Will that really be a big factor in petroleum use? My driving is not dictated by the cost of Petrol (not yet anyway). I am not going to start driving twice as much if my car suddenly only uses 50% as much fuel.
Plenty of people might buy a large hybrid SUV if its fuel consumption was closer to that of a conventional compact car. Fuel cost goes into the calculation of whether to drive instead of taking the train, or carpool, or how much you're willing to pay in rent to live closer to where you work (or to mass transit), or how much you're willing to sacrifice in fuel to get home sooner, or whether to take a road trip, etc. etc.
It's also not all about cars. Coal and petroleum are used to generate electricity and heat homes. Make it cheaper to move the thermostat five degrees in the direction of comfortable and people will.