After further testing using known factors - in the case 3.70 rears - I have pinpointed exactly what I am working with.
I have a 13 double over .73 and .87 with direct being 11th. Obviously..
By my calculations currently I should have a road speed of 65 in top gear aiming for a target of 1400 RPM. For the ISX engines, Cummins recommends 1300-1350 for optimum fuel performance with 1400 being the max for the higher HP engines.
I am all but certain the rears are playing a large role in reducing my economy. Here is part 1 of my potential solution.
Swapping to 3.08's would put me at 66 mph in 12th leaving me room to go up to top at 73 @1400.
Swap to 2.64 would put me at 67 @1400 in direct. That right where my engine is most eco. These cummins you can run all the way down to 1100 with peak torque of 1850@1200. Direct I would need to downshift at 53. Having the 2 OD gears would give me the option of 66 @1200 on the flats in 12th and 13th at 73 roughly around 1300 if needed.
Note: I like the idea of running in direct mostly to minimize power loss and maximize eco. Having the double over available for those times I have to go is good but I would rarely use 13 unless I'm in west TX.
Swap 2.85's. Direct will give me 62 @ 1400, 12th 66 @1300. and 13th 73 at 1200.
Note: I do not like the idea of my direct being 62 at the top of my optimal torque range. I do like having my OD at good cruising speeds for when I need it at lower RPM.
So, what do yall think? I'm more looking towards option 2 or 3. I know going faster is going to blow my fuel mileage to !&&* to I will avoid it as much as possible. Just sometimes I am going to have to go go go! On a side note, ordered the scan gauge today.