I have done some testing with E38 economy tuning. The general consensus is to do the following for best economy.
1. Lean afrs anywhere from 15.5:1 to low 16's from 1500-2500rpm less then 60kpa.
2. A higher amount of ign timing at lower cylinder pressures
3. Quality fuel
4. Of course a steady right foot.
My car weighs approximately 1750kg with driver. Its modifications are exhaust, headers, 218 224 high lift camshaft and increased compression ratio.
On a 100km+ trip of open road driving at 100km/h in cruise control @1500rpm the car acheived 9.5L/100km running in open loop mafless with an afr of 14.8:1 and 40 degrees of ignition timing. The second half of the trip sore the economy creep to 9.9L/100km as I turned the a/c on. As I drove through the built up area it increased to low 10s then quickly returned to high 9's.
I applied a change to the VVE which leaned the mixtures to 15.7-15.8:1.
On the trip home a/c off economy dropped to 9.2L/100km at the lowest, generally it stayed at 9.3l/100km. Any slight increase in hill or head wind and the car felt a substantial drop in torque and hence the economy would increase to high 9's/low 10's instantly, going back on flat road or slight dip it would go to low 9s but generally stayed in the high 9s low 10s. With the a/c on usage increased to 10.5L/100km/h.
It appeared that the engine had lost torque with the leaner mixtures and hence the economy suffered. If I had a lighter car it would obviously gain from leaner settings but not in the heavy weight VE ute.
I flashed the previous open loop tune in that maintained slightly above stoich mixtures and the cars vacuum was lower in similiar situations then the leaner settings, economy stayed solid at 9.5L/100kms slight gradients didn't bother it. With a/c on fuel usage was some .6L/100km lower with stoich mixtures.
Main thing I learnt from this lesson was open loop tuning/ mafless is good for economy and if the VVE is correct afrs are stable.It eliminates switching of O2s which can increase fuel usage slightly. Using stoich afrs increases torque and makes the engine more efficient at cruising speed.
