Looking for a check on calculation

[chmhasy]

So here is what I am trying to do. I am trying to calibrate my speedo using an Arduino
I am programming the Arduino to start the calibration. Send the 2 miles of pulses to the speedo. Then end the calibration
I am looking for a check on my calculation for how many pulses I need for a mile
my car is not finished yet to hit the road
according to the instructions you calibrate the speedo by getting into the ppm menu. press the button to start the calibration, driving for 2 miles, and then press the button to stop the calibrations.

The speedo takes a signal off the electronic sending unit from the transmission

So here is what I am looking for, Pulses Per Mile (PPM)
I came up with 33,469.48473 PPM

here is what I have

Nitto NT555 315/35ZR17. Tire Diameter from Nittos website of 25.67

rear gear ratio of 3.55
Tremec TKO TCET5008 has a output shaft with a sending ring with 12 teeth, 12 pulses per one revolution of output shaft
FFR speed hut speedo

Thanks

[Papa]

Edit: (I can't do math today) A tire that is 25.68" tall will make 785.32 revs/mile. Divide that by 3.55 to get the number of revolutions of your output shaft in a mile. Multiply that by the 12 pulses per revolution of the output shaft and I come up with 33,454.632 pulses/mile. Your number and mine are pretty close.

[karlos]

I love word problems :p. If the tire diameter is 25.67", then it will travel a distance of pi*25.67" = 80.64" in 1 revolution. There are 12*5280 = 63360" in 1 mile. So the number of wheel revolutions in 1 mile is 63360/80.64 = 785.7. In that same distance the output shaft will turn 3.55*785.7 = 2789.3 times. With 12 pulses per 1 revolution of the output shaft, the total number of pulses per mile is 2789.3*12 = 33471.

You guys are right on.

Got any more??

[CraigS]

That tire diameter correct for the tire w/ the weight of the car on it?

[NAZ]

Listen to Craig - the published diameter is not the same as rolling diameter. Best way to get rolling diameter is to air the tire to the pressure you will be running and put a mark on the sidewall extending down to the tread. With the tire mounted on the car at it's running weight, place a corresponding mark on the shop floor and roll the tire one revolution and mark the floor again. Measure the distance between the marks. You can convert this to diameter if you want but since you're already trying to calculate distance traveled from diameter use the raw "distance" number in your calculation to determine how many pulses per mile.

[Jeff Kleiner]

If you want to split hairs the difference between chmhasy, Papa and karlos' calculations is less than .0005%---maybe not quite close enough to bring the Space Shuttle back into the Earth's atmosphere probably plenty good for driving down the interstate :)

Jeff

[chmhasy]

Craig, I briefly though about weight on the tire . so I just went with the published tire diameter.
but I think I should be in the ball park.

It might be a fun project to take an Arduino and count the pulses from the tranny and drive a mile to find out what the exact number of pulses would be but I will have to wait to my car is done

[Big Blocker]

Once more, I have to go along with Jeff on this one . . . .0005% won't let the space shuttle land on the landing strip, more-than-likely on the grass to the left - but for us guys driving a Cobra (or for that matter, any other car that we have assembled), it'll get you down the road with just about spot-on accuracy without the fear of tickets for speeding. That last part will be all on you . . . Ha!

Doc