Over Drive

The real advantage of an over drive like Gear Vendors is the ability to split gears. They want to sell you on the MPG increase, but that will take a lot of miles. A friend of mine put a three speed behind his four speed in his gas powered truck. Darn thing pulled like crazy and a Lot of fun to drive, but file mileage increase? Not so much.

Diesel engine applications are not applicable to gasser issues. To maximize an OD change a tighter torque converter is required. Been there, a lot. Not a matter of saving fuel, more a matter of saving accessories, AC compressor, water pump, PS pump, etc, happier @ 2500/2800 than mid 3000 r's.

going to the forum for your engine... find out the rpm other owners of that engine gets for mileage considering the various loads.
my 6.7 cummins got it's best millage at 1950 rpm. which put me over the speed limit in oregon... so when towing i would drop a gear or two when climbing passes. maintaining that sweet spot. regardless of road speed. as long as i wasn't over the limit or the limit of the trailer tires (most trailer service tires are rated at 65 mph max).. the 5.9 cummins sweet spot is around 1800 rpm. so some research for that sweet spot will determine what gear ratio you should have in your final drive, considering transmission having an overdrive or not..
with the 8.1 vortex on flat land you could drop below the sweet spot, but if you get into a head wind. you would need to change gears to keep the load off the engine and to bring the rpm up..

tempforce wrote:
what rpm is the engine turning at 60 mph?

My 8.1 Chevy Vortec in OD (Allison Trans), I get my max fuel milage at 1900 RPM, 59 MPH, hope this helps you!

cncsparky wrote:
8mpg is an average over the entire trip/tank of gas.

Ok, then obviously your OD will have to do better than an increase of 1mpg.

2oldman wrote:

cncsparky wrote:
On a 100 gallon trip, would save about $50 @ $3.50/gallon. Is my math close?

Ya, provided every single mile was driven in OD.

8mpg is an average over the entire trip/tank of gas.

cncsparky wrote:
On a 100 gallon trip, would save about $50 @ $3.50/gallon. Is my math close?

Ya, provided every single mile was driven in OD.

I get about 7mpg. If I added an OD transmission, would 8mpg may be out of the question? That is a %14 increase in mileage. On a 100 gallon trip, would save about $50 @ $3.50/gallon. Is my math close?

Like someone said, another benefit would be less engine wear and lower noise.

Sully2 wrote:
You'll never save enough $$$ to pay for the OD unit

On this one I'm not so sure, If I put the fuel consumption blinders on you are right...

To the O/P.. no 10% RPM reduction will NOT = 10% MPG improvement, WILL improve MPG but not 10%, closer to 1 or 3 would be my guess.

BUT: Removing the Fuel Consumption blinders:

IT WILL give you greater engine life,,, Likely 10% or more, bearings will not wear as fast, Rings and valves will not wear as fast and if it makes the difference between Major Overhaul, and no major overhaul.. BARGAIN it is.

Buster...it appears that I am the only one so far that has actually added an under/overdrive. Several years ago, I did this on a 1988 33' motorhome with a 460 Ford.
Before the overdrive was added, the engine was turning 3000 rpm at 60 mph. With the overdrive, I was able to cruise at 65 mph at about 2600 rpm.
Another benefit of the U/OD, is that I could split gears and have a 1st over, and 2nd over too. That was a great advantage going up hills where 2nd was to low, and 3rd was to high.

I'm pretty sure that I didn't recoup my cost in gas mileage, although I did get a couple of mpg better because of the lower rpm when cruising. The main reason that I bought the unit was because of less wear and tear on the engine running at lower rpm's, and I could cruise at higher speed limits and not have the engine roaring.