2002 2500HD 8.1L/Allison Questions

snyps · July 26, 2005

I am new to this board and read as many posts about the 8.1L and Allison Tranny combo, but I still have a few questions.

I am looking to buy a 2002 Chevy 2500 HD LT, with the 8.1L and Allison Tranny in it. It currently has 34K on it, and seems to ride, and run great. I looked for the Piston Slapping, and Honking noise from the tranny that seems to be a common problem with this truck, but it isn't present. But my questions are.

1) What kind of MPG can I expect without modding the Chip, Engine, Exhaust or Intake? I realize this isn't going to be too fuel efficent, but seing it is exempt from the EPA regulations and the dealer has no info on the MPG, true numbers are hard to find.

2) Small cluck when shifting from Drive to Reverse, is this normal? I have heard this on a D-max with the allison before, and that owner said the dealer claims it is normal (Could be a line of bull from the dealer not wanting to warentee it!). The clunk comes from under the mid to rear of the truck, but isn't loud enough to hear from inside the vehicle.

3) Any other problems that are common with this Engine/Tranny combo.

Thanks

BigBlockHead · July 26, 2005

I have an '03 but I would expect similar results from an '02. I got 12mpg on the highway driving it home when I bought it and haven't seen double digits since, but I've never run another tank through that didn't have at least some towing. Mine's a crew cab, long bed, 4X4, 4.10:1 The only mods are 265-70X17 tires and a Flowmaster exhaust. Hope this helps.

Gary8.1 · July 26, 2005

I have a 2002 2500HD 8.1/Ally with 4.10's with 255/85/16 tires and a hyptertech programmer.

In regards to milage I get about 9.5 average in town and 13.5 on the highway. If you keep the RPM's at or below 2000 you can do ok, but if the RPM's go about 2000 you can start to see the gas gauge move.

My truck also makes the clunk from shifting from drive to reverse. It seems to be alittle bit more noticeable on the 8.1 versuses the Durmax because the 8.1 idles a little higher.

My truck also developed an oil consumption problem at about 7000 miles. Finally got it resolved by having new intake manifold bolts put in that could be torqued higher and the valve covers started to leak. It know only uses 1/3 qt every 3000 miles.

Hope this helps,

Gary

Bart2500HD · July 26, 2005

Mine with 4.10 gears gets an average of 9-9.5 MPG in town and around 12.5 MPG on the freeway at 72 MPH. Towing is anywhere from 5.2 to 8.5 MPG depending on load and speed. I agree, if you can keep the RPM's low, gas mileage will be better.

The power is great and I absolutely love the Allison. Mine uses about a quart of oil between oil changes. I don't have a noticeable clunk when shifting.

A couple weeks ago, I pulled this setup at 65-70 MPH against a headwind of about

20-25 MPH and got a whopping 5.12 miles per gallon!

snyps · July 26, 2005

Oil consumption is normal in any vehicle, just as long as it doesn't drip all over the place. I have to say, my current vehicle (soon to be traded in) a 94 Jeep Wrangler with the 2.5L and about 100,000 miles hasn't leaked a drop, but then again hamsters don't require oil..

Seems like the clunk might be a normal thing..

snyps · July 26, 2005

Mine with 4.10 gears gets an average of 9-9.5 MPG in town and around 12.5 MPG on the freeway at 72 MPH. Towing is anywhere from 5.2 to 8.5 MPG depending on load and speed. I agree, if you can keep the RPM's low, gas mileage will be better.
A couple weeks ago, I pulled this setup at 65-70 MPH against a headwind of about

20-25 MPH and got a whopping 5.12 miles per gallon!

<{POST_SNAPBACK}>

Not bad, with fuel prices, that is about $.48 a mile..

ABQFirefighter · July 26, 2005

I get around 9.5-10 in town, 6.5-7 towing my 33' 11,000lb 5th wheel. I do have the "clunk" you are referring to. I was concerned when I first bought it (used), but I watched my buddy's 2003 Allison video and it addressed all of my concerns that I had with the tranny.

Now don't let the mileage scare ya. There is nothing like a BIG block. The sound and power are just incredible. It might get bad mileage while towing, but I never have a loss of power (can't answer for the 6.0). Good Luck on your decision, I know you will make the right one!

01Sierra · July 26, 2005

Well, I only have about 100 miles on mine so far. I haven't noticed any clunk, at least not yet anyway. As for mileage, too soon to tell, but I bet mine can't be much more than 6 or 7 to this point. It's hard to get good MPG when you're having a blast with your foot in the pedal.

FlaMan · July 26, 2005

I have a 2002 LT with the 8.1/ Allison, 4X4, crew cab, loaded, leather, etc. that just turned 50K but I have babied it and it is in great shape. I have had no problems and the MPG is between 12-14 most of the time. I live in North Fla. so if you are interested, I could E-Mail you some pics! Call me at 850-997-6396

Thanks! Brad

I am new to this board and read as many posts about the 8.1L and Allison Tranny combo, but I still have a few questions.

I am looking to buy a 2002 Chevy 2500 HD LT, with the 8.1L and Allison Tranny in it. It currently has 34K on it, and seems to ride, and run great. I looked for the Piston Slapping, and Honking noise from the tranny that seems to be a common problem with this truck, but it isn't present. But my questions are.

1) What kind of MPG can I expect without modding the Chip, Engine, Exhaust or Intake? I realize this isn't going to be too fuel efficent, but seing it is exempt from the EPA regulations and the dealer has no info on the MPG, true numbers are hard to find.

2) Small cluck when shifting from Drive to Reverse, is this normal? I have heard this on a D-max with the allison before, and that owner said the dealer claims it is normal (Could be a line of bull from the dealer not wanting to warentee it!). The clunk comes from under the mid to rear of the truck, but isn't loud enough to hear from inside the vehicle.

3) Any other problems that are common with this Engine/Tranny combo.

Thanks

<{POST_SNAPBACK}>

Bart2500HD · July 26, 2005

I get around 9.5-10 in town, 6.5-7 towing my 33' 11,000lb 5th wheel.

Hey ABQFierefighter, what Flowmaster cat-back setup you got? How loud is it? Is it really bad when the truck downshifts for hills while towing that big 5er? I am thinking about getting a cat-back. My current factory system is going to mysteriously get taken out by a rock while deer hunting this year.

SandZilla · July 26, 2005

I own a 01 4x4 8.1/ally 3.73's and 265 tires with 65k miles on it now. Mine also has the clunk, been there since I go it. It will burn about a quart every 3k miles. This past weekend we took a trip to Las Vegas and I got between 14.2 -14.7 mpg (using the milage from the gps) with the cruise set at 73 mph.

When towing our loaded 19' 7400lb toy hualer I get 6-6.9 mpg.

ABQFirefighter · July 27, 2005

Bart,

I believe that the cat-back has the 3-chamber Flowmaster muffler. I don't think it's too noisy, but some people might. I like to hear that big block rumble! As far as towing, I just installed my O/D lockout switch and I'm getting ready to go to the lake tomorrow. We'll see if the downshifting improves. I does like to kick into 3rd though while climbing long grades (a little noisy with those RPM's), but it still has the power.

Bart2500HD · July 27, 2005

Bart,
I believe that the cat-back has the 3-chamber Flowmaster muffler. I don't think it's too noisy, but some people might. I like to hear that big block rumble! As far as towing, I just installed my O/D lockout switch and I'm getting ready to go to the lake tomorrow. We'll see if the downshifting improves. I does like to kick into 3rd though while climbing long grades (a little noisy with those RPM's), but it still has the power.

<{POST_SNAPBACK}>

I hear ya! I don't especially like it downshifting to 3rd on those long hills either.

But it is the best auto tranny I ever had.

moo snutz · July 27, 2005

I don't have the Allison but with 4:10s and running synthetic oil, I average 11.7 now on my 10 mile commute into town via interstate. I'm at 7,000 feet elevation FWIW.

I got just shy of 15 mpg on a recent trip to CO. with cruise set at 70mph.