Transmission Opinion Question

[sjarrett71]

My 72 GMC currently has a granny low 4-speed in it. Eventually I want to go back to an automatic (it was originally an auto from the factory) but I found a 3 speed manual close to me that is reasonably priced. I don't know the different gear ratios in the transmissions and I understand the granny low is basically like a 3 speed with the low gear.

My question is, would going to the 3 speed benefit me in any way such as speed or gas mileage or lowering my RPM's?

Thanks.