Fuel economy BAD - Change gearing?

[ssybert]

Hello everyone.. I've got a Chevy Dually 3500 Diesel. Currently I'm getting about 12-13 mpg. I do 40% highway and 60% city driving and need to get better mileage. I run 2350RPM's @ 65 on the highway. Based on the fuel economy section of this site, Click Here (http://www.62-65-dieselpage.com/mileage.htm) I think that means I've got 4.10's in there. I'd like to change the gear radio to something which will get me better gas mileage. I bought the truck because I like the big feel of the dually. I dont NEED that kind of towing power right now and dont know if I ever will. I tow a large popup in the summer, 2 snowmobiles in the winter and possibly a 16-18foot boat this summer as well. I dont think any of them are that heavy. When I go camping I will be taking my motorcycle in the back of the truck as well. Does anyone have any recomendations on what I should do here? I as thinking I could go with 3.08's and would like to hear any input anyone may have. The truck is 4WD and I'll need to know what I should do up front as well. I've never tackled something like this before but am mechanically inclined and will be able to figure it out. I just need to know what I should buy and hear the positives and negatives to a decision like this. Also, if anyone has any other ideas, I'll all ears. I've had recommendations such as adding an overdrive... My transmission HAS an overdrive. Is the suggestion to add an ADDITIONAL overdrive? Also, they are very expensive and I will most likely not see a return on my investment for a LONG time. As far as I know, gears are cheap. I'd rather go that route if possible.

Thanks, Scott.

[jbplock]

Scott,

Welcome to the forum! http://forum.thedieselpage.com/ubb/icons/icon14.gif I'm guessing your 3500 is the previous body style and has the 6.5L turbo diesel (?). The 2001 and up 2500/3500HD trucks come with 3.73's and turn about 2000rpms at 65mph. I used to own a 98 K3500 6.5 TD dually with 4.10's and my mileage ran about 16-17 empty and 10-12 towing. If your truck is indeed a 6.5 you might also try posting your question in the Towing Forum, 6.5 forum, and/or Member's Forum. (This forum focuses on the 2001 and up 2500/3500HD trucks). Also try using the forum search feature for info.
smile.gif

[Heartbeat Hauler]

What JB said. Also, what size tires are you running? You can lower RPMs by as much as 100 - 200 revs by going to a taller tire, i.e. 235/85/16. The other thing you might try is changing all fluids over to synthetic. Engine oil, trans, and especially the front & rear diff. and xfer case. All of this might get you 1 or 2 mpg, but it is pretty cheap compared to the alternatives. If you change rear end gears you are looking at about 700 - 800 bucks each(frnt & rear). The other alternative is to install a Gear Vendors Overdrive unit. While this is muy expensivo you get the best of both worlds, good towing power and double overdrive when empty. Here is the website if you wanna check it out:

http://www.gearvendors.com/

One other thing, if you plan to tow, haul, or carry anything other than groceries, 3.42s are to high and I wouldn't even consider 3.08s in a dually. Just my $.02 worth. Good luck. smile.gif
JP

[ssybert]

Thanks for the input guys. I took a look around eBay for a used overdrive and they look to be going for about 1200.00. There is one up there now for a Dodge... I'll keep my eyes open. According to the specs @ the Gear Vendors site I should be able to go from a 4.10 to a 3.08 equivelent gear ratio with the overdrive. That should make a tremendous improvement on the highway. Now I just need to calculate and figure out how many miles I need to drive before it pays for itself. :)

[JimWilson]

Originally posted by ssybert:
I as thinking I could go with 3.08's and would like to hear any input anyone may have.You do that and you won't be able to tow anything anymore. It will be like a Saturn dually! :D

[Heartbeat Hauler]

SSYBERT,
I think the GV unit will do wonders for what you are wanting your truck to do. It also has the advantage of splitting the gears, it reduces the rpm drop between shifts which keep the motor in it's power range.
Also, as far as overdrive goes, the trans OD is .75 x 4.10 = 3.08(rounded up), then you have .78 GV OD = 2.40(rounded). That will be some serious OD reduction in RPMs :eek:

If you tow, it is recomended that you don't tow in OD(trans), but you can put the trans in Drive and use the GV OD which comes out to 3.20. That should be good for mileage.

I would also make sure that I have a good exhaust system (read larger) and an after market air intake filter...Amsoil, K&N, UNI, etc. And, last but not least, good guages...trans temp and EGT. Definitely a must if towing.
JP

[Turbo Al]

Hi Scott, and welcome to the forums, sorry had to move your post. I have 3:43 gears (stock) in a 3/4 ton and have no problems at all towing what you list & since the truck is empty 90% of the time your overall mileage will improve -- it will be worse when you tow. If you need to tow something big once in a while just start out in low range -- that's what I do when I haul gravel.
Al