Because the fuel prices in Europe are very high I would like to use the 4th gear of a 518 and in this cae a 46RE.
From personal experience, don't expect the A518 to improve your fuel-economy. I didn't gain anything going from a 904 & 7-1/4" combo to a 518 & 8-3/4".
It does however make the car more fun to drive because of the added transmission gear, which lets you install a deeper rear-axle gear and still maintain normal highway-rpms.
