Chev 350 or 250

Question to all, how much better gas mileage would a 250 get compared to a 350, thinking about doing a swap in my chev p/u
?, Thanks

 

A 250 and 350 share the same gear ratios. The 250 has smaller capacity clutch packs and a second gear band instead of an intermediate clutch pack and sprag as the 350 does, so there may be some very small frictional benefits. I don't think the trade off in durabilty for no perceptible difference in economy is worth the effort.