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Old 04-09-2001, 08:37 PM   #1
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How does it work? How can you tell what is the corrsonding speed to the ratio?

Example:
1st:3.14
2nd:2.52
3rd:1.92
4th1.45
5th:0.94

Final drive:4.14

How can you tell? Somebody please explaine.
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Old 04-11-2001, 12:13 AM   #2
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You also need to know overall tire height to calculate everything, so here's all the math...

Tire diameter= ((width in mm x aspect ratio % x 2) / 25.4) + rim diameter

That will give you the tire diameter in inches, which is exactly what you need for mph calculations. Example for a 225/45 R 16 tire:
((225 x .45 x 2) / 25.4) + 16
which equals 23.972 inches. So for every rotation of the axles, these tires cover 23.972" of road (when they aren't slipping of course).


Now, apply that to this formula with your gear ratios to figure out actual mph in a given gear at a given engine RPM....

MPH= (engine RPM x tire diameter) / (gear ratio x final drive ratio x 336)

Which for your gearing, my tire size and say 6000 RPM in 1st gear, you would get....

MPH= (6000 x 23.972) / (3.14 x 4.14 x 336)
which when simplified is...

MPH= 143,832 / 4367.87
which equals 32.9 mph.

And there you have it, gearing!


Ps- Torque multiplication theory is basically the same, just take the torque output of the engine at a given RPM and multiply it by the overall gearing ratio of a specific gear. That figure will give you effective torque to the wheels.
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Old 04-11-2001, 01:44 AM   #3
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damn, you smart man.
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Old 04-11-2001, 07:20 AM   #4
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Quote:
Originally posted by texan
So for every rotation of the axles, these tires cover 23.972" of road (when they aren't slipping of course).
.
I agree with the ending answer but to be more correct....

For every rotation of the axles, these tires cover 75.3" of road....

You need to multiply the rolling diameter of the tire by Pi in order to determine the rolling circumference. In your formula the "336" includes the "pi" factor as well as other conversion factors.
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Old 04-11-2001, 09:15 AM   #5
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ya, you guys are smart. Seeing as I'm posting here, What's all of the info on the tires mean?
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Old 04-11-2001, 09:37 AM   #6
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Thanks I under stand know
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Old 04-11-2001, 09:39 AM   #7
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Talking

"understand" I how can I do that
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Old 04-11-2001, 09:42 AM   #8
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How do you know all this? Did you read book about this? What are the title of the book you read if you did?
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Old 04-11-2001, 01:11 PM   #9
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Would you do the same thing for the 2nd gear and add what you got to the number for the 1st gear?
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Old 04-11-2001, 02:28 PM   #10
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If you want to find the max speeds in the other gears:

All you have to do is to substitute the gear ratio for this number and recalculate....

MPH= (6000 x 23.972) / (3.14 x 4.14 x 336)

Change the number 3.14 to the new ratio..

Example: For 2nd gear...

MPH= (6000 x 23.972) / (2.52x 4.14 x 336) = 41mph

This is what you generally learn in your Physics and math classes. I couldn't recommend a specific book, but look for a physics text.
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Old 04-11-2001, 02:57 PM   #11
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CraigFL- Doh! I knew I would forget something, I was typing that out at work . Thanks for keeping me on my toes .



SolReaver- There are no shortage of books on the subject of cars, the trick is finding one that caters to your specific level of knowledge and builds upon it. For entry level mathematics....

#1- "Auto Math Handbook" (it's not perfect, but will give you plenty of the formulas you need and has a nice, HUGE, conversion table index of nearly every form of measure in the back)

#2- "Fundamentals of Vehicle Dynamics" (reads like a physics doctoral thesis, but contains more information per paragraph than I can easily digest)

I myself have over 20 books on various car subjects, from braking systems to model specific tuning. The more you read the more you will know, but you should also try and balance that out with practical know-how.



Porsche- What did you want to know exactly? If it's just the breakdown of the various information on the tire's sidewall, go here... http://www.toyo.com/tire_basics/index.html
Hope this helps, peace.
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Old 04-11-2001, 06:24 PM   #12
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Where would you find these books? An automotive store like Auto Zone maybe? I also want to say thanks to texan and CraigFL
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Old 04-11-2001, 08:27 PM   #13
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try amazon.com first. I got mine at a well known automotive performance book shop in Hollywood, so unless you live near here you should check there first. You could also always check your local book stores and libraries, but I doubt anything like this will be at an auto parts store.
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Old 04-11-2001, 09:13 PM   #14
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Well, I live out in the country so local book sotre are out. Maybe the local library. Amazon sounds like my best bet though
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Old 05-03-2001, 08:52 PM   #15
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I have a spread sheet in excel that you can use with gear ratios to show their effect on MPH and a graph that shows a line, the more vertical the line, the faster the car will accelerate but the lower the top end, the more horizontal the line is, the less acceleration but the higher the top speed. I have a formula that gives 0-60 mph times that are going to be with in 10% of the true times, but this formula will NOT give you the perfect run, it will give u a run that is lower than the peak. I haven't intigrated it yet into the spread sheet but I'm close. anyone interested in having a copy?
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