We're guessing you've heard of throttle body spacers by now. They work a little differently than the old phenolic spacers your dad used to stick on his carbureted hot rod. Phenolic spacers are used to minimize heat transfer, and throttle body (TB) spacers are used to improve airflow into your engine (therefore increasing efficiency). And increasing the efficiency of your engine doesn't take any more fuel, but instead improves the energy extraction from your fuel, offering you more power from less fuel. And this results in better fuel economy and more power from your engine.
Sounds nice in theory, doesn't it? Especially when considering that throttle body spacers cost under $100.... The problem is that not all throttle body spacers are effective. Every company has their own secret method on how to create a better airflow path, and different TB spacers have different effects on different engines. Without going into a huge discussion on the design of intake manifolds and airflow passages, let's just say that some engines will see mileage increases but no power difference; with some you'll feel a power difference but notice no fuel consumption improvement; and with some you might as well not even have them on the engine. We've done a brand-X TB spacer on a '94 Chevy 350ci engine in the past, and noticed nothing at all except a cough at wide-open throttle. That was a waste of $100. But we have noticed a remarkable difference with [specifically] a Poweraid TB spacer on a Jeep 4.0L engine. Not only did we feel an off-the-line improvement, but we noticed our Jeep would stay in gear a little longer before downshifting during hill climbs, and our onboard computer tells us we're getting two more miles per gallon at cruising speed! There wasn't any big horsepower gain, but there was a definite improved throttle response, and we're loving the extra miles we get out of each tank of fuel.