255/70R16 to 235/70R15

I have a stock 96 EB Explorer.
What would be the results of switching from the stock 255/70-16 tires to Explorer/Ranger 15" wheels with 235/70-15 tires?
Speedo going to be off? Better/worse mileage or acceleration?

Need a couple of new tires and found a decent set of 15" aluminum wheels with near new tires off a Ranger.

 

[Mbrooks420]

Speedo might be marginally off. Not likely enough to be a problem. The smaller diameter of the tire will probably be offset, mileage wise, due to less weight. Acceleration should improve, but also just by a small margin.