VSS Calculation

[FUBAR]

Does somebody have a formula or calculator for figuring out the number of pulses per mile for our VSS's? {E6508} -Thanks

[Motorzane]

Check this web site out.
http://www.miata.net/garage/tirecalc

Or you can mark your tire with a piece of tape at the 6 o'clock position (side of it) and then make a mark on the ground next to it (starting point). Then roll the truck forward or back one full turn so the mark is back at the 6 o'clock position again. Make another mark on the ground. Now measure how far apart the 2 marks are apart in inches i.e., 102 inches. There are 63360 inches in a mile so divide 63360/102 which equals 621 inches and that tells you how many revs per mile the tire will make.

[FUBAR]

Thanks Motorzane. I have figured how many tire revs per mile for my tire, but what I do not know is how to calculate pulses per mile for my Vehicle Speed Sensor. I know LBZ Duramax/GM has a speedo calculator built into V7.5, but oviously their sensor does me no good on my Dodge. Thanks though!

My way of thinking is, for example if it takes 100 (numbers are purely used for examples and reasoning) pulses for my 35 inch tire to equal 1 mile, then stepping down to a smaller 32 inch tire would mean it should take more pulses..say 125 pulses to equal one mile?? Right? But I don't have a starting point though. Even my fresh flash from the dealer, according to it, is different than other stock file VSS values.

[LaserBob]

I am a rookie at this EFI Live, so forgive me if this is just not right.

It appears to me that {E6500} is a tire Revs Per Mile table. This should do the job in my estimation.

LaserBob

[FUBAR]

You would think....I just wonder if the few who get the tire size to work via EFILive are changing both tables instead of just revs per mile table. The final outcome the ECM sees is what it gets input from..a sensor (common anatomy of the CAN system) [mail routing essentially]...the VSS. So unless we alter it via ABS/(ECM?) flash from the dealer or get it to work via EFILive. Unless the ECM can retain its original pulse per mile and take in the sole alteration of the revs per mile value, and adjust from there...kinda like when we rescale axis's. Someone with better understanding of the ECM computation process may be in order here to help or someone who has successfully calibrated the speedo on a 5.9L.

[FUBAR]

I just don't see how if you only alter revs per mile value, but the actual pulses per mile value stays the same, the output changes. If the value in the ECM is 800 pulses per mile, for every 800 times it passes the vehicle speed sensor, its gonna be a mile, period. That's what's got me hung up trying to figure out how many pulses for my given revs per mile.

[dsltnr]

I just don't see how if you only alter revs per mile value, but the actual pulses per mile value stays the same, the output changes. If the value in the ECM is 800 pulses per mile, for every 800 times it passes the vehicle speed sensor, its gonna be a mile, period. That's what's got me hung up trying to figure out how many pulses for my given revs per mile.

FUBAR, I am struggling with the same thought. I installed 20" inch tires and wheels and input my revs per mile of 590 but the speedometer is not correct. This has to be due to what you are talking about.

Do you set the pulses per mile to determine the speed or is that simply the number of times it will pulse to interpret speed...does that make sense?

[FUBAR]

I may be wrong, but I believe it's simply setting the value how many pulses per mile. So for every 'x' amount of pulses I set in EFILive, it will acknowledge that has being one mile traveled.

[dsltnr]

I may be wrong, but I believe it's simply setting the value how many pulses per mile. So for every 'x' amount of pulses I set in EFILive, it will acknowledge that has being one mile traveled.

If that is the case then what does tire revs per mile have to do with it? For example, if the revs per mile were correct but the pulses is wrong then you are saying the speedometer would be wrong SO if the pulses was correct then would the revs per mile even matter or are the two referencing each other and they both have to be correct?

[LaserBob]

Well, to maintain my simple approach, I drove OTR trucks for 23 years. When I wanted to change tire size, I would change the revs per mile in the electronic speedo head.

I could actually compensate for tire wear as they wore down. The tires would have as much as 18/32" of tread depth when new and we would run them down to 4/32". That is a big difference on a 24.5" diameter rim plus the tire height.

As long as I knew my tire revs per mile and adjusted for that, my speedo was spot on.

Maybe math no longer works like that. Please explain to me why.

I may just go and change my revs per mile on my 06 Dodge to see if math still works.

My last thought on this, The number of VSS pulses does not need to change as it references the transmission output shaft rotation vs rear end ratio, correct?

LaserBob