the easiest ways to help improve gas mileage are full tune up (plugs, wires, fuel filter, gas treatment to clean injectors) replace air filter (i'm a fan of the K&N) make sure your tires are at the right pressure. these won't give you a whole lot of extra milage but it will restore any you lost with wear... other things would be looking online for different electronic components to control the engine preformance more exactly... couldn't point you in a good direction for that... since my truck is an 88 nothing just hooks up easily...
there's always the more expensive means to improve mileage significantly such as trading in your 22re for a 22rte (same thing only thicker cylinder walls and turboed) or special order a diesel 4banger from japan you also would gain more power from these upgrades
basic maintenance and keep teh vehicle clean.. and less weight basically
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