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

[16gaSxS]

Hi DA BIG ONE;

Thankes for all the great input about gear changes. Your shift points changed on a per MPH basis but I bet are about the same on a RPM basis, that's what happened with me when I changed to 3.42's. This makes sense for the tranny controller isn't concerned about speed across the ground but engine speed for shifting. OD lock up happens about 1600 RPM if I am using very modest acceloration. That was about 47 -48 MPH with the 3.73 gears now OD lockup happens about 52-53 MPH with 3.42 gears but still 1600 RPM.
If lock up happened for you still ~ 45 mph that would be too low of RPMs and would "lug" the engine. It appears your thining in MPH's instead of RPM's.

I hope this helps you out. I appreciate you tire size observations it has helped me from going to a larger tires.

[JohnC]

Yeah, what 16ga said. If there was any increase in RPM relative to shifting it was due to the fact that you were harder into the pedal as a concequence of the taller gearing.

I just find it ironic that you worked so hard to get the shift points back up to the original speeds and now are contemplating driving around in 3 instead of OD. Could it be that your upshifts are happening too early?

RJ, you may be concerned with doing work faster. I gave that up a while ago... If time is an issue these days, I usually opt for less work.

a = dV/dt

[TurboDiverArt]

Does anyone have any experience changing the gearing using tire only but not reducing the effective gearing past 3.42. I think what DaBigOne was saying is that reducing the gearing to 3.08 is too much. I could choose a tire that reduced my gearing to about 3.50-3.55 I estimate. Stock tire is 30.5" tall and a 255/75/17 is 32.0 (but .4

[moondoggie]

Good Day!

"I think what DaBigOne was saying is that reducing the gearing to 3.08 is too much." (Following based on 3.08 gears, NOT what DaBigOne may or may not have meant.) For my 95 pickup, with 4L80-E in OD, GV (Gear Vendors Aux. OD) engaged, & 4% oversize tires, that's my effective ratio. It pulls it just fine. In fact, the last couple races we went to, I started pulling our trailer (looks like the US Cargo SM727TA2 [Click in colored text] except it's 25' long instead of 27') in double OD * with no problems; I only did this if cruising 65 mph or more. As long as I anticipated the roll of the highway with my footfeed (the cruise would let it lag too much, causing a downshift, causing my tranny to code due to how I've got the GV electrically hooked up (non-standard)), it worked great, & my truck's bone stock - it's never even been timed in my ownership (80K miles to present).

I don't recommend doing this, but from time to time I'll be in double OD at 50 mph, which is ~ 1250 rpm. Within the range the PCM allows before commanding a downshift, it cruises & accelerates without a hitch.

You guys are just jealous because us GV folks are only turning ~ 1750 rpm @ 70 mph.

Blessings!

(signature in previous post)

*double OD: What us fortunate GV owners call it when the tranny's in OD & the GV is engaged - 0.75 (I think) for the 4L80-E, 0.78 for the GV.

[DA BIG ONE]

It's been a costly ride changing out gears, wheels, & tires.

I'm thinking 3.42:1 w/stock sized tires is about the limit w/everything going down hill from there.

The third gear on highway idea came from the fact that the motor is no longer in it's best rpm range w/my setup. Defeated, or?

[moondoggie]

Good Day!

"Biggest lesson learned with much $$$$$ in my gear changeout is that you get good mpg until you start going larger tire diameter & width, then you arrive at the same spot you paid so much to get away from in the first place." I ran 235/85R16's on my pickup for a few years, which are ~ 4% taller than stock & 10mm/.4" narrower. As much as I take care about recording mpg, I can't determine what if effect they had on my mpg. So, I'm switching back to stock (245/75R16's) on both 95's.

Blessings!

(signature in previous post)

[Warren96]

'' As long as I anticipated the roll of the highway with my footfeed (the cruise would let it lag too much, causing a downshift, causing my tranny to code due to how I've got the GV electrically hooked up (non-standard)''

Moon ....,
What does that mean ....isn't the Gearvendors made for your truck?

[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?

[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.

[JohnC]

...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....

This is the setup I had in my '93. It was OK on flat ground towing a 5000' trailer. First is too low for most starts and 2nd is too high if you're on much of a grade. Also, all of the gears are spaced too far apart and you have to really flog it before shifting.

The mileage was great, though. However, if I had it to do over, I think I'd opt for the 3.73 or maybe the automatic.

Running empty it was fine.

[332bill]

For those of you that have gone to 235/85/16 tires how do you like them? Does the taller side-wall lead to more floatiness (is that a word), or other undesireable handling characteristics? What about 265/75/16's? The 265's are about the same diameter as the 235/85's and would look more manly, but I am concerned about the wider tire having more drag. So, for better MPG, with no other changes, should I go 235/85 or 265/75? Of course, the other option is stock - 245/75. BTW, I have 4:10's

Bill

[TurboDiverArt]

For those of you that have gone to 235/85/16 tires how do you like them? Does the taller side-wall lead to more floatiness (is that a word), or other undesireable handling characteristics? What about 265/75/16's? The 265's are about the same diameter as the 235/85's and would look more manly, but I am concerned about the wider tire having more drag. So, for better MPG, with no other changes, should I go 235/85 or 265/75? Of course, the other option is stock - 245/75. BTW, I have 4:10's

Bill

For what it's worth, tomorrow I'll have 265/70-17's mounted on H2 rims. It's about an inch taller than stock (31.6) and about 1/2" wider section width than stock (10.4

[TurboDiverArt]

[QUOTE=TurboDiverArt]For what it's worth, tomorrow I'll have 265/70-17's mounted on H2 rims. It's about an inch taller than stock (31.6) and about 1/2" wider section width than stock (10.4