ORIGINAL: Twister

Let's face it, there is a lot of mis-informationabout what a rear gear is and the ratios...A rear gear converts the revolutions of the driveshaft into revolutions of the rear wheel/tire. In other words, a 4.10:1 rear gear turns 4.1 turns of the driveshaft into one turn of the rear tire. The larger the first number is, the lower the gear ratio is, and vice versa.

A low gear ratio is for acceleration, while a high gear ratio is for a top speed. Drag racer's usually want to have as low of a gear as posible, since they are only worried about getting to the finish loe that is 1320 feet away, so a 4.10:1 is usually ideal for them. If you are driving your bone stock 1987 5.0 5 speed 200 miles on the freeway, the stock 2.73:1 rear gear ratio is better suited for you since your speed will be higher at the same RPM than a car with 4.10:1's.

No matter what, there are some constant things when it comes to cars, and they are all tied into math. Let us take that bone stock 1987 5.0 5 speed mentioned before and look at what it will do in 4th gear with a set of 225/45R17 tires. Now, we are also going to set another guideline before chaging the rear gear ratios, this car is going to be running at a constant 3000 RPMs. I chose 4th gear since it is a 1:1 gear, meaning that each time the flywheel of the engine turns, the driveshaft will turn once also.

The new 225/45R17-sized tire has a calculated diameter of 24.97", a circumference of 78.44" and will roll 835 times each mile, no mater what gear you are in, what your gear ratio is, or your RPM that the engine turns.

2.73:1=81.67 mph
3.08:1=72.39 mph
3.27:1=68.18 mph
3.55:1=62.80 mph
3.73:1=59.77 mph
4.10:1=54.38 mph

now, lets mix it up a little, and see what RPM that you are running with the same car, tranny gear and tire size with different gear ratios at 65mph....

2.73:1=2388rpm
3.08:1=2694rpm
3.27:1=2860rpm
3.55:1=3105rpm
3.73:1=3263rpm
4.10:1=3587rpm