |
| Size catalogue | Median for the size in the catalogue, when the manufacturer's own value is missing |
Same size, different rolling circumference
| 650/65R42 | Rolling circumference |
|---|---|
| Michelin Multibib | 5,722 mm |
| Michelin RoadBib | 5,733 mm |
| Michelin Multibib Plus | 5,752 mm |
| BKT Agrimax RT 657 | 5,810 mm |
Multibib and BKT differ by 88 mm, about 1.5 %. On a Case IH Puma 215 that is enough to move lead from 0.72 % to −0.80 %.
What changes rolling circumference?
- Wear – large agricultural tyres have several centimetres of tread, and as it wears the radius shrinks.
- Inflation pressure – lower pressure means more deflection and a shorter distance per revolution.
- Load – a higher load has the same effect.
- Surface – on soft ground the tyre sinks, and slip also affects the measured distance.
Measure it yourself – step by step
- Set working pressure per the manufacturer's table, ideally with the tractor's normal load.
- Choose a flat, firm surface such as tarmac or concrete.
- Mark the tyre at the bottom with chalk or tape, and mark the ground at the same point.
- Drive straight ahead slowly without slip for exactly 5 or 10 full revolutions.
- Mark the ground again and measure the distance with a tape.
- Divide the distance by the number of revolutions.
- Repeat front and rear, ideally twice, and use the average.
Example (invented figures): 10 rear revolutions measured 57.05 m, so 5,705 mm per revolution. 10 front revolutions measured 43.40 m, so 4,340 mm. With a gear ratio of 1.324, lead is (4,340 × 1.324 ÷ 5,705 − 1) × 100 ≈ 0.72 %.
Common mistakes
- Using π × diameter instead of rolling circumference.
- Measuring on grass or soft ground.
- Measuring at the wrong pressure, for example with tyres hot after long road runs.
- Measuring only one revolution, which makes marking errors large in percentage terms.
Rolling circumference and wheel slip
Rolling circumference is also needed to calculate wheel slip, how much the wheels turn without the tractor moving the corresponding distance:
Slip (%) = (1 − actual distance ÷ (revolutions × rolling circumference)) × 100
Example (invented figures): rear wheels with a 5,705 mm rolling circumference make 10 revolutions while ploughing. That is 57.05 m without slip, but the tractor only moves 50.2 m. Slip is (1 − 50.2 ÷ 57.05) × 100 ≈ 12 %.
For heavy draft work about 10–15 % slip is commonly quoted as reasonable. Much higher slip suggests too little weight or too high a pressure, and much lower slip can mean unnecessary ballast. See axle load and ballast.
Rolling circumference for common sizes
| Size | Smallest | Largest | Difference |
|---|---|---|---|
| 480/70R28 | 4,155 mm | 4,374 mm | 5.3 % |
| 540/65R28 | 4,145 mm | 4,276 mm | 3.2 % |
| 540/65R30 | 4,317 mm | 4,483 mm | 3.8 % |
| 600/70R30 | 4,630 mm | 4,867 mm | 5.1 % |
| 650/65R42 | 5,722 mm | 5,911 mm | 3.3 % |
| 710/70R42 | 6,025 mm | 6,288 mm | 4.4 % |
Differences come from tread pattern, construction and whether the tyre is standard, IF or VF. See for example Nokian, Trelleborg and Continental.
Related questions
Is rolling circumference the same as circumference?
No. Circumference is π × diameter of the unloaded tyre; rolling circumference is the distance under load, about 6 % less in the examples above.
How much does rolling circumference drop with wear?
It depends on tread depth. Measure before and after a season. The table in the lead guide shows the effect of a 2 % reduction.
Why is the speedometer wrong after a tyre change?
The speedometer often counts wheel revolutions with a set rolling circumference. Change the circumference and the reading changes.
Worked example: theoretical circumference vs databook
Trelleborg TM900 ProgressiveTraction 710/70R42:
- Rim: 42 × 25.4 = 1,067 mm.
- Sidewall: 710 × 0.70 = 497 mm.
- Outside diameter: 1,067 + 2 × 497 = 2,061 mm.
- Circumference: 2,061 × π ≈ 6,475 mm.
- Databook rolling circumference: 6,190 mm, about 4.4 % less.
That is why you should not calculate lead from diameter × π. The difference is not necessarily the same for other tyres, since deflection varies.
Step by step: measure it yourself
- Inflate to your working pressure with normal load.
- Mark the tyre and the ground.
- Drive straight on firm, level ground, with 4WD disengaged if possible, for at least 5 revolutions.
- Measure the distance and divide by revolutions.
- Repeat and use the average.
Troubleshooting: measured value differs from the databook
- Measured value lower: worn tyres, lower pressure or higher load than the reference.
- Measured value higher: higher pressure or lower load. Also check the revolution count.
- Large left/right difference: different pressures or uneven wear. Check pressures.
710/70R42 from different brands
| Tyre | Rolling circumference |
|---|---|
| Michelin RoadBib 173D/170E | 6,025 mm |
| CEAT Farmax R70 | 6,119 mm |
| Vredestein Traxion XXL | 6,140 mm |
| Mitas SFT | 6,150 mm |
| BKT Agrimax RT 765 | 6,174 mm |
| Trelleborg TM900 ProgressiveTraction | 6,190 mm |
| Continental TractorMaster | 6,191 mm |
| Michelin MachXBib | 6,195 mm |
| Nokian Ground King | 6,245 mm |
| Firestone Maxi Traction | 6,288 mm |
Even one brand can differ: two versions of Michelin RoadBib differ by 152 mm. Pick exactly the right tyre in the app, including load index.
How to do it in the app
Find the tyre in the Catalog to see its rolling circumference, SRI and source. If you have measured your own values, enter them in the Lead tab. To find what rolling circumference you need, use Theoretical lead.
Open Catalog in the appFrequently asked questions
What is rolling circumference?
The distance the tyre travels in one revolution under load. It is smaller than the geometric circumference because the tyre deflects.
How do I get rolling circumference from SRI?
Rolling circumference ≈ 2 × π × SRI. An SRI of 911 mm gives about 5,722 mm.
How do I measure rolling circumference?
Mark the tyre and the ground, drive 5–10 full revolutions on firm level ground without slip, measure the distance and divide by the number of revolutions.
How much does rolling circumference vary within one size?
In our catalogue 3–5 % depending on size, for example 5,722–5,911 mm for 650/65R42.
How do I calculate wheel slip?
Slip (%) = (1 − actual distance ÷ (revolutions × rolling circumference)) × 100.
Last updated 2026-10-15. General information. See disclaimer.