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Here is the way to hand calculate your Revs Per Mile (the most accurate measurement). This is not my idea, I'm just passing the info along as it was given to me.


I know alot of us use the calculators out there to figure out the number for the sct or rely on the tuner to figure it out from tire size... well they are off. Lonnie brought this to my attention.

After correcting the number the shifting got alot better then what it was..


Here's the simple way if you don't already know how..

Put a mark or piece of tape on the 6 o'clock position of your tire(side of it) and then make a mark on the ground next to it(starting point) , 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 are in inches.. mine was 99"

There are 63360" in a mile..

so 63360/99 is 640 thats how many revs per mile the tire will make..

Iv been putting in what the calculator told me and that was 617.. Tires are 285/70/17s.

Yah that's a bit off now isnt it.. ;) :beer:

Go try it out for your self it really made my shifts alot quicker and nicer. only took my 2 years to learn this :hehe:
Ill add there was never any problems but this made a change that i like.
If a Moderator could please make this a sticky, I would greatly appreciate it, as I have gotten a ton of PM's asking for this information. :thumb:

Thanks!
 

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I used a dab of liquid soap on one of the tire lugs, rolled truck on concrete then measured between the two marks on the driveway.
 
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I do think hand calculating is always going to be more accurate.

I do have a question about the quoted section above. Do we know how much wear the tires had when he made the ground measurement? That measurement will change as the tire wears.
 
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Sure that tire measurement cant change that must as far as entering tire rotation in the SCT program. Or I guess you could measure once a year and reenter the RPM at that time. I would love to know of a actual measure test of how much rubber was left on the road after a year of driving, burnouts, and ricer whipping. Steve
 

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I do think hand calculating is always going to be more accurate.

I do have a question about the quoted section above. Do we know how much wear the tires had when he made the ground measurement? That measurement will change as the tire wears.
I would just measure about every 5-6 months, depending on how you drive! :evillol:
 

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I do think hand calculating is always going to be more accurate.

I do have a question about the quoted section above. Do we know how much wear the tires had when he made the ground measurement? That measurement will change as the tire wears.
That's the beauty about hand calculating every few months. You get an accurate measurement that's not dependant on tire size or wear and you can go a few more rev's per mile on your SCT program tweaks to compensate for wear.
 

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I have a question about the measurement. I went out and measured my tires and they are 285/70/17 Nitto's. They measured 102.5 inches. When I did 63360/102.5 I came up with 618. Now when I check Nitto's sight they show my tire at 634. I used 1010tires.com tire calculator and it shows 636. On that sight it shows the circumference at 102.7 which is very close to what I had at 102.5. Now if we are both using almost the same circumference measurement how is theres 636 and when I used the 63360/102.5 I come up with 618? :dunno:
 

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I have a question about the measurement. I went out and measured my tires and they are 285/70/17 Nitto's. They measured 102.5 inches. When I did 63360/102.5 I came up with 618. Now when I check Nitto's sight they show my tire at 634. I used 1010tires.com tire calculator and it shows 636. On that sight it shows the circumference at 102.7 which is very close to what I had at 102.5. Now if we are both using almost the same circumference measurement how is theres 636 and when I used the 63360/102.5 I come up with 618? :dunno:
Depending on the wheel it's mounted on it could change those numbers.

If you play around with the numbers a bit you will see what happens with the tire revolutions. Since the SCT jumps in 5 rev increments I figured the tire needs to loose 1" in order to require a change in the SCT user adjustable settings. I set mine to 595 running 35" Toyos and it makes a HUGE difference from my settings from the Toyo website.
 

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Depending on the wheel it's mounted on it could change those numbers.

If you play around with the numbers a bit you will see what happens with the tire revolutions. Since the SCT jumps in 5 rev increments I figured the tire needs to loose 1" in order to require a change in the SCT user adjustable settings. I set mine to 595 running 35" Toyos and it makes a HUGE difference from my settings from the Toyo website.
So is the 595 what Toyo's website said you should be running or did you just try different settings until you found the best one?
 

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I did the math as described above a few times. Took the average and tried the two closest settings that the SCT could program. 595 has the best shifting feel from what I could remember when it was stock. It tows my boat just fine or anything else I want to do for that matter.
 

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Ok I think I will give it a try. Thanks for the info.
 

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Well after going to Nitto's website and seeing what their revs per mile was listed for my tire (285/70/17) they showed 634. I entered the tire size in the online calculator that alot of the people on here use and that came up to around 615 I believe. So I went out and marked the tire and the driveway and used that measurement and used the 63360/? method and came up with 635. So I guess that does show at least for me that the best method is to measure your tire.
 

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i have never messed with the revs on the sct, what does that change? I thought it was for speedo read out, and that option did not work in our years.... it does other stuff?..... interesting
 

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i have never messed with the revs on the sct, what does that change? I thought it was for speedo read out, and that option did not work in our years.... it does other stuff?..... interesting
It changes the way your trans shifts, accounting for the taller tires.

OP, good write up, i've done several posts for this issue and they always get lost/forgotten.
 

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It seems to me that I learned somewhere that circumferance=diameter x pi. If you know that and a little more math to determine actual tire diameter you can calculate rpm without chalk marks and measuring. Of course that wont account for tire wear.

I never really saw any difference in shifting when changing the revs/mile on my truck.
 

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It seems to me that I learned somewhere that circumferance=diameter x pi. If you know that and a little more math to determine actual tire diameter you can calculate rpm without chalk marks and measuring. Of course that wont account for tire wear.

I never really saw any difference in shifting when changing the revs/mile on my truck.
Thats true, but its been proven that the operating revs per mile can be quite different.

I noticed a better shift when I put the correct revs in mine. Overall more predictable shifting, as in, there was no occasional weird situation shift.
 

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Is all the REV per mile setting in the SCT and when do you set these parameters? I just got mine yesterday and the email from Eric with my tunes today and will be downloading and installing them tomorrow. I have 35X12.5 R 20s with Toyo M/Ts and want to make sure I am putting in the correct settings. I am new to the programmer side of the diesel world.
 

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Is all the REV per mile setting in the SCT and when do you set these parameters? I just got mine yesterday and the email from Eric with my tunes today and will be downloading and installing them tomorrow. I have 35X12.5 R 20s with Toyo M/Ts and want to make sure I am putting in the correct settings. I am new to the programmer side of the diesel world.
When you go to load the tune it should give you the option to change parameters or something like that...you want to hit yes and goto the revs per mile option. Change that to the number you come up with using the above method.
 

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When you go to load the tune it should give you the option to change parameters or something like that...you want to hit yes and goto the revs per mile option. Change that to the number you come up with using the above method.
I also set my axle ratio while I am in the setting to.
 
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