You can use a Hydrogen system which costs about $1000 - $1500 and if you are in the USA there is a federal incentive that will give you a tax credit of up to $4000 for hydrogen fueled vehicles.
These systems if install properly will increase your fuel economy 40%-70%.
But, now you have to ask yourself, if you will own your car long enough for this to pay for itself.
Here is a website that sells these kits, but be careful as I have just started studying these myself and checking pros and cons of these systems.
Make sure that any kit you buy includes a Oxygen sensor conversion which will boost you fuel economy far greater and is needed for hydrogen boosted cars.