i dunno if the system im bout to write bout is the same as adam's post but heck more info the better.
If you are looking for better fuel economy there is one thing you migth want to consider. LPG DRIVE
i dont know how common it is in the US but over here in europe its pretty common.
so what is LPG drive?
well basicly its an addon kit you can install in your car that lets you run on LPG gas, (propane)
-the most basic systems is almost a hose into the intake (uses a mixer u install on the tube between trottlebody and air filter)
and vacume valve that feeds gas according to map pressure. (stay away from theese kits they use a lot of gas and are way outdated)
Theese kits shoud only be used on carburettor cars, (or so says all the pro's ive talked to)
- the next step up the ladder is a oxygen sensor (hego) system. and this is a good option for us bronco pilots.
The basic idea is a gas drive ecu that feeds of the stock petrol sensors and the calkulates how much LPG to send to the mixer.
Theese kits work wery well on early (pre 1995) efi cars, and on carburettor cars,
but shoud NEVER be installed on a car with a plastic intake maifold due to possebilety of backfire.
both of these systems uses a unit called a Emulator to fool the stock petrol pcm to think the injectors have been fired when they have been closed.
and is reliant on the stock pcm to control ignition idle and so on.
The last type of system is a VSI system (vsi = Vapor sequential injection)
Theese sytems are pretty adwanced and works in a very simular way to our stock petrol efi PCM's
with iduvidual gas injectors for each cylinder.
the vsi systems usualy uses a hego sensor and input from the stock sensors. BUT require programming of the vsi pcm to work properly.
most vsi systems uses emulators to turn off stock petrol injectors and fool stock pcm to think nothing is wrong.
all theese systems lets you drive ur car on propane,
Yes, with the first 2 systems you will loose some power when ur driving on gas, but hey its damm cheap.
with a properly programmed VSI system you will not loose much power and they use less gas then the other 2.
On ALL of theese systems you can change from petron to gas and from gas to petrol wo any issues and while driving.
all modern systems have decent emulators and control units.
some other considerations when running a car on lpg,
1 ur engine shoud be in good working condition.
2 if you have a tiny crack or a developing crack in ur cast iron headers, exspect it to get bigger.
lpg burns a little bit hotter and on lpg ddrive old cast iron headers migth crack. myself im getting tubular headers for my bronc.
3 LPG gas has a octane rating of 110, but all engines ive seen that runs lpg runs great and smooth.
http://en.wikipedia.org/wiki/Octane_rating
4 u shoud read about it and make shure u know what ur doing if you are doing it urself.
A badly installed lpg system that leaks can be very dangerus. so best thing is to have a pro either install it or test it for you before first fillup.
5 I have NO idea on how inshurance companies in the us thinks about lpg converted cars.
6 LPG drive only works as on petrol cars, on a disel car you have to mix disel and lpg, and is mostly only used to help reduce fuel cost.
I hope my post has been useful, and sorry for bad grammar and spelling error's my native language is Norwegian not english
Edit:
as an added bonus, your petrol system is untouched, so if you want some performance, just press a button and boom ur back to the good old petrol,
