318, auto, interstate driving, road trip vehicle.. 70 MPH absolute max speed..
I have a 2.2 rear gear right now. Would a 2.7 or 3.23 be better rear gear?
Would it be the same answer for a 360?
Ima guessing your 2.2 equipped 318 might have say 205/75-15 tires on it, which have a roll out of about 84.8 inches. Then 70= about 1918 in loc-up mode.
For straight hiway use, this is a pretty hard to beat starting point.
How much fuel economy it gets will depend on the current state of tune. Or maybe how good a tuner you are.
Tuning with a vacuum gauge is as good as hopeless. Rather, it takes forever to see results.And you will never be 100% sure if it's better or not.
What you need an on-board adjustable timing control.
Then get her up to temperature and speed on a hard, flat, level hiway, and run her for a mile or two. Then retard the timing 3 degrees and watching the speed-O, wait. If the car loses speed, then whack the 3 degrees back in, and watching the speed-O, wait. If the speed recovers, whack 3 more degrees in, and watching the Speed-O, wait. If the car picks up speed, reduce your speed back to the chosen cruise-speed, and let her settle there for a mile or two. If you have a vacuum gauge installed, note the difference in vacuum, from the starting point to the new, more closed, throttle position. I bet you cannot see a difference. But if you do, it's only because you saw it move just a tiny bit. Had you not seen it, you would never have known that it moved.
Ok so keep driving at the new throttle position, at the chosen roadspeed and let the engine settle into it. Then whack 3 more degrees into it, and wait. If the car slows down, retard it 3 degrees to the previous number and call it done. But if the car picked up speed, reduce the throttle back to the chosen roadspeed and drive again, until the car is settled, then whack 3 more degrees of advance into it, and wait. Repeat, as many times as necessary, until the car slows down, then back up the timing 3 degrees and call it done.
Now, you need a tach to see at what rpm your chosen roadspeed occured at.
Say it was 1900@70 mph
Stop the car.
Rev it up to 1900 and read the timing. What you read there, is your best cruise timing for 70mph @1900rpm. Now you just gotta make it happen. Butum, if your chosen roadspeed is not 70@1900, then use the numbers you recorded.
Butum,
if you change ANY parameter; like engine temp, inlet air temp, elevation, cam-timing, compression ratio, I mean any parameter; then you gotta start over. So leave that timing control installed.
In this way, you can determine your best cruise timing in a matter of a couple of hours, instead of weeks and months.
To make this go even faster, before you even start;
Just rev it up to 1900 and set the (cruise) timing to 45 degrees advanced with the vacuum advance
can fully advanced. Then go cruise at 70mph and work the Dash-mounted, dial-back, timing retard box, as already described. Maybe you'll get it in two or three steps.
Now you can start to work on the carb..........
and after you get the cruise circuit leaned out, then you gotta redo the timing again.
Pretty soon, she will have the cruise tune to 85/90% of perfection, maybe you'll stop there...... calling it good enough.