Racking layout is usually fixed before the trucks are chosen. That order of operations quietly sets a ceiling on storage density for the life of the building.
The arithmetic
In a 60 m deep building with 12 aisles, reducing working aisle width by 400 mm frees roughly 4.8 m of floor across the layout — often enough for an additional racking run once the cross aisle is re-planned.
What narrows an aisle
Turning radius, chassis length ahead of the fork face, and how much clearance the operator actually needs to work confidently rather than the minimum on the datasheet. A truck that fits in theory but forces three-point turns will not deliver its cycle times.
Test with the real pallet
Aisle figures assume a standard pallet at a standard load centre. If your unit load overhangs, or you handle a mix of pallet types, the working figure moves. Run the trial with the loads you actually handle.