To add to what Blue Bowtie said, also the weight of the vehicle, gearing etc would help us make recommendations that are tailored specifically to your application, in order to extract the maximum amount of smiles-per-gallon that can be had.
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1988 9C1 - Modified LM1 @ 275HP/350TQ - TH700R4 - 3.08 8.5" Disc Rear - see it at
http://www.silicon212.org/9c1!
2005 Crown Vic P71 - former AZ DPS - 4.6 liters of pure creamy slothness!
1967 El Camino L79/M20 old school asphalt raper
Remember - a government that is strong enough to give you everything you need, is also strong enough to take everything you have.