The RPM Formula! - Lateral-g Forums

rockertype · #1 10-12-2005, 03:49 AM

Does anyone know where I can find that chart thap helps you figure out your RPM's according to tire size, gear ratios, ect.? If anyone wants to have a stab at it: 31" diameter tires, 4:56 gears, trans in overdrive (I think .75) Thanks guys!

**XcYZ** · #2 10-12-2005, 05:29 AM

Rockertype, I have a great Excel spreadsheet where you just plug in the tire radius, gear ratio, and rpm and it gives you everything you ever wanted to know for every gear in the tranny. It has the most common transmissions already in, so you just give it the value of which one you want. It's a great tool for comparing transmissions as long as you know how to use Excel.

TravisB · #3 10-12-2005, 05:53 AM

http://www.et-studios.com/motorsports/gears/gears.html

try this......seems to work ok!

Clockwork · #4 10-12-2005, 06:10 AM

I thought this one was pretty cool as well, check it out.

http://www.geocities.com/z_design_studio/

JamesJ · #5 10-12-2005, 05:17 PM

http://www.5speedtransmissions.com/gear_calculator.html Best one, IMO

rockertype · #6 10-13-2005, 12:52 AM

Man, you guys are the best! Thanks again for the info!!!

uneasyrider · #7 10-13-2005, 06:16 AM

Here is a calculation you can use.

mph = (rpm x tire dia.)/(gear ratio x 336)

Note: Tire diameter in inches.

Below is a ROUGH calculation you can use to find out the hp it will take to drive your car through the air at a given speed.

hp = mph^3 x frontal area/262500 and add about 30hp for rolling resistance.

Note: mph is to the third power.

Note: Figure the frontal area as the width of the car times the hight of the car in feet.
Note: The constant (262500) factors in the Cd (coefficient of drag) using a Cd of 0.4 as a generic estimate of a typical modern passenger car Cd. If your Cd is known multiply the constant by 0.4 and divide it by the known Cd to get a closer number for the constant.

This one is for 1/4 mile ET

ET = [(weight/hp)^(1/3)] x 5.825 (This formula is good. It gets close.)

Note: It's the quantity of weight devided by hp to the cube root and that quantity is multiplied by 5.825.

or it can be written as

hp = weight/(ET/5.825)^3

This one give you mph in the quarter mile

mph = [(hp/weight)^3] x 234 (This formula is also good.)

Here is an old drag racer's rule of thumb, the best shift point is the peak hp rpm plus 10%. Example: If your car makes peak hp at 5000rpm shift at 5500rpm. That is realy just a good starting point but NOT that EVERY set up works there.