some numbers:
last full system (gas and CNG) fill up 1/27/12
today I refilled both systems
mileage since last fill 506
3200 lbs of CNG
18.8 gal of dino gas
dino gas cost 3.55 pg
cng cost 1.50 pg
conclusions:
after driving 21 days without any fill up I am quite happy with the preliminary findings. typically my car would have produced 22264 grams of co2 gases but with the CNG in place it burns 90% cleaner so the gases produced would be far less than with normal fossil fuels. My goal in all this was primarily to reduce my emissions as much as possible and secondarily to increase range. My mileage is a hard one to calculate as you can't run on just one or the other fuel type, they both work in tandem. I really want to take a road trip now to see what the variable is on the highway but depending on how you calculate it I am seeing 26.9 to 29.1 mpg combined fuels, as I said it is not a direct gas or CNG conversion so a continuous highway trip will keep it in CNG for the whole trip rather than having it shut down then restart on gas and running 20 seconds before switching to CNG (typical in town scenario) the above figures were 100% stop and go in town driving and IF the impending doom of the $5.00 per gallon gas predictions hold true this summer I will be very happy that I did this conversion although I would rather not have that be the outcome.
Is it worth it??.....time will tell. the next phase is going to be a 2 month test average to see what it is doing over time. I was going to be more scientific but I didn't expect to be able to run 21 days between fill ups! technically I still had 6 + gallons of dino left so the range of the combination would have been 120 (+ -) miles before I needed to fuel up.

 

For any remaining skeptics out there I submit two charts. One is wholesale natural gas.

http://futures.tradingcharts.com/chart/NG/W

The second chart is RBOB Regionaly Blended Oxegenated Blendstock. 10% Ethanol Unleaded Gasoline to the rest of us.

http://futures.tradingcharts.com/chart/RB/W

NOTE the trends

 

...still no odd issues! everything works as advertised. I am still fighting the low fuel pressure code...it seems to happen randomly. When I drive the car normally it will throw the code but strangely enough when my wife takes it no codes appear...I am kind of stumped by that! I won't need a fill up for at least another 3 weeks based on what the gauges read...still no long road trip.

 

Jet,
Saw this today you might find it interesting
Enjoy
http://media.gm.com/media/us/en/gm/n...fuel_silverado

 

Just wanted to let you know I've been following this thread with interest and appreciate all the feedback, jet1! Feel free to send me a private message anytime if you have any questions along the way.

Best,

Katie
GM Customer Service

 

thanks! I appreciate the offer, and I may take you up on it. I also see in a recent WSJ that chevy is going to offer CNG as an option in the new pickup. I would be very interested in the research and development of this.

 

the pesky fuel low pressure...so far no joy on sorting this out...since it was deemed redundant on the 2009 and later cars I may just have to live with it. the other cel is the low cat efficiency measured on the cat outlet and I think it would take GMC boffins to adjust the threshold in the computer to sort this. simple solution...just buy an Ipod and software! It actually gives me a better handle on everything that's going on under the hood anyway!
The really other odd thing is that I haven't bought gas or cng since the last fill up last month! I am used to buying gas every two weeks and I am beginning to wonder if the cng actually gives better mpg than an equal amount of gas...there is no way to test this however. ran out of cng last Sunday and still have 1/4 tank of dino fuel!
to be continued.....

 

In the long run it's the bucks per let's say 1000 miles driven that matter.

 

filled the cng only a few days ago (3-14) and still have 1/4 tank of dino fuel. at this rate in town I will only have to buy dino fuel once a month and drive on 1.50 p.g. cng most of the time. I think I am sold on this! had several interesting conversations at the fill up station and found out EXACTLY what people who should know, DON'T know about the GMC hybrid products....at least they leave edjumakated and impressed!

 

It would be nice to see some type of official retrofit option for our existing 6.0L engines.

Spare parts kits will probably be available to the creative dealers anyway