How To Calculate Your Car's Running Speed

This post is basically to help you answer this kind of questions:
"How fast am i going on 4th gear at 6000 rpm?"

There are so many ways to answer above question. Starting from the simplest to the most complex way. First one is of course by peeking at your speedometer, silly! Jumping to the third way, since i think it's the most complex and expensive way, is to have your car dynoed. There are of course other absurd ways like holding out a portable anemometer outside your window etc but why bother when you can do it this way..

The second method is by using a Gear Ratio Speed Calculator. This post will discuss this method. First of all, why would i bother doing this when my Charmant (or nearly all cars) already has speedometer? The reason is that i am going for a custom cluster gauge and as some of you may have experienced, aftermarket speedometer is one of the most difficult gauge to install with the gear ratio, speed sensor and all. So i am considering of not installing it. However, i also want to know if, once finished, The Dog can reach 200 kph or not. You know, just for my personal justification of the price i am paying for the engine ^ ^ So this method would help me pinpoint the 200 kph mark on my tachometer numbers..

Gear Ratio Speed Calculator is something i found on the net not so long time ago. You can download it here. I forgot where i got it so if you're the one making it, pls let me know. It's basically a tool which calculates these three: angular velocity, linear velocity and gear ratio. I won't bore you with math and all so i'll just explain how to use it. You need some inputs and they belong to the yellow and green fields as seen below.
The inputs are:
1. Rim diameter in inches
2. Tire profile and width
3. Engine rpm in question (on our case above, it's 6000 rpm)
4. Gearbox ratio for each gear position, usually found in owner's manual, brochure or just google it
5. Final gear or diff ratio, also usually found in owner's manual or google

In my case, i am planning to R14 175/65 tires so that's 14 for rim diameter, 175 for tire width and 65 for profile. Charmants use T50 gearbox ratio and by googling i found the following ratio for each gear:
1st = 3.587
2nd = 2.022
3rd = 1.384
4th = 1 --> meaning the drive shaft is turning as many RPM as the engine
5th = 0.861
For the last input which is the final gear ratio i got a help from a fellow Charmant owner here that it's 3.909..

So with those as the input, i can answer the question above: On the 4th gear at 6000 rpm, my Charmant is running at 168 kph!

I then managed to find that my Charmant is running 200 kph on 5th gear at approximately 6100 rpm. So, once i finish my custom dashboard panel, i'll know where to put the 200 kph mark on my tachometer.