As far as MPG, I rely on the Odometer in my '71 GMC Jimmy [K/5]. [350V8/SM465/NP205/3.73 posi rear].
The gas gauge is out of order, and replacing the in-tank sender is more than I want to deal with right now, so I keep a logbook and record my fill-ups and trip-run miles. I figure on an arbitrary 10 MPG, so I try to top off the 20 gallon tank every 100 miles or so.
The fuel tank is covered by a steel skid plate with about 40 bolts holding it on, and additionally there's a big Class IV hitch bolted on under that. I've thought of cutting a patch out of the floor panel on the back deck and bolting a repair plate over it --for access-- but plunge-cutting a grinder thru the floor on top of a stock fuel tank is its own kinda thrill.
Also the speedo and ODO read slow with 33x12.50x15LT tires. I use a GPS for speedometer sometimes. Or I just drive 43 indicated in a 50 MPH posted zone.
I have spare Borg Warner compensator widget, but it needs a pro speedo shop to correct it.
