I was lookin in a JC Whitney catalog and it says these Throttle Body and Carburetor spacers will increase gas mileage 4 miles to the gallon or 10%. Is this true?

If it was true, don't you think Toyota and every other car manufacturer would already have this added to their engines? It would cost them next to nothing.

So your saying this wouldn't work and it's a waste of money?

In a word, yes

Logically, if something potentially this cheap and easy to add on does what it is supposed to do, and provide the OEM with such a marketing benefit ("Our truck now gets 4 more mpg and costs you as a consumer no more"), it would be adopted by OEM across the board. SInce it hasn't been adopted at all, it is not worth what the OEM would pay for it, which is pennies per vehicle.

Carburetor spacers do provide a benefit in that they can, if designed correctly, insulate a carburetor from the heat of the intake manifold and prevent vapor-lock and other heat-related problems with stored fuel (carburetors store fuel in the bowls). These benefits do not add mileage, but simply make the carburetor more reliable. They do nothing for a fuel-injection system.