How many MPG should a 4.3L engine get on average?

[stillstrong170k]

I have a 1990 k1500 full sized 1/2 ton 2wd P/U that gets about 14 mpg on average. To me this seems pretty lousy. I've done everything possible in the way of a tune-up and using higher octane fuel and nothing changes. The truck is hands down one of the best and most reliable trucks I'll probably ever own but the oil companies are making it pretty hard to feasibly drive the 100 or so miles a day I put on a vehicle. But luckily I have an alternative to this problem, or I should say I have about 95 percent of the alternative. The question I am about to post probably belongs somewhere else and I realize this since it's not directly related to chevrolet trucks (unless some came stock with the same bi-fuel system), but I do not know where to start. At least not on the internet anyway. So before I take the truck in to a specialist in this area who will probably try to gouge me for all he can, my question is this: What type of tank and where could I find a tank (A good used tank preferably)for an aftermarket CNG (NOT PROPANE) alternate fuel system? As well as anything else I made need to know about running a vehicle on CNG instead of gasoline. I bought the truck with everything needed to burn compressed natural gas EXCEPT for the actual tank needed to hold the fuel. I am not thrilled about taking up precious bed space since I need the truck to work but I am even less thrilled about the rapidly approaching $3.00 for a U.S. gallon of gasoline. Can anyone help me out? Would be much appreciated. Thank you.