mpg--4:10 vs 3:42?

[mad rad]

Thinking of changing from 4:10 to 3:42's anybody know mpg of 3:42's.I get around 15 with 4:10's and a 5 speed (1 ton dully) thanks

[moondoggie]

Good Day!

mpg? You'll be unlikely to see 2 mpg improvement. I got about that in my pickup going from 3.94 effective ratio (oversize tires) to 3.08 (oversize tires & Gear Vendors Aux. OD). My Sub (3.42 gears) has been running 21.5 mpg lately, but it runs mostly empty, at the posted, granny (me) driving.

Noise? It'll help.

You might want to look at Looking for a 3.73 rear & front end (http://forum.thedieselpage.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=1;t=007122)(Click on the colored text.)

Try searching on mph

[Dvldog 8793]

Howdy
I went from 4.10 to about 3.48 with my Gearvendor. AT A GIVEN SPEED of 65mph I noticed an increase of about 5mpg. Overall average is on par with what Brian said, about 2mpg. It depends allot on your driving habits. Also if you get the higher gears and then drive faster it really doesn't help at all. My truck runs the same rpm at 70 as it did before at 55, but with wind and rolling resistance MPG is probably lower. It is nice to be able to keep up with the flow of traffic without SCREAMING my motor! :D My truck will normally average about 18MPG with about 40% city driving. I have gotten as high as 24 on the highway with IDEAL conditions. Lowering your average operating RPM is also going to save wear&tear on the engine and driven components.
Hope this helps!
L8r
Conley

[jdmetcalf57]

DvlDog
I want to know all of the specifics about your truck. I have a 99 K3500 AT CC drw LB. I to went from 4.1 to 3.73 and 265/85 tires which make the equivalent of 3.4. In near ideal conditions I can sqeeze 17-18 mpg but seldom do. With a little city driving its more like 16 and usually about 13 pulling a 4k hilo type camper. I drive 70 on the Interstate but 65 only changes mpg a little. Changing my gears at most gained 1-2mpg but is great for quietness and piece of mind with the engine running slower.

I have been thinking about trading for a cummins for the extra mpg and would like a manual trans. However I do like my truck and would otherwise like to keep it. I have done some almost identical driving test on my truck compared to a manual trans cummins and they get 2-3 mpg more pulling my camper.

[Dvldog 8793]

JD-
My truck-
1999 Crew cab long box SINGLE rear wheels, about 8000lbs with me and 3 kids. NON EGR HO motor, 4l80e trans, locker rear diff w/4.10 gears, 245/85r16 LRE.
new pump at 120,700, new Block at 144,000 (so I'm not sure what mileage I will get with new block, should be about the same)
GM8 turbo, High pops from JK, JK exhuast NO muffler, mandrel bent ceramic crossover, JK boost control set at 14psi max, Synthetic everything EXCEPT motor oil, UNI air filter w/ cleaned up air path, Gearvendor Aux OD, timing set to -1.70, Powerstop cross-drilled rotors, Powerstop EMS calipers and pads/shoes, Stainless brake lines, EMS hightemp DOT4 fluid, All the normal gauges by cyberdyne and isspro. Maghytech trans pan & Diff cover. Amsoil remote filter w/ bypass. Gregs cooler lines. I use a mix of 3gal Powerservice white, 1.5gal FPPF +8, 2qrt synthetic twc-3 2 stroke oil. About 18oz per 30 gal.
The truck gets its best mileage w/ outside air temp of around 35-40*. When winter hits andI have to use 1-2 fuel the mileage suffers.
Hope this helps
L8r
Conley

[mad rad]

Thanks for all the info,I was hoping for more mpg than 2-3.

[DA BIG ONE]

I had changed from 4:10 to 3:42 front and back milage did not change at first.

I had a hell of a time getting the speedo calibrated until the members here directed me to hack the VSSB myself. Once I had speedo calibrated, I still had about the same MPG, then did the top dead center relearn and milage went up. How much varies, but have seen 24+ on long trips, average is 20+. If I dog it, no mpg gains!

I very seldom tow anything, and usually load this truck w/4 big guys, hunting and camping gear.

[jdmetcalf57]

Dvldog
Your usage of Powerservice doesn't make sense to me. Do you mean 3oz per gal of fuel? That's what they recommend. You're also using 18oz of 2 cycle oil per 30 gal right?

Is the GM8 turbo the stock turbo?
What boost do you see at normal interstate cruising?
What mod do you think made the most differece in mpg, the high pop injectors or the increased boost?

My engine runs about 2100 rpm at 70mph which I usually run on interstates.

The main differences in my truck and yours is; duals, high pops, 14psi boost and I don't use the 2cycl oil.

[Dvldog 8793]

JD-
I mix all the additives together in one container. When I'm done I get the following mix per tank of fuel:
PS white- 12oz for cleaning/some cetane boost
FPPF +8- 6oz more cetane
2cycle oil- 1oz for lubrication (see Dr Lee)
If I'm doing allot of pulling I'll double the dose. It really is a work in progress and I'm trying to find a good happy medium that is readily available. There is some debate on whether we need additive in our fuel, it makes me happy to put it in! :D
I would have to say the single best modification that I did was the exhaust/airflow. Next would be the Gearvendor.
The highpop and extra boost are for more power when I pull and were probably detrimental to MPG.
Also I noticed an increase when I put the synthetic in the diffs. I really think that duels make a big difference in MPG. It would be interesting to see a study done on that. does your truck have a visor or bug deflector? They can suck up MPG. Also I think the factory air dam that comes on some trucks makes a difference. Mine did not have one, it does now.

[jdmetcalf57]

Dvldog
I'm not sure what the FPPF is. Also the 1oz of 2cycl oil doesn't seem like enough to do any good but I will reserch Dr Lees e-mails to see what he has to say.

I've been wanting to change to SRW but haven't found a good bed yet. I did a mpg test just taking the outer dual off and it made at least 1mpg difference. Getting ride of the fender flares, I'm sure would make some more difference. I also took the bug sheild off a while back and it seemed to make about .5mpg difference.

How much difference do think a manual trans would make? I think it would be significant pulling because thats when the torque converter does a lot of slipping wasting energy. I want to change to a manual just because I don't like the way the auto pulls.

[Dvldog 8793]

JD-
the 2cycle oil probably should be increased a little. I think Dr Lee said to use about 1oz per 10 gallons. As I said my recipe is a work in progress :D
The info can be found in the DR Lee Forrum, do a search for Sulfur and it will be Good bad or Ugly.
L8r

[moondoggie]

Good Day!

[Dvldog 8793]

Brian-
So who are the other two??? :D
L8r
Conley Janssen

[kowsoc]

I always use a fuel additive of some kind.....tried alot of them. Most claim to increase fuel economy but they vary in result. I even tried one that boosted cetane 7 - 10 points and claimed up to 20% fuel savings but real world was about 10%. PowerService claims to reduce fuel economy by 5% but I see about 8%. It makes the truck run better too and I use it in every tank.

I also use 2-cycle oil but I usually put 16 oz./tank. It doesn't make the engine run any differently but I bet the injection pump is alot happier.

[Marty Lau]

MoonDoggie;

I think if you can get close to 20mpg and to be getting about 23 mpg of a full size rig that is great. Gear vendors eh????? How long have you had that?

[moondoggie]

Good Day!

Dvldog 8793: OK, OK, I