Higher RPM's = lower MPG
Lower gear ratio = higher RPM

Therefore....

Lower gear ratio = lower MPG

You're right though....it has a LOT to do with how you drive.

This was recently done by someone.....around here somewhere. As I recall.....it averaged about 1.4 MPG per every 10 MPH....which corresponded to some RPM numbers that I don't recall.

 

[quote author=Posm link=board=20;threadid=17922;start=0#msg168446 date=1060097434]
The differences between OD and direct would be much great than between 4.10 and 3.73s. I submit the difference in wind drag at a higher speed makes the majority of difference. It takes much more HP to run the rig at 75 than it does at 65. [/quote]
bingo. the 3.73/4.10 difference itself is about 10%, while the OD/direct difference is 37 percent (on the 6 speed). Thats not even a fair test and makes some assumptions (that the economy/rpm curve is linear) that are just not valid.

While I tend to agree with the wind drag theory, here's some other things to consider when comparing trucks. We already know that the 3.73/4.10 difference is about 10%, but consider:

1. The auto versus manual transmission contributes another ~5 percent to the rpm equation because they have different overdrive ratios

2. Unless I'm mistaken, doolies have shorter tires (235/70 17, no?) which contributes another ~5% in rpms over the 265/70 17 on the SRW.

3. The CTD fuel economy might avalanche (get really bad really fast) at or near ~2200 rpm, as upposed to being "linear" (gradual economy loss with increase in rpms).

Consider two trucks, one with 4.10, 6-speed dooley (30" tires) and a .73 OD ratio, and the other, a 3.73 auto SRW (31.5" tires) and a .69 OD ratio. After compensating for actual rolling (loaded) tire diameter, my calculations show that at 75 mph, the dually is going to be in the 2500 rpm region and and the SRW is going to be more like 2100. thats 20%!

So just be careful what trucks you compare, and note that there could be a substantial economy difference between 2100 and 2400 rpm. don't know that for sure, just my $.02.

 

The dooly comes with 235/80R17E tires with a diameter of 31.8". With 4.10s it is turning 2175 rpms at 70 mph.
Should have added that is with the 6 speed.
48RE Overall top gear ratio, 2.57 with 3.73 and 2.82 with4.10
6spd Overall top gear ratio, 2.72 with 3.73 and 2.99 with 4.10

 

ok, thanks for the clarification on the tires. we're at least close, now, in terms of rpm . However, for the 4.10 6-speed doolie, the rpm at 70 mph would be:

2.993*336*70/31.8 = 2213 rpm

However the tires actual rolling diameter is probably only 31" when running empty, which makes the rpm:

2.993*336*70/31.0 = 2271

And driving 75 mph, this truck would have to turn 2433.

 

So, If I found someone who wanted to do the gear swap, what exactly would be involved to do the actual swap at the garage?

 

I have the 3500 SRW, with the BFG Rugged Trail T/A 265/70R17's and the tires themselves are about 30.5 inches from the garage floor to the top of the tire (I guess that would mean about 31 inches for an unmounted tire)

If I need 10% larger tires, would that mean going to a tire that was:

31 * 110% = 34.1 inches?

 

LatrobeHO,

A SRW tire will loose about 1 to 1.5 inches from theoretical height to actual loaded height (a number of variables there). the 265/70-17 is a 31.6" tire so you're just about right.

A pair of DRW tires do not loose as much when loaded and I expect will loose less than 1". Thus, a pair of 235/80-17s are 31.8" (unloaded) and will be approximately 31" tall when mounted.

yes, 10% taller is ~ 34 inches. 315/70-17 will be close.

 

It makes no difference what the load is on the tire the circumfrence will not change, and that is what is really used to calculate the RPM's

Buy the way I checked today and 70 is 2240

 

[quote author=Mopar_Mudder link=board=20;threadid=17922;start=15#msg168970 date=1060187043]
It makes no difference what the load is on the tire the circumfrence will not change
[/quote]
seems that way, to be sure, but actual effective rolling circumference does indeed change, depending on pressure and load. It is always different from unloaded measured circumference.

chalk marks on the driveway will measure loaded circumference, probably more accurately than directly measuring diameter.
Then your loaded tire diameter is 31.4", approximately 1/2 inch shorter than published tire size suggests.

 

So, If I found someone who wanted to do the gear swap, what exactly would be involved to do the actual swap at the garage?

Well, you'd have to take both trucks in at the same time. They'd have to pull all 4 diffs, and then set up all 4 in their new home. I'm not familiar with the new axles, but if they are like regular axles, any shop that sets up diffs should be able to do it. I'd tell them you have The new AAM? or AMM? axles, just to make sure they can do those. You'll probably be charged double shop time. I can't remember, but I think it is common to figure 6 hours per diff. So, that's pretty big $$, but you both would pay half, i'm guessing. The only other cost would be diff fluid, and maybe some shims. Bearings should be fine on trucks that new.

 

Why not just disconnect the brakes and shocks unbolt the U-bolts holding it to the springs in the back, the front would be more work but probably easier than taking the R&P out, and swap the entire axle assemblies? Of course you would not need to do the front if only two wheel drive. That would be the fastest way with least chance of problems down the road.

 

definately, if you have two trucks ready to swap gears, don't bother swapping the gears themselves -- swap out the entire axle. if you want to trade gear ratios, then the only thing you save by swapping the actual R&P gears is the cost of the gears themselves, which is small compared to the total cost of the job.

When you trade axles, isnt there an alignment consideration? seems like you'd want to go have a 4 wheel alignment "done" (measurements made) to make sure everthing is in spec.

 

I didn't even think of that. Yeah, you'll definately have to get an alignment, but that still doesn't even come close to the $$ involved in swaping the gears. Heck, if you've got an auto, try to find a someone to trade with a stick, that way, you'll get the gears you want, and a bigger axle. ;D

 

Hoss

I guess I did not make my point clear. The truck will use more fuel the faster you go. That is a function of drag. The resistance increases exponetially. The motor does not nessasarally use more fuel at a higher RPM, it uses more fuel at a higher load. A 3.73 truck and a 4.10 truck require the same HP to move a truck along at a given speed, say 70MPH. The TDR article I referenced stated that on constant load Dyno, the truck used about the same amount of fuel per hour from 16 or 1700RPMs all the way to 2500RPMs.

Slight mechanical loss aside, the engine will not use much more fuel within its designed RPM range. At truck in N at 2500 RPMs will use a lot less fuel than the same truck in OD at 2500RPMS.

Some one who has a 3.73 that runs the same route day after day should try my experiment. Run in high gear for two or three days or even two or three tank fulls. Then run in direct drive for the the same trial. If the speeds are the same I would bet a ponderosa steak dinner the MPG would be within 1 or 1.5MPG.

My current truck varies from 9.75 MPG all the way to 21MPG and load is the only factor that I can see. I have ran through 450 tanks of fuel in the past three years and have checked 90% of them with a calculator after filling the truck neck full.

 

[quote author=LatrobeHO link=board=20;threadid=17922;start=0#msg167885 date=1060006983]
Someone mentioned that I could put a set of 35's on the truck and that would negate some of the effects of the 4.10 gears. What exact tire(s) are you referring to?[/quote]
Hey Latrobe, go look at some BFG 315R70/17's; these are the tires that come standard on the H2 Hummers. They'll do a great job in raising your effective gearing and look good too. There's a few guys on this site running them on stock wheels and no leveling kits just fine.

BTW, I just returned from a trip down to the Outer Banks, NC with my truck. On the way down I was in a hurry and ended up @ 17.5 mpg; and today on the return trip I was slowed by rain and traffic, and messed around in Virginia Beach for a couple of hours and ended up at 19.5mpg. I'm not complaining. These figures were hand calculated with my HP 28S, and not the overhead. I did lose a rear hub cap on the way down - If anyone finds it, could you mail it back to me???

All this talk about swapping gear sets and clusters; you're begging for Murphy's law to engage. Give it a rest for about 6 months, then revisit it.

Edit: my truck has the 265R70/17 BFG's; and in a hurry translates to about 2450rpm @75 where possible - other than the gridlock situation in Kill Devil Hills area. Note: you should really use tire revs/mile in your calculations rather than tire diameter; .... but I don't either. The revs/mile number (circumference) will never change due to load, air pressure, etc.; where diameter can vary due to load, tire pressure, etc.