Engine/Transmission (1994 - 1998) Looking for the Pros/Cons of Underdrive/Overdrive Unit

[Delta King]

Hi all. Due to the increase in Diesel prices, I am thinking about a underdrive unit. I have 4:10's and don't really want to change the ratio to a higher set. Any ideas on the pros/cons of a underdrive unit? What about the cost? Has anyone had bad service?

Thanks in advance for the help.

 

[Forrest Nearing]

the money you spend on one will buy a LOT of fuel, and they often run pretty hot under load...



you'd be better off w/ a switch to 3. 54's IMHO

 

[hsmith]

The Gear Vendors unit like mine is a costly project. Installation, drive line mods. etc. probably close to $3k. Hard to justify for the price of fuel only. Bought mine for the advantage of splitting gears while towing heavy. Works great for that, but probably not a gain in mpgs.

 

[Extreme1]

A few years ago there were a few posts about wear issues on the shaft connecting the two units. Not an easy or cheap fix.

 

[RJPotts]

Get some bigger tires, cheapest way I know of to get lower rpm at higher speed (lower numerical ratio).

Otherwise, I din't think a gear splitter will give any more top end gearing, just more options.

-R. J.

 

[JFitzgerald]

When I watch the overhead fuel computer, I don't see much difference whether I use double OD or just 6th, unless I'm cruising with a tailwind or downhill, or drafting on someone. It is nice to drop the rpms on the highway though. Makes it quieter. If I tow heavy in hilly areas, the truck seems to like 5th over. Just a slightly lower gear than 6th. It's a neat toy, but I wouldn't spend the $$ looking for mpg. My truck already had it when I bought it. BTW, my truck is for sale.

 

[CJ Lagos]

You want an overdrive, not underdrive.



JFitz: Have you used double overdrive for long periods on the highway and measured the fuel mileage? I can't image NOT having better mileage when comparing 2300rpm to 1900rpm for the same speed?

 

[TRCM]

And with a 4x4, it isn't as easy to do as with a 4x2. And if it ain't easy, it ain't cheap.



changing the gears would be significantly cheaper.

 

[JFitzgerald]

You want an overdrive, not underdrive.

JFitz: Have you used double overdrive for long periods on the highway and measured the fuel mileage? I can't image NOT having better mileage when comparing 2300rpm to 1900rpm for the same speed?

No, CJ. Have not taken any long trips with it. Most of the miles I put on my truck were driving to and from work, 40 miles each way, on a 4-lane divided highway with just a few stoplights. I usually run 70 mph. I tried it in 6th and 6th over, and by watching the fuel computer, there was little or no difference. It seems like it's more a matter of engine loading than rpm that effects the fuel usage in diesels. That may be why I do see a real difference using the OD in tailwind, downhill and drafting situations. The fuel computer would show in the upper 20's and low 30's in those situations. RPM definitely has some effect, but I don't think it's as big a factor a we might think, in some situations. The truck will get an honest 20 mpg driving to work. This is by real figures, not the fuel computer, which runs about 2. 5 mpg higher than actual. Just a couple months ago, I got a company car to drive, so the truck sits home waiting for some towing to do. #ad
Sure is good to burn company gas right now. #ad