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Mr_D · 01-15-2017, 04:24 PM

Quote:

Originally Posted by dbarton291

The sixth gear was there in the transmission from the beginning. It's only a programming change in the TCM. The reason a lot of vehicles get the five speed program is normally due to the vehicle's driveline not being able to handle the increased speed. The other reason not to have a six speed is the RPM the engine turns at the vehicle's cruising speed. You want the engine turning at the bottom of its fuel curve when cruising down the highway.

The fuel curve is curve that shows the brake specific fuel consumption of the engine in pounds of fuel per hour. The bottom of that curve is the engine's most efficient point for fuel consumption. It's also call the sweet spot. You want to cruise at the sweet spot if at all possible.

Yep, ya beat me to it!! And, as is also noted the 6 speed ones got different rear end gearing.

tmw188 · 01-15-2017, 04:30 PM

So can you explain the chart above a little better?

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Arch Hoagland · 01-16-2017, 09:51 PM

Quote:

Originally Posted by 4x4van

My 2004 WH chassis (W22) has the 8.1L V8 with the 5-spd Allison 1000. Yes, the 5th gear is considered an O/D. The gear selector and dash display shows P-R-N-D-3-2-1, then there is a switch on the dash for turning O/D on or off. In the off position, 4th (D) is the highest, in the on position, it will shift up to 5th (O/D).

My 2004 is the same. You need to find out if the owner changed the fluid every two years and 25,000 miles. The 2004 originally came with Dexron iii and the spin on filter should have been replaced after the first 5,000 miles.

I change all my oil and transmission fluids myself and it is really simple to change the transmission fluid as it has a drain plug just like the oil drain plug. It also has a spin on filter just like an oil filter.

I recently switched to Transynd fluid which is a synthetic made for the Allison transmission.

dbarton291 · 01-17-2017, 12:00 PM

Quote:

Originally Posted by tmw188

So can you explain the chart above a little better?

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That chart is a torque curve for an engine. It shows the engine torque produced at full throttle at a given RPM. They are normally produced by running the engine at full throttle, no load RPM, and then adding load and measuring the torque. It's also called a lug back torque curve.

That chart does not have the engine fuel consumption curve on it.

tmw188 · 01-17-2017, 12:37 PM

Quote:

Originally Posted by dbarton291

That chart is a torque curve for an engine. It shows the engine torque produced at full throttle at a given RPM. They are normally produced by running the engine at full throttle, no load RPM, and then adding load and measuring the torque. It's also called a lug back torque curve.

That chart does not have the engine fuel consumption curve on it.

Ok that's what I couldn't get from the chart was the fuel consumption curve. If I'm on a nice flat highway I just try to use the on dash consumption reading. Accurate probably not but it feels good trying.

dbarton291 · 01-17-2017, 12:45 PM

Quote:

Originally Posted by tmw188

Ok that's what I couldn't get from the chart was the fuel consumption curve. If I'm on a nice flat highway I just try to use the on dash consumption reading. Accurate probably not but it feels good trying.

I do the same thing in my pickup. I try to see what kind of mileage I can squeak out of it. It's kinda fun to see what I can get out of it when I try.

terry735001 · 02-27-2017, 03:36 PM

Quote:

Originally Posted by tmw188

At 65 and normal flat highway no wind I'll be at 2250rpm I thought too that maybe the rear end is geared different for the 6 speed.

it is .. they had to sizes on the rear end .. a 5 some thing and 6. some thing

i have a 4.7 so i am 1800RPMs at 65 and it helps me get 8 to 9MPG with no tow

they removed 6 speed over cruise control that the throttle computer would kick a MLD light on the dash when on any kind of hill as the computer would give it what it think is to much gas to not added speed so it would think some thing is wrong

there is a hack to get it 6th back on some RVs but i would not do it as most go back as the trans seems to skip 5 and go right to 6th and in the wrong drive speed would hold you in 4th so MPG would take a hard hit

tmw188 · 02-27-2017, 06:09 PM

Quote:

Originally Posted by terry735001

it is .. they had to sizes on the rear end .. a 5 some thing and 6. some thing

i have a 4.7 so i am 1800RPMs at 65 and it helps me get 8 to 9MPG with no tow

they removed 6 speed over cruise control that the throttle computer would kick a MLD light on the dash when on any kind of hill as the computer would give it what it think is to much gas to not added speed so it would think some thing is wrong

there is a hack to get it 6th back on some RVs but i would not do it as most go back as the trans seems to skip 5 and go right to 6th and in the wrong drive speed would hold you in 4th so MPG would take a hard hit

Ok? I didn't quite understand that, could you explain it again.