I have a 85 bronco in which I swapped out the 300ci and slapped in a bored out 302.

then came the tranny swap. out with the crappy stock one and slapped in a lower geared cast one to pump up the strength.

now my question: I drive a decent distance with it. I want to keep the torque benifit I have with the cast tranny. but I run 3,000 rpm @ 75mph. I currently have stock tires. something arounf 27.5" tall. if I swap them for a 31x13.5 super swamper TSL giving the lower rpm's I will be turning 300-400 lower from the quick and dirty tire and gear ratio guide. will I optain better, worse, or the same gas milage?

 

You have to add up all the factors. Minuses: more rolling resistance (inflate tires to max PSI), more rolling mass, etc. If you don't recalibrate speedo, don't forget to factor when calculating mileage otherwise you definitely get equal or worse gas mileage. Most often I have seen little or no improvement in gas mileage just changing tires. I have even seen worse mileage especially with changes similar to yours. One big factor is where the power curve lies for your engine. If there is sufficient power available at the resulting RPM's you may overcome the minuses. Bottom line: if what you want is the mileage, then change the axle gears. If you are looking for an excuse for bigger tires, just get them.