How can I calculate effective gear ratio's on my Yukon and what RPM's will be at certain MPH's with certain tire and gear combo's? I'm thinking either 4.10's or 4.56's with my 35" tires......

 

4.56 would be best

 

At least 4.56, if you do much off road I go to 4.88s

 

i wouldnt go with 4.88s unless i had 36-37" tires.... but its only a little more torque, so it is deff an option.... the 4.88 r&p is weaker than a 4.56 pack, so if u have plenty of power with those 35s i would just swap rear ends... then gear accordingly

 

What RPM's do you turn with 4.56's on 35" tires at about 80 mph? Also, what does the true gear ratio of 4.56's become with the 35" tires?

 

What size tire do you have now? Divide your new tire height by your old/stock tire height, then multiply your stock gears by that number. So 35/32=1.09 So if you had 3.73 x 1.09 a 4.10 would put you close to stock, or if it was 4.10 you would need about a 4.56. I dont know how tall your stock tires were, or what your gears are now, but that is how I figure it. If it is half way between 2 ratios, I go with the higher numeric to give it more power to drive the heavier tires. As far a "true gear ratio" there is no such thing. The only thing that matters is your truck, its mods, and what you intend to do. If you want this thing ot have good umph, do the 4.56. As far as RPM you would be looking at about 2450 in OD at 80 mph.