Can your slanty pull the gearsplits?
The ratios are 3.09-1.67-1.00-.73od.
Splits of 54-60-73 percent. Meaning, at each shift, the rpms will drop to those % splits. On the 1-2 shift, whatever you rev it to in first, the Rs will drop TO 54% in second and so on.
Lets say your current gear is 2.94. and with an 80"tire roll-out (25.5" tires), your rpm at 60Mph will be 2330 rpm in direct. Shifting into overdrive, the Rs will fall to 1700. Can your slanty pull 60@1700?
One thing I can guarantee you,
guarantee you,
is that your engine will not have nearly the required amount of ignition timing to make any kind of fuel economy, at 1700=60mph.
But suppose you already have 3.91s.
Now your cruise rpm will be 60=2260, and you will likely still not have nearly enough cruise-timing.
At 2400 your engine will want about 54 or more degrees of cruise timing. There is no way to get it with stock parts. So if yur thinking to install the OD box for fuel economy alone; it ain't gonna happen, or at least not at 1700rpm.
Of course running 3.91s is a huge advantage, lol.
To see where you are at;
> go rev your engine up to your cruising rpm and record your timing with the Vcan hooked up. I bet you can't get anywhere near 54*. If you can't get at least 28* at 2400rpm, check the Vcan and see if the diaphragm is ruptured.
If your diaphragm is good, then you will need to figure out how to get some decent cruise timing into the current combo, which will already increase your fuel economy substantially.
To run the OD box;
IMO
The smaller the engine is or the lower it's cylinder pressure, the more likely that engine is to require a change in rear gear.
But no matter what, the cruise timing is gonna need work.
