The 'gear ratio' is what everyone has said: It is the number of times the engine turns to make the tires turn. With a 4.10:1 ratio, the engine has to turn 4.1 times to turn the rear tires one revolution.
Inside the rear differential there are two gears. A ring gear and a pinion gear.
The ring gear is attached to the axels by means of a 'carrier.' If you lie the ring gear on the ground it will have teeth on one side and be flat on the other.
The pinion gear looks like a christmas tree, if the green part were gears and the tree part was attached to the drive shaft, (which connects to the transmission, which connects to the engine)
If you open up the rear differential and look on the outside edge of the ring gear you will see some numbers stamped on it. Those numbers tell you where it was made, the serial number, and the number of teeth the ring and pinion gears have. If you divide one number into the other you will know the gear ratio. For example, you may see the numbers that are on the ring gear in my 12 bolt posi:
2 GM 3862325 11 41 9 69
The 2 GM probably means the plant it was manufactured it (Eaton I'm guessing), the 3862325 is the serial number.
Th 9 69 is probably the month and year of the production.
The number of teeth on the ring gear is 41 and the number on the pinion is 11.
If you divide 41 by 11 you get the 'gear ratio' or the number of times the engine has to turn the pinion to turn the ring gear once. In this case 3.73:1.
There are other ways to get the rear differential ratio. For example, raise both the tires off the ground and turn the drive shaft by hand and count the number of revolutions it takes to make the one tire go around 10 times, then move the decimal point. I use ten revolutions to make it an easier number, like 41 or 37 (4.1 or 3.7) I also mark the shaft and differential with chalk, and the rim and brake backing plate to make it easier.
Another way is to open the rear differential and count the teeth on the ring and pinion gears and divide.
Knowing the gear ratio doesn't tell you every thing you need to know, but it gets you a long way.
Once you know the gear ratio for the rear differential and the gear ratios for your transmission you can calculate 'final gear ratio' for each gear. Most 4-speed and 5-speed transmissions have a 1:1 ratio in 4th gear. So with a 3.73 'final gear ratio.' With .72:1 5th gear ratio, like many of the t-5s have, your final gear ratio would be 2.69 (3.73 x .72 = 2.69).
You also have to take into account tire height to figure out the RPM's it'll take to go a certain speed. And with those bits of info and the max RPMs for your engine you can figure out the max speed.
When you know the 'power band' of your engine(i.e. what RPM range it produces the most power in), you can work backwards and figure out the best rear differntial gear ratio to use for your transmission and engine combo.
Here's A (just one) formula for RPMs. There are also many 'caculators' on line you can just plug the numbers into:
RPM =
(336)x(Rear Diff)x(Tranny Ratio)x(MPH)
_____________________________________
(Tire Diameter in Inches)
FOR EXAMPLE:
Rear Diff: 4.11:1
4th Gear: 1:1
Final Ratio: 4.11:1
Tire Hight: 29"
Speed: 65 mph
(336)(4.11)(1)(65)
__________________
(29)
89,762.40
_________
29
3,095 RPMs
The 4-speed with 3.08 rear differential ratio and same tires would need 2,300 RPMs, roughly!
The same rear differential with a fifth gear transmission ratio of .72, which is common on the t-5, would need 2,475 RPMs at the same speed with the same tires!
A five speed with a 3.42 rear differential would only need 2,300 rpms to go 75 mph!!!!! That means faster, less gas, less noise and stress on the engine, plus a big chunk of the power band still available to pass or speed!
