Is it worth changing gears for better MPG's???

[16gaSxS]

Time too time the topic of switching rear end ratio's comes up to save fuel and there for $$. Here is a little though and calculation I came up with on this topic. The conclusions will vary based on your cost and if you have 2 wheel drive vs 4 wheel drive. The calculations are based on 4x4.

If you pickup 2 mpg going from a 4.10 to a 3.73 ratio which assume you average 16 mpg and then would average 18 mpg. 16 MPG would take 62.5 gallons to go 1,000 miles 18 MPG would take 55.5 gallons per 1,000 miles. That is a savings of 7 gals per 1K miles or $21 per 1k at $3 per gallon. So if it cost $1800 to swap gears ($800-900 per axle) $1800/$21=85.7k miles or 85,700 miles to break even. Now unless you tow heavy and a lot say over 7,000 lbs I would consider going up too 3.42 ratio the cost would be about the same and if you pick up an extra 1 MPG that would mean it would take 52.6 gallons per 1K miles or 9.9 gallons less or a saving of $29.70/1k miles. $1800/$29.7=60.6k miles or 60,600 miles to break even.
So anything over 61,000 miles and your ahead of the game. The key is HOW LONG do you plan on keep your truck or really how many miles do you plan on keeping your truck??? If you keep the truck for about another 100,000 miles you would save ~ $1200 in fuel cost @ $3/gal and 3 mpg increase.

Gear changing was something More Power talked about more in the early days at The Page.
I did this calculations back when I had about 30,000 miles on my truck and did the swap at 38,000 miles and figured it at $1.35 per gallon for Diesel. It worked out that I need to keep the truck at least 60,000 miles to break even. MY goals/was to drive the truck too at least 300,000 miles. With about 110,000 miles past break even on my truck and with fuel cost well above the $1.35 I used I am way ahead.

So. I believe it is feasible to change axel ratio's out and save enough to pay for it self PROVIDED you drive the truck enough.

So if you only keep the truck for 20 or 30,000 miles after changing gears your only wasting money and helping the next owner.

A couple other thoughts about changing rear end gearing, More Power said in another thread when he did it on his old K-5 Blazer it made the truck much more pleasant too drive.I found this true also, lower engine and drive train noise. Jim in the early days talked about extending engine life by going to 3.42's from 4.10's by an estimated 100,000 miles and maybe 50-75,000 miles going from 3.73's (my old ratio) too 3.42's. At the time with a New 6.5 this was important too me. Maybe it's not as important to folks who buy a now well used 6.5TD as it was then. I do think there maybe few folks like myself who buya truck for a long time and want to keep their long term cost and headaches to a minium.

In conclusion I think changing gears is a good option for some but not all 6.5TD owners. If your considering this consider your towing profile, type of usage, and expected ownership time frame.

[john8662]

I just wanna go faster!

Didn't like the 75mph interstate traffic with me driving 70 and getting looks from other drivers like that was the fastest my rig could go. It could go faster but at the cost of fuel economy and my ears.

6.5 4.10 gears..

Planning on the 4.10 to 3.42 swap, doesn't make sense to go 3.73 since I don't pull too often and too heavy anyways (just the weight of another vehicle occasionally).

This way, all RJ has to do is follow the black cloud

J

[More Power]

I agree with John & Marty... Switching gears from 4.10 to 3.73 might not be worth the investment. If your use of the truck is primarily non-towing, then 3.42 is your best choice.

I encourage people to look at how they use their truck to determine whether they should invest in a gear swap. If you use your truck to tow heavy, then stick with 4.10's. You need the gear reduction. Otherwise, you'll notice a big improvement in driving pleasure as well as a bump in fuel economy when going from 4.10's to 3.42's for a non-towing or light towing use.

The cost of a gear swap is highly variable - depending on whether you can do the work or if you hire the work out. The parts aren't really all that expensive, but getting the gears set up correctly is vital for a long and noise-free gearset life.

To get an idea of what the driving experience will be like when going from 4.10's to 3.42's, take your truck up to 65 mph out on the highway, then put the trans in neutral with the engine idling. After switiching to 3.42's and while running in OD, the engine noise almost completely goes away....

Back in the mid 90's I added an OD auto (it had a non-OD auto) and a Banks turbo to a 6.2 GMC K1500. If I were forced to choose just one of these two upgrades, I would choose the OD auto - just for how nice it made the truck to drive on the highway - regardless of how it affected mpg (although it added about 5 mpg). The truck already had 3.42 gears.

Jim

[DA BIG ONE]

It would be 3.73 for me the next time around because of the autotrac. What would be nicer is a stand alone tranny controller so I could stay w/3.42's.

[moondoggie]

[font=times][size=+]Good Day!

I picked up ~ 2 mpg going from ~ 3.80 effective ratio (4.10 gears w/ 8% oversized tires) to 3.08 effective ratio (4% oversize tires + Gear Vendors Aux. OD).

To figure out when a modification will pay for itself, put this in your spreadsheet program (do NOT input the quotes

[bmp34]

How about the 5sp manual trannys. They about the same ratio as the automatics?

[TurboDiverArt]

It would be 3.73 for me the next time around because of the autotrac. What would be nicer is a stand alone tranny controller so I could stay w/3.42's.

You and I have similar trucks. Why is the auto track a problem with 3.42

[DA BIG ONE]

[QUOTE=TurboDiverArt]You and I have similar trucks. Why is the auto track a problem with 3.42

[MartyB]

I thought the OD ratio's for the auto vs manual were opposite of what John wrote, ie the auto was a .71 vs tha manual at .74. Where the auto lose's, is the first gear effective ratio. The manual depending upon yr, is 5.64 for later NV4500's or 6.34 for the early IIRC pre 94 NV4500's or ANY of the dodge tranny 4500's. Where as the auto first gear effective ratio is about 4-1. A 1.64-1 difference, quite abit when it comes to take off! You can have 3.08's with a manual, and still have better take off than a auto with 4.10s, and if you can drive in OD, what should be great MPG's!

If you tow, be carefull about changeing out gears, you may lose all you take off ability if you go too tall.

[bmp34]

what is lowest ring and pinion you can get for a 4x4? The lowest i have seen is 3.42.

[JCM5]

In short
Lowering the ratio, lowers the RPM - this does not mean lower MPG
This can and will be detrimental to the engine to lug a motor.
Every engine out there has a "sweet" spot for power and were it like to run.
Outside of this area is wear either by friction or poor lubrication, yes even at low RPM
The 6.5 pulls best around 2700RPM
Put in a gear the you can live with and run the engine as intended.

I run 4.56 gears with stock tires, I pull 8000 lbs. However I can only go 75MPH top end, fast enough for me. When the engine runs at 2500 to 2800 it takes alot to slow it down, hills etc. Run the engine at 1500 and your foot will be in the gas pedal more often

Key in all engines for MPG is vacuumm not gearing.

[Bnave95]

JCM5
Key in all engines for MPG is vaccum not gearing.
How are you thinking this? Turbo's have no vac. Added boost up to 10lbs. 12lbs. max W/O intercooler helps with EGT and better fuel burn.

[Bnave95]

Your running the same tire set up as I am. Gearing one ratio down will help in the RPM area. This will work with lite towing and needed can tow in 3rd for hills, slow driving, ect. Running with out load will help with mpg. Diesel like the tourqe range over rpm's. I wish I could get the mpg you are seeing Though I have bed mounted side rail tool box with ladder rack and in bed slide out box. Lot's of weight and not at all areo when it comes to drag. My best is 14mpg.
I do like 85's when it comes to tires. What air psi do you run?

[ronniejoe]

Diesels don't make vacuum...

Gearing is the answer...

The engine operates with maximum efficiency near its torque peak... For best fuel economy, operated near 1800 rpm. Gearing will let you accomplish this.

Also, slow down. Don't run much over 65 mph.

Why do you think all of the late 70's and early 80's cars suddenly started using really tall gearing?

[HH]

I calculated with my tires, I will be at exactly 1800 rpm with 3.42 gears at 70 mph, and at 60mph I would be at 1540 rpms using the 5th OD. In the 4th gear at 60 mph the engine would be at 2175 rpm, which is close to what I run now at 70 mph in OD

With 3.73 gears at 70mph the engine will be at 1960 rpms, and at 60mph the engine would be at about 1675 rpms in OD, in 4th the engine rpm would be about 2375.

From what I have read, it appears that I will gain driveabilty, but lose some towability, (is that a word?). Since I have the low-low 1st gear I should be good with the 3.42s as long as I do not increase tire size. Since I am running the NV4500, I can always drive in 4th, or select the gear I need.

It appears the 3.42 gears run the engine in the sweet spot (1800 rpm) at 70 in OD, and at a good (familiar) spot at 60 in 4th gear (2175 rpm) and using OD in town at 35mph it will be close to idle.

Are these calculations correct? Again my tires are 235/85/16, approx 31.75" tall. I usually run the fronts about 60psi and the rears at 65-70psi.

[Stratosurfer]

My question to the group is this: I can get a Gear Vendors installed turn key in Houston for $3,400, about double the price to lose my 4.10 gears. I have only heard/read glowing reports on how the 6.5 TD responds when given an 8 speed transmission that is corrected down to 3.08 final drive when not towing.
I am on the fence as to what to do with all this data y'all are pouring out on installing smaller gears against the future possiblilty that I will obtain the bumper-pull portable Texas Ranch House/pontoon barge (Texsas Reservoir Yacht) of my dreams and need to tow 9,000+ lbs once in a little while.
I like the idea of the gear change for the component of simplicity, I lile the idea of the second transmission for the reportedly phenomenal results.
Anyone opine on this conundrum?
As well Texas DPS is slowly changing the speed limits in the 'remote' areas (I live in a remote area) to 80MPH. I will somehow feel deprived if my RPM's are so high at 80 MPH as to force the stereo to maximum DB's and I still can't keep up with the lawyers from Austin in thier sporty German Turbo Panzerwagens.
Towing, MPG and MPH; is Gear Vendors the only true solution to all these?

[moondoggie]

Good Day!

"...I've got the GV electrically hooked up (non-standard)." I didn't expand on this previously, in the name of saving space in case no one was interested.

Normally when a GV is installed, there is a second VSSB used. One supplies the PCM with the shaft rpm signals received from the tranny input & output shafts & massaged by the VSSB. The 2nd VSSB is fed from an additional shaft rpm sensor, identical to the GM units, on the GV output shaff. Then the wire that feeds the speedo/odo is disconnected from the 1st VSSB & connected to the 2nd.

Mine's wired different. I only use one VSSB & don't use the tranny's output shaft sensor, just the GV output shaft sensor. Because of this, I have to make sure I never use the GV except in OD (tranny). When I mess up on this & downshift the tranny while the GV is engaged, the PCM figures there's slippage (input & output shaft rpms don't match the gear ratios in the tranny, which the PCM knows) & commands full tranny fluid pressure, causing hard shifts.

Why does this work? For some reason, the PCM never checks for slippage when the tranny's in OD.

Why did I do it? In the GV-approved hook-up, as soon as the GV is engaged, the PCM thinks the vehicle is going 0.78x the speed it's actually going. This tends to make the truck downshift a lot more than I like. As mpg is my only hobby, I like my truck to get into top gear & stay there.

"I can get a Gear Vendors installed ...Anyone opine on this conundrum?" Send me a private message with your real email address & I'll send you an email with a spreadsheet attached that will allow you to make a reasonably well-informed decision. This spreadsheet allows you to type in what ANY mod you might be considering costs, what you get for mpg now, what you guess you'll get after the proposed mod, & the price of fuel, then returns exactly how many miles it will take to pay for your mod. This works REGARDLESS of what mod you're contemplating.

My best guess is that my mpg increased ~ 2 mpg after installing the GV. The added benefit is that you literally can't tell you're driving a diesel at highway speeds. With the addition of 4% oversize tires, I'm only turning 2000 rpm @ 80 mph. (A GV in front of 4.10 gears will give you 3.20 effective ratio, NOT 3.08 - 3.08 is what I've got with the addition of the 4% oversize tires.)

Another alternative is to build a mondo strong & powerful 6.5 like RJ's done, then you can have taller gears all the time, although I'm not sure anything taller than 3.73 is readily available for 14-bolt full-floaters. (I'd suggest a search - this has been hashed out B4; I didn't pay much attention as I already had my GV.)

Blessings!

(signature in previous post)

[DA BIG ONE]

My question to the group is this: I can get a Gear Vendors installed turn key in Houston for $3,400, about double the price to lose my 4.10 gears. I have only heard/read glowing reports on how the 6.5 TD responds when given an 8 speed transmission that is corrected down to 3.08 final drive when not towing.
I am on the fence as to what to do with all this data y'all are pouring out on installing smaller gears against the future possiblilty that I will obtain the bumper-pull portable Texas Ranch House/pontoon barge (Texsas Reservoir Yacht) of my dreams and need to tow 9,000+ lbs once in a little while.
I like the idea of the gear change for the component of simplicity, I lile the idea of the second transmission for the reportedly phenomenal results.
Anyone opine on this conundrum?
As well Texas DPS is slowly changing the speed limits in the 'remote' areas (I live in a remote area) to 80MPH. I will somehow feel deprived if my RPM's are so high at 80 MPH as to force the stereo to maximum DB's and I still can't keep up with the lawyers from Austin in thier sporty German Turbo Panzerwagens.
Towing, MPG and MPH; is Gear Vendors the only true solution to all these?

If you can use the GV it would be a better option, and doubtful the tranny would search for the right gear like it does with only the 4L80e and 308's. The gear vendor was my first choice but never an option for me because of the NP246 AutoTrac.

I'm thinking the GV w/4.10's is the right route.......

Posted: 80mph= 90+ for most

[HH]

If I understand right, my 4.10 gears effective ratio with 235/85R16s is about 3.73 (plus or minus). And if I change to 3.73 it would be about the same as 3.42 and going to 3.42 it would be about 3.08? Currently at 70 mph I am turning about 2200 - 2300 rpms.

My K3500 dually has the NV4500 with the low 1st gear. I recently found a couple large chipped pinion gear teeth and I am contemplating changing to 3.42, but I am wondering if that will be too high with the 235/85R16 tires. Would I be better off at 3.73? I only tow boats and an open car trailer once in a while.

I would like the GV but it appears the payoff time is very long, the gear sets are about 1/3 the price and a shorter payoff, and I need to do the rear end anyway. I am currently averaging about 19mpg (summer blend) and about 14 mpg towing, and wondering how much better can the mileage get, maybe 2 - 3 mpg?