searched this out from the past , but am dealing with the same sanario right now. i already have had 35's on my 06 250 3.73's for some time. problem is it cant tow squat undder 80 mph and mantain speed. only at 1800 rpm at 70 (corrected speed) already made up my mind to regear but dont want to over gear either. looking to be in the sweet spot 2100-2200 rpm at 75. 4.10's or 4.30's. anyway to calculate this before the swap.
That generic calculator Troutzilla linked will give you a ballpark number. But if you want a more exact number you have to do the math yourself. For example, that calculator says my rig should have 1,947 RPM at 70 MPH. But it has 2,000 RPM at 70 MPH. That's about 2.65 percent error.
The key to the math is the percentage change in tire revs/mile, and the percentage change in rear axle ratio.
Stock tires (265/75R17 on a 2006 F-250) and 3.73 ratio with calibrated speedo should be about 2,150 RPM @ 75 MPH.
Stock tires have 660 revs/mile. 35s have 599 revs/mile. 660 minus 599 = 61, divided by 660 = 9.24%
3.73 + 9.24% = 4.07, or call it the common ratio of 4.10.
So you would need a 4.10 ratio to get back to the "feel" of power of stock-size tires if tire diameter and rear end ratio were the only factors. And the 4.10 ratio would give you very close to 2,150 RPM @ 75 MPH.
But other factors include wider and taller tires that have more mechanical and aerodynamic "drag" than stock-size tires. And that increased drag means that with 35s and 4.10 ratio, you still won't have quite the feeling of the power and torque you had with stock-size tires and 3.73 ratio.
So you'll probably be happy with 4.30 gears if you want the "feel" of a smidgen more power than the truck had with stock-size tires.
The 4.30 ratio is about 6 percent shorter legs than the math shows you need, so you have to add 6 percent to the 2,150 RPM you had with stock tires @ 75MPH. So 75 MPH with 35s and 4.3 rear end should be close to 2,275 RPM.
The numbers assume your speedo is calibrated to 601 tire revs/mile. You really need about a 4.20 ratio to nail your goal. But I don't think anyone makes that ratio, so you have to compromise with either not quite as much feeling of power as stock, or with a bit more power along with a bit higher RPM than your goal.