Firstly you need to read and reflect on all the issues mentioned above in respect of counterbalance forklifts, and take heed of advice about getting a rerating from the manufacturer because ratings are complex.

For a reach truck you need to measure the load distance L from the outrigger wheel centres to the rated load centre with the forks fully extended - the overturning moment related to the load is then the rated load by L.

Then determine the new distance L distance = L1 at 30 or 36 inches load centre.

The adjusted load is then the RL * L / L1.

As an example a Clark NPR 22 is a single stroke reach truck and has a rated load of 2000 kg at 24 inches. The distance from the load centre to the centre of the outrigger wheels when the forks are fully extended is 28.6 in.

For a 30 in load centre that distance becomes 34.6 ins and the rated load = 2000*28.6/34.6 ~= 1650 kg.

And for a 36 in load centre that distance becomes 40.6 ins and the rated load = 2000*28.6/40.6 ~= 1400 kg.

Similarly for a Clark NPR 15 D, a double stroke reach truck (and because of the greater fork extension has a lower rating even though the reach truck is heavier) has a rated load of 1350 kg at 24 inches. The distance from the load centre to the centre of the outrigger wheels when the forks are fully extended is 50.67 in.

For a 30 in load centre that distance becomes 56.67 ins and the rated load = 1350*50.67/56.67 ~= 1200 kg.

And for a 36 in load centre that distance becomes 62.67 ins and the rated load = 1350*50.67/62.67 ~= 1090 kg.