First I need to find out whats in there. I jack up the 89 suburban and mark the drive shaft and tire and count the number of times one turns compared to the other , then divide??

I have made some up grades in performance and added some 30" tall tires and have a 700r4 for a trans. I need to find out what gears I need to make all this work better with each other. This surbuban will mainly be used for high way trips pulling a trailer. Once I get the gears in there I will start to get the computer tuned to get the most mileage I can out of it but thats another story. Any links on calculators that will get me the gearing close to where I need to be? Or any advice, thanks