double overdrive

[Jurek]

I have gear vendors seating on the shelf. My truck is 97 2500 gmc sierra with 4l80E 2wd 3.73 rear 33'' tires. Would double overdrive /GW .78/ benefit my mileage, running empty on the flats without a trailer at 65mph. On this speed I get about 18mpg right now. I don't have any problems to run on flats same speed with loaded truck and 7000# loaded flatbed trailer with transmission on the overdrive. In double over, rpm would be around 14-1500. Would engine have enough power to move empty truck on the above speed and give me better fuel economy. How much torque would engine have at those rpms. Thanks for your opinion.Jurek

[phantom309]

If it were me,. i,d look at the shorter tire to go with the double OD,.it takes a lot of torque to turn big rubber,..
I have 3:42's and 285 75's, motor gets its best mileage right at 1700 or so,..
If you are just on real flat ground with not too much wind,. it,d be interesting to see if you gained any mileage,..

[Warren96]

Two things I have learned about a 6.5L diesel here that has been most benificial is 1800rpm and 2800 rpm. 1800 rpm for max MPG, and 2800 rpm for pulling a load on a hot day. If you want the best mileage, what speed do you mainly drive? At 14 or 15 hundred thats too low for best MPG.......You have already installed taller tires than stock, so that lowers your 3.73 rear axle ratio down closer to my 3.42 ratio. Can you tow a load without slowing down too much?

[Dvldog8793]

Howdy
I have a gearvendor. My truck has 4.10 gears. I still get my best MPG at about 1700-1800 rpm...unloaded normal driving. The main reasons I installed the GV was to be able to maintain with the flow of traffic on highways and also to improve life of driven accessories and engine. At those MPHs(70-75) you will also experience more drag. My truck(dually CCLB lifted with 285/75) gets about 15-16mpg at highway speeds on a good day. I got those same MPGS...maybe 1-1.5 better without the GV at the same RPM. The GV alone doesn't really pay for itself unless you drive in a way that the goal is to pay for the GV. If you follow the maintence schedule and still drive in the same rpm range that you always did the GV will end up costing you money(engine/accessory life not included). an oil change on the GV normally cost about $40-50 every 10-12K miles.
All that being said....if you have a gearvendor just laying on the shelf....IMHO:D....install it or sell it....they bring good money on the ebay market and I have several people just waiting around for me to get a new truck and NOT install mine.

Hope this helps!

[Dvldog8793]

Forgot to say that with my GV pulling my regular trailer(14k or so) I would normally run in Drive(not OD) with GV engaged. This kept me at about 2000 rpm at 65MPH. The GV supposedly has a higher torque rating than the 4l80e. Also the truck seemed happier at that range. No problem with power. I have pulled some hills and had to drop a gear as when my truck gets below 1600-1700 pulling a load it really starts to complain.