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Ditto. I did this several times on two Volvos- our 89, 240 and 91, 70. Both cars will give 29 - 30+ mpg. I found the following to be contributors for and against mileage issues, including all of the above: I also found, to my surprise that when I wasn't disconnecting the battery as much, and allowed the ECU's to learn my driving habits, or whatever it is they do, that the mileage increased unexpectedly.
Use same gasoline
Fill it completely and consistently for accuracy
State of tune: Cap, Rotor, Air Filter, Plugs
Air conditioning on or off
Tire pressure
Dirty Fuel System - Use a Cleaner (Techron, Regaine, BG44K, Slick 50 w/Polyetheramine)
High or low Carbon deposits - Run with Overdrive off to clean off carbon
Thermostat
PCV hoses and/or flame trap
Vacuum leaks
Condition of O2 and Coolant sensors
Air filter thermostat
Timing
Cat Converter
Valves adjustment
Injectors condition
Fuel Pressure Regulator
Coil output
Belt tension
Dragging brakes
Front end alignment
diyer
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