Gearing Math Where did I go wrong?

[Achelea]

Alrighty, I decided to dust off the old brain box and try to calculate my mph for RPMs. I tried entering the numbers, but I think I might have erred... I'll give you the info I started with and maybe some one can tell me where I buggered up. ((This will be TLR length))

I looked up the gearing for my car on Tranny gear ratios for DSMs

1st 3.083
2nd 1.684
3rd 1.115
4th 0.833
5th .666
R 3.160
Primary 1.275
Secondary 3.866
Transfer Case 1.090
Final 4.929
Rear Diff 3.545

And at the bottom it adds in:

The "Primary" ratio is the Intermediate Shaft driving the Center Diff on an AWD or the Output Shaft on a FWD.
The "Secondary" ratio is the Output Shaft driving the Ring Gear on the Front Diff - the actual ratio of the axle shaft rotations compared to the tranny internals.
Primary x Secondary = final - this affects the rpm of the engine vs the rpm of the front axles.
Secondary/Transfer = Rear diff
The two must equal each other for an AWD car to roll without burning up the center diff (which is why you shouldn't run tires with different diameters F/R on an AWD).

Now I looked on Wiki to find out the way to do it.
Gear ratio - Wikipedia, the free encyclopedia

And below is where I thought I found out how to find my mph for rpms

With the gear ratios of the transmission and differential, and the size of the tires, it becomes possible to calculate the speed of the car for a particular gear at a particular engine RPM.
For example, it is possible to determine the distance the car will travel for one revolution of the engine by dividing the circumference of the tire by the combined gear ratio of the transmission and differential.
d = (Ct)/(GRt*GRd)
It is possible to determine a car’s speed from the engine speed by multiplying the circumference of the tire by the engine speed and dividing by the combined gear ratio.
v_c = (Ct*Ve)/(GRt*GRd)

Obviously I'd need my tire's Circumference to get started, so I looked up my tire's Diameter, which is 25.4 inches

25.4*3.14=79.756 inches around.

Hmm.. So it looks easy enough. Since my car is AWD, I thought I would need to add in the transfer case (Since it is between the trans and the diffs) And since I need mph, so I tossed in /12/5280*60 to change the answer from inches to feet, and then Feet to Miles. And then to find out how many miles in one hour.
So in the end, it looked like this.
Vc=(Ct*Ve)/(GRt*GRx*GRd)/12/5280*60
MPH=(79.756*RPM)/(GR1*1.090*3.545)/12/5280*60
And this is the result.
http://spreadsheets.google.com/ccc?...U53LU9nUF96cTZiaFdtb3c&hl=en&authkey=CKOn6PED

The portion that makes me think I might have buggered is the entries seem to be skewed later on. Look at 5th gear for example. I know the top speed for my car should be 140... so why does my math show 5k being 146?

5th Gear
1000 = 29.3482
2000 = 58.6965
3000 = 88.0448
4000 = 117.3930
5000 = 146.7413
6000 = 176.0896
7000 = 205.4378
8000 = 234.7861

Did I use the wrong number? Or is it an order of operation's error?

 

[19Eclipse90]

The method you give seems sound, but if I understand it correctly, you have used the wrong number in your calculations. I ran the numbers myself to double check and I obtain the following:

1st Gear - 3.083
(RPM) - (MPH)
1000 - 4.97
2000 - 9.94
3000 - 14.91
4000 - 19.88
5000 - 24.85
6000 - 29.82
7000 - 34.79

2nd Gear - 1.684
(RPM) - (MPH)
1000 - 9.09
2000 - 18.19
3000 - 27.29
4000 - 36.39
5000 - 45.49
6000 - 54.59
7000 - 63.69

3rd Gear - 1.115
(RPM) - (MPH)
1000 - 13.74
2000 - 27.48
3000 - 41.22
4000 - 54.97
5000 - 68.71
6000 - 82.45
7000 - 96.19

4th Gear - 0.833
(RPM) - (MPH)
1000 - 18.39
2000 - 36.78
3000 - 55.18
4000 - 73.57
5000 - 91.97
6000 - 110.37
7000 - 128.76

5th Gear - 0.666
(RPM) - (MPH)
1000 - 23.00
2000 - 46.01
3000 - 69.02
4000 - 92.02
5000 - 115.03
6000 - 138.04
7000 - 161.05

The way I look at it, ideally the wheels should be moving at the same speed, thus taking into account the transfer case is not necessary. You can simply use the final drive ratio and calculate the speed of the front wheels. The results I obtained seem very reasonable so I am fairly confident in their correctness. All you need to do to your code is replace: (B#*1.090*3.545) with (B#*4.929) and you should be set.