I have noticed various posts stating what your rpm's will be at a given speed with a certain gear ratio. Since I don't know the formula, I'm hoping somebody will let me know what these changes would be...

I currently have '99 F-150, 4.6, 3.55 LS, 4x4, stock size tires.

If I were to install 4.10 gears, what would my rpm's be at say, 70mph?

Also, how much higher would the rpm's be at 70mph if I went with 4.56's?

If anyone wouldn't mind sharing, please just give me the info I need to figure this out for myself. I want to look at a dyno chart or two and try to decide what combination would be best for my driving habits and vehicle usage.

Thanks in advance!

Mike

 

Need tire size for calc's. Here are a few sites that do the math for you.
http://www.csgnetwork.com/multirpmcalc.html
http://www.thedirtforum.com/rpmcalc.htm
http://wahiduddin.net/calc/calc_speed_rpm.htm

JMC

 

Thanks JMC!

 

Gears are just percentages, so if you post what your current RPMs are at a given speed, we can calcualte what they will be with new gears.

(This assumes you change only the gears. Since it's an equation, everything else remains unchaged so they just cancel out. If you start changing tire sizes or other variable on us then you'll need the online calculators)