This may be a good case to point out an idea of mine.
As I See it, there are three categories of things that affect our results.
1. The Vehicle Condition.
2. Driving Conditions.
3. Driving Style.
Everything we Gadgetmen do, the Groove, all other mods, vehicle age/ maintenance, fall in #1.
Driving conditions,#2, that's the roads, traffic, weather, temperature, wind, altitude, baro. pressure. Everything about the environment being driven IN.
#3 Driving style- HOW it's driven.
We, and our Customers, can alter and learn how to change #1 and #3. #2 is a lot harder. Basically you can decide whether to drive here and now, or not. You could change the route used, and wait for better weather, etc. But, of course we usually don't do that. We want/need to go wherever, now.
Realize that today is January 11th. Middle of winter. Cold, rain, snow, ice. Often slower speeds used. Water in any form on the road increases rolling resistance. Cold temps reduce MPG. Wind adds to vehicle/engine loads.
When I did the MPG test drive in Subie 4 1/2 years ago that resulted in a 72 MPG result--that was a calm summer evening on a flat hiway with very little traffic and no wind. Test track like conditions.
Remember, we are striving for gains in efficiency, and we usually get them. Realize also, that we have to accept #2, conditions, as they are. Another idea re: MPG results is: you NEVER take the same trip twice.
So, Michael, keep this all in mind as you go forward.
I'll add this: use a scantool or equivalent, check LT/ST trims, O2 voltages, in real time while it's running/driving. Have the owner drive as you watch the readout. Watch to see that the upstream O2 V switches hi/low fast enough, and that downstream O2(s) V stays in the right range. Greg's right- tire press drops in winter, needs checked w/ cold tires. How many miles on this truck? Are the O2's all original? Overall engine/vehicle condition and maintenance?
EVERYTHING matters. Stay on Target, Michael. Doin' good, Buddy!
Tracy G