need help re 400E

[blk300E]

I have a few questions and have not found an answer on this site or from a couple of local techs.

History: Owned a 92 400E for awhile but began to have transmission & rust problems when I decided to replace it. I owned the replacement vehicle (92 400E) for two months now and the performance of the 2nd vehicle is nothing like the 1st. During the 2 months I replaced cap, rotor, plugs, wires, throttle cable, kick down switch, fuel filter and @ least 4 oil changes.

1. What could be the cause for veh 2 to not perform as well as veh 1? By perform I mean acceleration. Veh 1 would pin you to your seat while veh 2 cruises up to speed. FYI: both vehs had ASR. Shifts in veh 2 are smooth and there is no discoloration in transmission fluid. Premium fuel only is used from Chevron, Exxon or Shell - 93 octane.

2. Why is performance slightly better when using Shell gasoline? However, it still does not compare to veh 1.

3. Gas mileage in veh 2 decreased from 300+ miles per tank to 200 per tank immediately after replacing the fuel filter. The only work performed after the fuel filter was throttle cable & oil change. Mechanic found no codes and states that the O2 sensor was working properly.

Thanks in advance for your help.