MPG Any thoughts?

[DA BIG ONE]

I have changed out my ring and pinion's from 4.10 to 3.42, finally calibrated the speedo with help from the members here, but the mpg is the same or worse. It is a 1999 GMC SUB 6.5TD L-65, INTERCOOLED, K&N, 3" exhaust, 4L80E and auto 4wd transfer case w/93k miles, it runs great but I am confused as to why the mpg is the same or worse?

Any thoughts?

[G. Gearloose]

Because it doesn't matter much unless you spend your day at 70 mph+, even then the difference is minimal and rarely worth the cost of gears.

[Scooby]

What is your MPG ??? As in previous post, higher gears will rarely help mileage unless the engine is grosely over reving and out of it's torque band. It would also require lots of highway driving too, to see any difference. In my experience, I have maintained the SAME mileage by higher, such as from 3.08 to 4.10, and that is because with the 3.08 the engine was not in it's torque band with the tranny in OD at highway speeds. Driving around without letting the tranny go into overdrive, yielded no change in mileage, so when I put in the 4.10 gears, mileage stayed the same when using overdrive. It really depends how you use the truck too. Heavy loads and lots of hills will do much better with the lower gears, and help the tranny not shift so much too. Higher gears are ok for flat land ( I live in Florida) and not heavily loaded. With the overdrive tranny, I would prefer the higher gears, and staying in overdrive longer on the hills. Thats my .02

[damork]

I run 3.73 gears in mine and over time I've observed this:

1. Economy is better with hotter thermostat - 195 is better than 180 (but not good for underhood temps with the PMD on electronic FI).

2. I got my best economy unloaded running 55-65, 195 thermostat on 105F day, probably at average of 60, where the torque band was at the peak - got 20.5 mpg.

3. At 75 I rarely got beyond 18.5 mpg.

4. I switched to DSG gear drive for the cam, but have not run the hotter thermostat to do a comparison.

[Marty Lau]

:confused: Well it worked for me....3:73 ratio I got 18.5 to 19 mpg at 63mph 1900~RPM at 75 mph 17 to 17.8 mpg. My last long trip I got several legs 75mph 19+mpg. :confused: DIIK!
I also pick up about 1.5 mpg in my "mixed" driving.
track your averages for awhile a couple of tanks don't tell the whole story.

[DA BIG ONE]

I was getting 17.0 highway with the 4.10's now 2 to 3 mpg less overall.

[Don Boland]

When this subject comes up, it seems the numbers and results run all over the board. Heck, even the RPM's don't match at least for me. I run about 2200rpm at 55? I get about 14.5/gal on the clock, but I know that I am running about 7% higher actual miles because of my tires so I figure about 15.5/gal actual. I don't think that is so bad with a 7200lb rig with 6 tires on the ground. I run 235/85's. Sure would like to figure out what the deal is on my RPM's though cause they never match anyone elses' that have 4:10's? :confused:

[moondoggie]

Good Day!

Speed vs. rpm? You might want to look at my 9 Mar 04 post in Speed vs RPM with 4.10 rearend (http://forum.thedieselpage.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=1;t=005936)(Click on the colored text.).

You might want to look at my post in What's your fuel mileage with a load? (http://forum.thedieselpage.com/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=1&t=004439#000014) (Click in colored text.) I'm pretty sure I'm getting 2 mpg more with the Gear Vendors, which takes me from 4.10 gears to 3.20 (4.10 x 0.78).

Blessings!

Brian Johnson, #5044

[Marty Lau]

Originally posted by DA BIG ONE:
I was getting 17.0 highway with the 4.10's now 2 to 3 mpg less overall. OUCH.......... :eek: :eek:
4:10 to 3:42 is a 20% reduction in RPMs, these engines or at least mine runs best cruising 1800-2000 RPM wich gives me 62-75mph. This should translate in something less than 20% increase in mpg at a given reasonalbe speed. You won't get all of the RPM reduction but 15% increase would not be unreasonable. I figured about 6%-7% increase with my axle change plus another 5%-6% increase with the exhaust. I calculated about 60,000 mile payback and I have more than double those on the change overs.

MAYBE MOREPOWER has some thoughts?

Tim posted this in another thread and I think he hits it well:

tswartos
Contributor
Member # 8901

posted May 10, 2004 09:42 AM
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I have the same gear ratio...your best mileage will be when u keep the RPMs in the 1800 range. I read it in a posting some time ago that also had a spreadsheet accompanying it...I believe its in the members area. It was a basic breakdown of tire sizes and rpm levels. That bit of info goes a long way in understanding how to drive the 6.5...once I figured it out, i was controlling my mpg instead of crabbing about it. I increased my tire size and backed my foot out of the gas pedal...that meant 16 mpg in town and 20 plus on trips. Now...if I dont feel like taking my time, I take a mpg hit and drop to 14mpg city/ 16mpg hwy while generally running at 2200 RPMs

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Tim Swartos
1995 Chevrolet Silverado K2500,6.5TD short box, xcab,73,000 mi, K&N air filter, 3.73 posi,265s

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Posts: 96 | From: Boise, ID | Registered: Dec 2002 | IP: Logged |

[ 05-10-2004, 01:50 PM: Message edited by: 16ga SxS ]

[patrick m.]

Don, i think everyone has different size tires. I have 265/75/16 and that is the factory tire for my truck.
some trucks came with smaller tires i guess.
at 75mph im right at 2500rpm.
and i quit checking my fuel milage, because sometimes a tank will vary 30 miles.
(i figure its my driving habits :D )

[whatnot]

Does it still have enough power to hold overdrive?
What size tires do you have?

Did you change to a different type of gear oil? I seem to recall seeing posts somewhere about synthetic reducing mileage. :confused:

[JoeyD]

Hopefully the gears were installed correctly and not dragging.

[DA BIG ONE]

Originally posted by whatnot:
Does it still have enough power to hold overdrive?
What size tires do you have?

Did you change to a different type of gear oil? I seem to recall seeing posts somewhere about synthetic reducing mileage. :confused: It has plenty of power, tires are 245/75/16 E, as for gear oil I do not know because a shop did the change over.

[DA BIG ONE]

Originally posted by Joey D:
Hopefully the gears were installed correctly and not dragging. How could I find out if they are dragging?