'06 Duramax MPG Question

[KellyD]

I don't know if anyone has noticed this or if it's just mine, but I've been watching my fuel mileage on the computer recently. I've been averaging around 14.9 - 15.8 according the "Avg Econ" computer readout for just putting around town and back and forth to work.

Well, I started playing around with the Tow/Haul feature, even though I'm not towing anything and just driving around. Turned it on and left it on. This appears to, among other things, keep the torque converter in a locked-up state the majority of the time for both accel and de-accel. Surprisingly my fuel computer started climbing my Avg Econ by quite a bit. After driving around for an hour or so, it went way up to 16.8. I've never seen it that high for driving around like that. Is this because of the shifting and converter lockup behavior changes? The engine rpm seemed to want to stay higher most of the time, which I would have thought would have negatively impacted fuel economy.

Anyone else tried this or experienced this?

-Kelly

[silverback]

I'm pretty sure your computer (DIC?) MPG readings are inaccurate due to the larger tire size. I may be full of BS but I recall other posts regarding the accuracy of the computer. Even if you have used a programmer to set the new wheel size I believe it only corrects the speedo and odometer.
-KL

[KellyD]

Originally posted by silverback:
I'm pretty sure your computer (DIC?) MPG readings are inaccurate due to the larger tire size. I may be full of BS but I recall other posts regarding the accuracy of the computer. Even if you have used a programmer to set the new wheel size I believe it only corrects the speedo and odometer.
-KL Why would this inaccuracy only show up when I hit the Tow button? It's had 35" tires since the day I bought it and it's appeared to be somewhat accurate so far.

Oh well, I guess if it truly is getting better MPG by changing the shifting/lockup behaviour then yay for me. I'll have to run a tank or two of fuel through it to validate it. I was just curious if anyone else has noticed this.

[KellyD]

I guess I'll add that the truck has 1750 miles on it now, and the computer is programmed with the exact size of the tire. (Not calculated like with the Edge product.)

[FrankA]

How did you get the 5's to fit? I just got a rough country 6" lift and the rub in (Front tire)back wheel well. I have the Dick Cepecks 35s on 16 inch rims.Did you have to use spacers or change the fenders and just start cutting plastic?
Thanks Frank

[KellyD]

Originally posted by FrankA:
How did you get the 5's to fit? It took just a tiny bit of trimming on the front fender. You really can't tell now, but they don't rub anywhere. Here's a pic I took... can't really see the fending trimming though. http://www.kellydrown.com/misc/kellysnewtruck1.jpg

[rob@rone.ca]

Be interested to hear if your tow/haul mileage improvement stays consistent, and bears up in actual calculated mileage for a tankful.

Regardless of tire size or other variables, the actual difference you've measured is amazing.

I have found though, on my 2005 that the DIC indications can vary quite a lot over short periods of driving, and it seems to depend a bit on the fuel level in the tank as well.

It seems possible that the lockup function would improve mileage, but one would think the higher RPM's would cancel out any gains.

Has anyone else tried running in tow/haul empty long enough to see if it makes any difference???

[Jim Brzozowski]

I suspect you may be right. The 35's changes your overall final drive ratio and makes the engine work harder to accelerate and of course cause more TC slippage than stock tire size. With the converter locking up you are probably seeing speed increases greater with the TC locked than without. Its interesting most of the time T/H mode causes it to use more fuel due to the higher shift points, but with the tall tire it probably helps move the truck forward and without TC slippage, probably less fuel, less heat buildup in the trans. fluid. Let us know how it checkes out at the pump.