http://www.rctech.net/forum/electric-road/361591-gearing-confusion-newbie.html

Gearing Confusion

This is all about need and speed and a bit of simple math. Before we can give you a suggested gear ratio we need to know the "INTERNAL drive/gear ratio" for your car. Your manual should tell you what that ratio is. Every car or truck has one. It's the ratio the vehicle was designed with based on the gears in it's drive train that are usually NOT changeable.

When you drive the car...what you see is the result of the "FINAL drive/gear ratio". It's the ratio created after you factor in the spur gear size and the pinion gear size. For instance: On a TC3 chassis the Internal drive ratio is 2.50. If I use the stock 72 tooth spur gear and a 25 tooth pinion gear. To know the FDR I would divide the spur tooth count by the pinion tooth count. 72/25=2.88.
Then multiply that number by the Internal ratio. 2.88 X 2.50= 7.2 FDR.

The question is....what the heck does a 7.2 ratio mean to me ?? Does it make the car faster or quicker ?? In general...the larger this number is, the slower the top end speed of the car BUT...it will generally accelerate quicker. 2 key words SPEED and ACCELERATION. Again...another thing you have to learn....what do you want the car to do?? Have higher top speed or accelerate quicker ?? As an example...If I race on a very technical, twisty race course with very short straight sections....would I be more concerned about top speed or acceleration?? Top speed makes no sense because there’s no long section of track to use it. Acceleration is more important as I want to be able to make the turns and ACCELERATE quickly to the next one. So, I drive my car with that 7.2 ratio and I find its really slow on acceleration....I would then consider changing the FDR to a HIGHER number. Its much more normal to change the pinion gear rather than the spur gear. Easier too. Follow the general rule of thumb...You should make small changes in gear ratios. Change the pinion by only 1 or 2 teeth until you find the appropriate ratio. In this example lets calculate.....72 tooth spur/23t pinion= 3.13. Then 3.13x2.50= 7.82 FDR. Higher number = more acceleration.
This is a pretty basic explanation. It all comes with experience. You develop a sense for what gearing is needed based on the track type. And yes, brushless motors have different gearing requirements. Many manuals don't even mention the suggested gearing for BL use. But...you can compare to the listed motors as a guide. For instance...if the manual shows a 27t brushed motor gearing and you're using a 17.5 brushless system (which is comparable in performance) then start with that number and be prepared to make changes. Generally, you can use less teeth on the pinion to achieve the same performance when you use a BL motor. Run the car for a couple of minutes...evaluate performance AND CHECK MOTOR TEMPERATURE...then make gear changes.
Hope I helped. PM me if you have more questions. If I don't have the answer I'll get you to someone who does.

Gearing is gearing....no such thing as a pinion that’s to big. At least in theory !! But...from a mechanical standpoint the answer is yes...because of limitations at the motor mount or chassis structure...you can run into problems with larger pinions. Especially with brushless. Large pinions can be hard to get… especially in 48 pitch. For instance...I run RCGT class. 17.5 brushless. The ratios required run anywhere between the mid 3's and up to around 5.0. To achieve these on many chassis (particularly older cars and those like the Flux that are built primarily for the fun market vs. true racing) you have to modify the chassis to accept the large pinions... sometimes as large as 50-52 tooth. Many of the newer cars have motor mounts that allow for this.
But...there are times when simply changing the SPUR size will solve the problem. Most ratio changes are done via the pinion simply because you can normally adjust it over a greater range that way. Given the size of spur gears...there's normally only a small variation available. Shaft drive cars are more difficult in that respect than most belt drive cars.
And yes.... a change in ratio by just 2 teeth can make a big difference. Then again...it depends how far off you start with. If i have a drift car using a 8.0 ratio and I want to change it to a 4.5 ratio for racing....that’s going to take more than a 1-2 tooth difference.
If I understand you....your car at OEM is geared at 8.33 fdr ?? I'm not familiar with the 5700kph motor. I'm thinking that’s somewhere around a 19-27 turn brushed motor in comparison. If so, I'd suggest making changes 2 teeth at a time. I pulled up the Flux manual and it's useless for gearing info. I wouldn't hesitate to call HPI or Castle for gearing help. Just tell them what you're trying to do. I'm curious why HPI geared the car that way. Then again...it's more of a basher/fun car....I suppose they wanted to keep top speeds down.

Just to add a bit of technical info here:

Final Drive Ratio = FDR, is a number that tells you the number of motor revolution for one wheel revolution

e.g. if FDR = 8.5, it takes 8.5 motor revolution for 1 complete wheel spin.

You may heard of the term Rollout. Rollout is the distance traveled in 1 motor revolution. and here is the relationship between rollout and FDR:

Rollout = Pi x Tire Diameter / FDR

e.g. if your tires are 60mm in diameter, then Rollout = 3.1416 * 60 / 8.5 = 22.17mm

There is an FDR Calculator at this link forum.iowafasttrackrc.com/viewtopic.php