Fuel Mileage & Overdrive

[pez]

Hello all. I just returned from a r/t run between Denver & Aspen [Colorado].  416 miles r/t, did it on 23 gallons of fuel exactly. Never went under 80mph, with long stretches at 85 and 90mph.  Averaged 18mpg, which so far has been great for me.  However, at 90mph, with the 3 speed auto and the 3.07 rear end, I am running close to 4000 rpm's.  PS - this was over two mountain passes [13,000 feet] and none of it was flat & level!

I am wondering if ayone has any feedback about fuel economy and also if anyone has any experience retro-fitting an external overdrive unit to the car [ala the Volvo electrically operated o/d unit that sat aft of the tranny on their later generation 240's].  I am just brainstorming here, but if I could drop another 1000 rpms, I think I could get 24mpg or so!

[wacotech]

arn't some of the diffs 3.69?

[Big_Richard]

.

[oversize]

Not sure if these would be adaptable without spending big $$$, but they're certainly strong enough....

http://www.gearvendors.com/index.html

[wacotech]

OK, so my 3.69 diff is making my revs higher.
Does anyone know what LOWER diffs are fitting my w116 1975 m110?
And where is the part number written on it?