Well it turns out that the tune-up actually resulted in a big jump in FE. The Winter baseline was 30-31 mpg, with the tune-up tank coming in at 34.4 mpg.
That's about a 10% increase in FE.
Subsequent tanks are required to get a final tally, here are a few hypotheses...
- The plugs and wires were in sad shape.
- The plug gap was increased over spec
- Seafoam was dumped directly over the valvetrain and run through the system for 3 miles, and then...
- The oil/filter was changed
I never got around to changing the air filter. Soon though...
Strangely, ambient temps were in the +5F to 25F range, with some slushy, Winter weather present. I even carried passengers. Most driving was highway. We'll see what happens with subsequent tanks -- it's warmed up out to around 30-60F, so we'll see.
As it stands now, the tune-up seemed to help FE.
Happy Hypermiling...
RH77