SCT PRP user...How to adjust for different tires.

[Intense RT]

First calculate the revolutions per mile of your tire.

Ex. 222/50/17 that is 27.1" tall.
27.1(pi) = circumference or distance traveled per revolution
27.1(3.1416)= 85.1376" per revolution

Revs per mile: 63360" in a mile so..
63360/85.1376= 744.2

To calculate your 3rd gear intial scalar ....Take 744.2 and plug this into the appropriate column in the Microsoft Works spreadsheet I will provide. Then enter your rear gear ratio. You do not enter the trans gear ratio as it would be 3rd gear and 1:1 anyway and thus multiplied by "1".

This will give you the N/V 48.6 for 3rd gear initial which is in the MISC category. You can go ahead and plug that number there into the scalar.

To calculate for the other gears all you do is multiply 48.6 by the gear that you are changing. This will be at the top in the 2-3 lockout category. Ex. 1st gear N/V....1st gear ratio is 2.45 so....
48.6(2.45)= 119 N/V for 1st gear
For Second gear, I wasn't as sure what to put since it was "2nd gear N/V for skip" so I calculated the 2nd gear N/V with the original "stock" N/V in 3rd gear which was 45. 45(1.45)= 65 but the default number that was in there was 89. That is ~37% more. So for now I took the new N/V and calculated this....
48.6 (1.45) + 37%= 97. I'll see if this works.

And "Max N/V for High Gear"
48.6(.69) for 4th gear = 33.5~ so rounded to 34.

The scalars only take even numbers so I rounded final values to nearest whole number.

Here's the N/V calculator given to me.

[emwhiteR/T]

I thought SCT couldn't do it at all?

[Intense RT]

My pcm # 56040145AG
Stock settings in my sct prp are:

Under 2-3 Lockout scalar
1st gear n/v : 111
2nd gear n/v for skip :89 - again not sure on this one ...read my first post

Under MISC scalar
Initial 3rd gear n/v: 45........double checked that
Max n/v for high gear: 32

The settings I have calculated for my 255/50/17 Riken Raptors.

1st gear n/v : 119
2nd gear n/v for skip: 97
Initial 3rd gear n/v: 49
Max n/v for high gear: 34

[hskr]

So since I'm running 4.10 rear gears, does that make a difference in the numbers, or could I tell Sean to use the value you have calculated since I also have the Riken Raptors?

[Intense RT]

Gimme a couple mins.

[hskr]

Oh, and just to get it into this thread, here are my "stock" settings Hemifever sent me

Here are your stock parameters :

Initial 3rd Gear N/V : 50
1st gear N/V : 111
Max N/V for high gear : 56

Which don't seem to match yours, but our PCM part numbers are the same.

[Intense RT]

3rd gear initial- 50
1st gear 125
2nd for n/v skip 78 and if you add 37% like I'm going to try, 107. I'm just not sure as, to be honest, I just noticed it last night. It saying "skip" threw me off and it's gonna be trial and error.
Max n/v for high gear 35.

All were rounded to nearest whole number.

[hskr]

The number you have for third gear initial matches what Sean says mine already is, which can't be right. And since our PCMs automatically adjust for different gear ratios, how would that effect the excel calculator which obviously wasn't written for a Dodge. Maybe that's why he doesn't do it anymore because there's no consistent way to set them from truck to truck??

[Intense RT]

Oh, and just to get it into this thread, here are my "stock" settings Hemifever sent me

Here are your stock parameters :

Initial 3rd Gear N/V : 50
1st gear N/V : 111
Max N/V for high gear : 56

Which don't seem to match yours, but our PCM part numbers are the same.Hell, not sure. I triple checked. That's what my default values are when I reload to stock settings in the scalars.

Try the ones in the above post I figured for you. It shows the same initial 3rd, 35. That's probably because n/v is multiplied by 1:1 and no need to factor in rear gear because the formula in that calculator accounts for it and would only multiply by 1 anyway leaving the same number.

[Intense RT]

The number you have for third gear initial matches what Sean says mine already is, which can't be right. And since our PCMs automatically adjust for different gear ratios, how would that effect the excel calculator which obviously wasn't written for a Dodge. Maybe that's why he doesn't do it anymore because there's no consistent way to set them from truck to truck??That's because it measures how many times the ring gear rotates and doesn't account, for example, your pinion rotatin 4.10 times to one ring gear rotation. I thought about this too and is the only thing I can figure. I'm not a math guru either.:D

Just want to clarify N/V is the ratio of engine speed in rpms to vehicle speed in mph. And if you consider the small amount my speedo is off with 27" tires instead of 28" )like going from 3.92 to just shy of 4.10s....then it makes sense that when I multiply the calculated n/v by the gear ratios, that it is a small change.

[Intense RT]

Here's a copy of a post on SCT PRP forum that got me onto this.

THIS IS HIS EXAMPLE HE GAVE. GEAR RATIOS, ETC WILL BE DIFFERENT.

This is how N/V is used.

I will explain it with my current transmission ratios and N/V.

Trans gear ratio
2.84 _ 1st
1.55 _ 2nd
1.00 _ 3rd
0.70 _ 4th

N/V
40.444

At any given RPM or MPH we can find the other.


To calculate MPH, when we have RPM, the formula would look like this.

RPM / (N/V * Trans gear ratio)

So if RPM is 2500 and we are in 1st gear, the approximate speed would be...

2500rpm / (40.444 * 2.84)

2500rpm / (114.85) = ~22mph



To calculate RPM, when we have MPH, the formula would look like this.

MPH * (N/V * Trans gear ratio)

So if MPH is 75 and we are in 4th gear, the approximate speed would be...

75mph * (40.444 * 0.70)

75mph * (28.31) = ~2120rpm
End of post.............................................. ...........


To use these formula's, use the calculator for n/v I gave you in my first post starting this thread. Plug in the revolution/mile of your current tire. Formula for that is above also.

[Intense RT]

I thought SCT couldn't do it at all?Man, mines been dead on since I adjusted it.

[hskr]

You using GPS to verify your mph accuracy?

[hskr]

All I'd really care about would be 3rd gear and OD anyways. Don't spend much time in 1st or 2nd gear where speed would matter.

[hskr]

Got this back from Sean

Also changed the n/v stuff. I'll be interested to see if it works. That's actually not the correct parameters to change. I did work with the Dodge dealership making tire changes and exporting the file data back to SCT. They see what parameter needs to be changed but its in a level they aren't writing too currently. If the n/v stuff works, great, but it's not the actual place that is changed when Dodge does it

[Intense RT]

Hmm, I wonder why they even put it there. I'm going out side in a few to do a trans filter change and adjust bands. Just got a gps recently(wife has it all the time) and if I get done quick enough, I'll take it down the road and see what the gps says compared to speedo.

[Intense RT]

Finished my filter/ fluid change and band adjustments and went for a drive with the Garmin GPS. I held first to 25 and held steady. Compared speedo and GPS and it was dead on. Did the same with second to 45mph, speedo and GPS were dead on. Turned off overdrive and held 3rd to 60, dead on again. Turned on overdrive and went 80mph, compared speedo and GPS and if it was off any maybe a half to 1mph. Not even sure it was actually off on these hilly highways and may have been going down in elevation a little bits sometimes. Sometimes it looked dead. Plus a factory speedo can be off up to 2mph at that speed according to some motor trend kinda mag. Anyway, mine works with the n/v adjustments.

[hskr]

Well, he made the adjustments to mine, so will see what happens tomorrow. Will have the steal the GPS from the wife for the day.

[Intense RT]

Well, he made the adjustments to mine, so will see what happens tomorrow. Will have the steal the GPS from the wife for the day.Same pcm, same tires...hope it works for ya.

[RJ_Hythloday]

wow, lots of Greek in here. I'm pretty confused, but good to know there's an SCT PRP forum, I'll see you guys there.

I've got 31" tires which would be 78.7 cm
3.92 gears
not sure what the aspect ratio is.

is section width my height? 787mm?
I've got 15" rims

So this will allow SCT to correct the tire size in the PCM to correct the speedo right? Pretty cool, I was told SCT wouldn't do it.

[RJ_Hythloday]

I get the feeling I haven't found the SCT PRP forum yet, this one (http://www.sctflash.com/forum/forumdisplay.php?s=&f=108&page=1&pp=20&sort=lastpost&order=desc&daysprune=-1) has no activity and shows no messages, says they are all private. Maybe I have to register w/ my PRP code?

[Intense RT]

I get the feeling I haven't found the SCT PRP forum yet, this one (http://www.sctflash.com/forum/forumdisplay.php?s=&f=108&page=1&pp=20&sort=lastpost&order=desc&daysprune=-1) has no activity and shows no messages, says they are all private. Maybe I have to register w/ my PRP code?Yep.:biggthumpup:

[RJ_Hythloday]

Yep.:biggthumpup:

I haven't actually opened the package yet, see you there soon.

[Intense RT]

I haven't actually opened the package yet, see you there soon.I'm rarely there at all. I just have an occasional question I need help with. Hardly any Dodge guys there and us being speed density, on a lot of things, they aren't able to help much. They were able to help with th n/v ratio for speedo calibration. You can get help with general tuning issues.

[RJ_Hythloday]

I'm rarely there at all. I just have an occasional question I need help with. Hardly any Dodge guys there and us being speed density, on a lot of things, they aren't able to help much. They were able to help with th n/v ratio for speedo calibration. You can get help with general tuning issues.

That'll work, I'm just looking for a general starting point, I can make small adjustments as I learn.