Since I have been looking at gear choices I wanted to add to this thread. The chart above is based on 70 mph. Seeing as not all of us cruise at 70 mph I wanted to add in the math so anyone could figure out what they could achieve at lower speeds. I typically cruise around 60 mph and I feel that I get the best fuel economy when my engine rpm's are around 2100 so i will be looking for a gear set that puts me in that range at 60 mph.
A couple key things to know are:
-There are 63360 inches in 1 mile.
-Knowing the actual diameter of your tires will give the best results. For the sake of this post I will be using my tire size. A Nitto Terra Grappler in 295x70r18 with an actual diameter of 34.41"
-The circumference of a circle= Diameter x Pi (it's a button on your calculator).
So we begin by finding the circumference of our tire.. 34.41 x Pi = 108.1
This says my tire will travel 108.1 inches in 1 revolution. I know that there are 63360 inches in a mile so by dividing that number by 108.1 tells me that my tire will rotate 586.12 times in 1 mile. At 60 mph that also equates to 586.12 rpm. (if you want to work at any other speed you need to multiply the number of tire rotations in a mile by the speed you drive. Then divide that number by 60 and you will have the number of rpm's for your tire).
Next is looking at the gear ratio for your ring and pinion. Remember, the ratio is pinion to ring gear. If i choose a 5.13 ratio that means the pinion is turning 5.13 times for every 1 time your ring gear turns. (or you can think of it as your driveshaft turns 5.13 times for every rotation of your tire). So the math would look like this: tire rpm x gear ratio = driveshaft rpm
586.12 x 5.13 = 3006.8
We know that 4th gear in my 08 auto trans is .69
So we take 3006.8 and multiply that by .69.
3006.8 x .69 = 2074.68
So with my tires, a 5.13 gear set and me driving 60 mph. My engine rpm's will be about 2075. Thats not bad.
If I step up to 5.38's the last bit of math looks like this:
586.12 x 5.38 = 3153.32
3153.32 x .69 = 2175.79
At 60 mph I would see engine rpm's at 2176.
If you have the manual trans you would change the .69 to your last (6th) gear ratio.
I will put all of this into a simple to use excel sheet. If anyone wants it I will email it to you. With the sheet you would just have to input your tire diameter, cruzing speed and axle gear ratio. It will do the math and tell you what your engine RPM's will be.
Hope this helps!