were not trying to put a a body rear under his b-body car. He is looking at A-body 'o' type gears thinking they will fit his 'o' type full size rear. THEY are not the same. full size 'o' type rears are 12 bolt guts a-body 'o' type guts are 10 bolt. They have aftermarket 'o' type a-body gears ...but as fart as i know at this moment in time there are NO full size b-body 'o' type gears on the market. JUST nos stock gears. wow I hope that clears the air...lol.
As for the gas mileage thing...I think your reading into that thread too far. I am sure the open rear that is being replaced in the above thread was a freeway gear and the posi was a steeper gear. and was wondering if the gear change was going to make a difference. Probably but will never know how much cause the original ratio was never provided.
Just because a thread is old does not mean it can not help someone into the future. Besides it got alot of people talking right brainstorm it and will get the right info HOPEFULLY.
? Jim