I took a trip down the I-5 with my camper, about 3k, doing about 70mph and got 12.5 mpg on the fuel. This was from Klamath Falls OR to Visalia CA. I could have gotten better fuel mileage if I had dropped the speed to at least 65. I have the 4.10s and wonder if it would be worth the trouble of getting the 3.54's; if it would make a lot of difference. at 70 the tach is at between 23 and 2400 rpm. I will be towing a boat also, about 2500 lbs, just not this trip. Or would it be better to put one of those in-line overdrives in. That way I could keep the lower gearing for towing up and down the mountains.

I guess I could just go slower, but that is very hard; also takes longer to get someplace.

Any ideas would be appreciated

 

Either, or both! In the end the gear change will be slightly cheaper.

By the numbers the gear change alone would theoretically drop your cruise rpm by ~360rpm (fixed!) but with the auto it will most likely be realistically closer to 300rpm.

The GV OD unit will drop it ~4-600rpm with the 4.10s.

Both would see ~700rpm drop.

I am sure someone who is running either or both can give you actual numbers.

 

You might get 5% or 6% MPG difference. 1-2 MPG maybe? It would take a lot of miles to pay back unless you found somebody that wanted to swap axles.

@BILTIT - how are you getting 900 engine RPM drop at constant ground speed? For 265/75-16 tires and .76 OD transmission ratio, at 70 MPH I am getting 2000 RPM and 2300 RPM for 3.54 and 4:10 respectively. What did I do wrong?

 

I did it wrong, used just the ratio difference as a percent. You are correct, doing it properly i am getting ~360rpm difference for the gear change alone.

Also GV site shows OD as .69?

 

.76 is what I think the OD is in my nv4500. Gear vendors may be better.

 

His truck is an auto so it will be a slightly different OD.

 

Is the Gear Vender able to hold up to hauling the weight???

 

"GEAR VENDORS provides overdrive product to racing cars and trucks up to 2000hp and trucks towing heavy RV's up to 25,000lbs gcvw."

 

I'll take that as a yes

 

No don't do an OD unit, they're junk and don't hold up to diesels very long.
The place I worked at was contantly changing those things out, the torque just shreads them up. Old rigs with 3-speed trans okay..but diesels no.

 

Wouldn't it be great if they made a two speed rear end for our trucks; now that would be cool

 

I have uswed the gear vendors in 4 different trucks. The one I have now has over 100k on it and still works perfect. All behind Dodge diesels. And I tow 25k per year pulling my 5th wheel. They will give you a big advantage in gearing for those 4:10's. They are a 20% reduction in rpm's at any given gear. You'll have all the power you need for hard grades and heavy loads, and greater mileage for the highway. I certainly wouldn't run an automatic without it, and even my 5spds.

 

For $650 I bought a good, used 2002 disc brake rear end with Posi and put it under my truck. It was a 3.54 ratio.

The torque curve on our engines maxes out at about 1,600 rpm--above that you lose efficiency AND air resistance increases.

 

i would say do 5spd change, thats about normal withthe 4.10s with the 3.54 doin aboot 70 im at 2000 rpms. the thing aboot 4.10s you can dam# near tow down a house with em. i think it would be worth it in the long run, it would be more of a highway rig and can still tow well

 

Nice steal on the axle - If I could have found one at that price I would have done it as well.

On torque, etc : yeah, that is the short of it. On most vehicles, cars/trucks/suvs whatever, the air resistance starts to outweigh the frictional losses somewhere between 45 and 55 MPH. With a camper, it will be sooner yet, and air losses stack up very fast. Speed reduction, and its counterpart of keeping the engine under 2K RPM's will help quite a bit. Under 1750 is even better, but may get you shot at on a Cali highway.

You have to find your own balance of sanity and cost : getting where you are going sooner vs having more to spend when you get there.