Fuel Mileage

2007 Classic Silverado Ext Cab Standard 6.5' box 4x4 Automatic w/ Allison + Duramax
When purchased (new)fuel mileage was about 12 mpg Than at 1,000 miles it jumped to 18.5 mpg
1,800 miles, fuel mileage dives to 8.5 mpg
Any Thoughts would be appreciated. Tx, Mike
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Was it all the same type of driving and was the tank filled to the same point each time? Also, was the mileage computed using actual mileage and gallons as shown at the pump or was it the average mpg shown on the driver information center?
I have an '06 Silverado 3500, crew cab, long bed, 4x4, dually, Duramax w/Allison. Over all, non towing, I have average 17.6 mpg. Summer driving in MT last summer I averaged 19.05 mpg for 6,000+ miles of non towing, but this past winter in south TX I only averaged 16.16 mpg for just over 5,000 miles of non towing. The difference is caused by summer diesel blend vs winter diesel blend and in MT it is almost all rural highway miles while south TX is almost all urban driving.
Ron
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I have calculated using the pump gallons vs miles. Fuel computer and gauge correspond to each other. It just seems awfully low. The power is great. But what caused the sudden change. I am using the 15ppm as the owners manual states. Your mileage seems great. Did you experience such low ><high changes? Tx, Mike

Allison
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I would not worry about it much at this stage as it is new and breaking in and idle time and AC usage play a role in MPG of it too. If it is running poorly or smoking black AND getting bad MPG, take it too dealer to have it checked out. ----------------- TheSnoMan.com
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I do realize it maybe breaking in , it just seems drastic. How long does it take for a diesel to break-in? My last diesel I purchased used with 9,600 miles and never experienced this situation. Idle time is minimum and yes the a/c has been used quit a bit but to lose 10mpg...... I did notice the tailpipe smoking (black) a bit but I had it idling when I was hooking up my trailer. Tx, Mike
wrote:

+
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THe MPG indication system though fairly accurate is not fool proof and if you are using it for MPG concerns, I would confirm the MPG with fillup data. ----------------- TheSnoMan.com
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Greetings,
I own a 2004 Silverado Crewcab Duramax with the Allison (5-speed) and have kept careful track of my mileage since new. In combined driving (city and highway commuting) I get a constant 18.5mpg - and I do mean constant. It's never more than one or two tenths off at every fill up depending on just how much city vs. highway time I get.
The only thing that has affected my mileage calculations is the change in tire size when I installed 265's as a replacement for the stock 245's. There is about a 3-4% difference in the riding diameter and now my mileage actually shows a little lower than before because the speedometer/odometer is reading lower than before (3mph - i.e. at an indicated 68mpg I'm actually doing 71mph per my GPS, and at an indicated 70mph I'm actually doing 73.) Other than that I've never had such a wild swing in mileage as you report.
The highest MPG I've ever gotten is just over 21 on a long highway trip up I-95 while empty, and the lowest was 16 while towing my camper and a truckload of camping gear and friends and family. If I ever got 8.5 mpg (or even 12 mpg) then I'd have that truck in the shop so fast it'd make your head spin.
Cheers - Jonathan

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I bought a 2003 Duramax new a few years ago. Despite the claims from the salesmen, it literally took 18,000 +/- miles for the engine to break in and for me to go from 13mpg to 20mpg highway.
Last September I replaced that truck with a new 2007 Duramax GMC Truck. I have approx 8500 miles on it and am averaging 15.7 mpg based on half towing and half nontowing and do not expect the mileage to upgrade and engine to be fully broken in until close to 20,000 miles regardless of all Gmc claims.
A short aberration in mileage like you described is common when I do straight nonstop highway miles in perfect weather conditions. Me driving, not my wife, noone else 65 degrees sunny no rain no stopping...Then it can return to the normal mpgs.....

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