False RPO rear diff ratio

[Gruffid]

I've been curious about my truck's mileage recently, as it hasn't been quite what I had expected. My first tank (from dealership) was 18.4 MPG (very light pedal). Since then I've been anywhere from 16.1 to 17.5. After doing the math (with the spreadsheet available from this web site), and re-checking the numbers with my own calculator & computer, it seems that I indeed have 4.10 gears - from the factory. Even though the RPO (GT4) shows that I'm supposed to have 3.73 gearing.

My RPM at 70MPH is ~2200. I understand most of y'all are running at ~2000 RPM - is that right?

The cheapest way to get better mileage is probably to go with bigger tires - are there any other suggestions?

Thanks, JT

[ 07-24-2004, 12:18 PM: Message edited by: JT ]

[Modified]

I

[saywhat]

I have the stock 245 tires and run 70mph at 2000 RPM.If the torque converter is not locked up, it would be turning around 2200 RPM at 70.Just made a fast trip down to Rockford ILL. on Saturday running 70 to 80 mph and got 20.1 mpg (395 miles).

[Runaway]

I'm running the stock tires and get the same rpm/mph as Modified as long as I compare to a GPS. Using the speedo, all rpms are about 100 - 150 less. My biggest complaint on the truck is the speedo being off by 1 - 2 mph.

[drgracr]

JT just went through the same thing with my new truck.. road tested the truck with tech2 and found that the ipc is out of calibration..the truck does have the proper gear ratio (3.73)
average mileage by the cluster is 15-20 empty hwy

[Spoolin'It]

Considering the rear in the diesel trucks is larger than all the other GM rears and only available with the diesel, I doubt there is even a 4.10 ring and pinon gearset made for it.

[Big Angry]

I thought the 8.1 gasser if equipped with the allison is the same rear (the 11.5") and is available w/ 4:10s from the factory. :confused:

[ 07-29-2004, 05:26 PM: Message edited by: Big Angry ]

[Colorado Kid]

It is, and it does. (The same in an 8.1L truck, and come with 4.10 gears.) Also our friends at Brand D use basically the same rear end and 4.10 is avaiable with the Cummins.

[DarylB]

When in doubt, pull the cover. 41 11 Teeth = 3.73, 41 - 10 = 4.10 ratio. Just 1 tooth difference on the ring gear on those two. If you're changing your rear lube after break in (500 miles or so) you could check it then.

I'd hate to imagine you got the wrong rear gears since your truck is a 4x4. Mixed ratios can be hazardous to your drivetrain! (and pocketbook)

[BassinRVer]

The Gas 8.1 rear end is not the same rear end we have. The 4.10 gears from the 8.1 will not fit our trucks. Believe me I know because I bought 4.10 gears off ebay (from a gasser 2500 HD) to put in my truck and they will not fit. I believe the gasser has a 10.5 inch rear end and we have a 11.5 inch rear end. And GM does not make 4.10 gears for our trucks. The front gears however are the same. I had to get aftermarket 4.10 rear gears for my truck. See this thread:

http://forum.thedieselpage.com/ubb/ultimatebb.php?ubb=get_topic;f=7;t=005901

[DmaxMaverick]

The 8.1L rear end is 11.5", same as the rear end the you get with the Duramax, no difference other than gear options.

Bassin...
You either got the wrong gears, or the truck it came out of was a 6.0L gasser. The 6.0L HD's have a 10.5" RE, and 5sp stick or 4L85E. If you received 10.5" gears, they did not come out of an 8.1L HD.

I have heard of several HD's that have been upgraded from 3.73 to 4.10 using OEM gears.

[Colorado Kid]

Bassin,
I've driven a truck with the 8.1L, ZF 6 speed and 4.10 rear end gears and it had the identical rear axle as my truck (see sig) and my in-law's (2500HD 8.1L/Allison). 4.10 was (is?) the only ratio offered with the 8.1L/6 speed. As I said above the 8.1L is offerred with 4.10 gears in the very same 11.5" ring gear 14-bolt AAM axel that our D-maxes use, and the Cummins 600 is offerred with 4.10 gears (I've driven one of them too) in an almost identical AAM axel (difference: no drain plug in the Dodge version).

The 2500HD and 3500 6.0L with either 4L80-E automatic or NV4500 manual transmission uses the 10.5" ring gear 14 bolt. . .same axel as a 2500 Suburban, 2500 Pickup and 2500 Avalanche, and similar to a 2500 Hemi Dodge.