Always been a fan of autometer gauges.
I assume the car is mainly stock? Why not a vacuum gauge? (Out of room, right?)
Reason I ask is that we used them their well-known functions (in the car world), but in the towing world -- 7,000# trailers behind a big block with 3.23 or 2.97 gears -- they were THE item for knowing when to downshift on a Rocky Mountain upgrade:
When, at wide-open throttle, instead of waiting for the rpms to drop, we'd shift according to the vacuum gauge dropping to maintain best power during those 20-minute assaults.
[the purpose of this was, of course, to get the engineer-gears to clicking on where to add on yet another gauge. Then, I looked at the size of the trailer . . oh, well
In a little more serious vein, have you ever had a look at the marine fuel flow gauges: that, along with a vacuum gauge and tach is the "ultimate" set-up in my mind for gasoline towing; monitoring the relation between three needles for best power, or best economy, or best road speed against a head wind, etc. Real time info on fuel use vs. road speed vs. engine rpm.
We owned a '77 Impala wagon that I wound up taking out to 240,000 miles/18 years on the original motor. Father-in-law had a 96 Roadmaster that was past 185,000 miles at his demise. Great acceleration for a small block car.
Thanks for the view