Get the circumference of your tires. Take a tire of the truck, and wrap a tape measure around it. That's how I did it, and came up with 73 inches on my 225/70R15's (which I have recently changed!)
Divide 63360....which is the number of inches in a mile by the circumfrence of your tire in inches. This will give you how many times your tire rotates in a mile...I got 868
Drive your van at 60mph in 4th gear for a manual, or 3rd gear for an automatic, and get a tachometer reading These are direct drive gears, which takes the transmission out of the equasion. In automatics, you may want to set the cruise control, and make sure you're driving on a flat piece of road to get an accurate measurment (torque converter locked). My tach reads a little over 2600 RPM
You see where this is going...60mph is a mile a minute...the tires are turning 868 times in a minute.
Here's what it looks like to find my ratios.
2650 tach reading
______________________________ === 3.05 axle ratio
63360in in mile
----------------- = 868tire rev's
73in tire circumf.
My numbers wern't perfect, because there is room for errors... speedometer calibration, tachometer calibration, tires compress a little under the vehicle's weight, etc.
I want that same RPM to occurr in 5TH gear, but I don't know the ratio of my 5th gear overdrive, I just picked .73 ratio for overdrive. But you can subsitute ANY gear you're in to find the RPM's at 60mph Here's the equasion re-arranged for finding an optimum ratio...
(3.42axle ratio) X (868tire rev's) = 2968 engine RPM's, direct drive
(2968eng RPM) X ( .73 5th tranny ratio)= actual RPM's
this equasion will give you RPM's at a differrent speed...I'll use 75mph, and 45mph for examples...
----- = .75 OR... ----- = 1.25
(2166RPM's at 60 mph) X (.75) = 1624 RPM's in 5th gear at 45mph!
(2166RPM's at 60mph) X (1.25) = 2707 RPM's in 5th gear
OK, OK, I know you aren't math majors, but that's what I did, I"ll post it on my web page too...