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Discussion Starter #1
Ok guys need some help here. I went to the options setting to adjust the gear ratio and revs. I am running 4.88's and 40's (actual 39.8). I see the sct only allows for 4.56's. I calculated the revs per mile to be 520. So here is my question, do i set the gears to 4.56 and then 520 rev or do I leave the gears to stock setting and adjust to 520? Originally the truck had 3.73 gears. How do I figure out how close to stock gear ratio my current set up is to what the old set up was. Meaning 4.88 vs. 3.73. I think the tires stock were 275's. How would you all set it up....... just set the revs or change the gear setting too. Let me know please.
 

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Are the tunes you using custom? If so I'd get with your tuner and have them write the tune with the gear size and tires you have...
 

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Yea, i'd get with who wrote your tunes, i'm pretty sure they can add the option for those gears in there, then you could use the calculated revs per mile for the tires.

But, in the mean time i'd set the rearend on 4.56 and then 520 for tire rpms, no idea if thats the best option, but its what i'd do...
 

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520 ? my 38 s are 550 rpm looked it up on there site. and i did the ratio thing online and with my 38's 4.46 would be stock so iam a little lower than stock. u r probably perfect
 

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ptr34, check out my post on tire revs per mile, manufacturer, "mathmatical", and guesses can be far different from the measured revs per mile...
 

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i read your post. i will jack it up and measure with a tire tape and see what it is. i would think the manufactor would be right .but it could be wrong thanks
 

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Measuring around the tire wont be very accurate either, when the tire sets down its squashed slightly, and an inch difference can make a big impact on your rev's per mile. I recommend doing it by rolling the truck forward and marking the ground...
 

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You need to set both the gears and the tire revs that you are running.
 

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Just email Eric and attach a copy(s) of the file(s) you want him to check out to verify that the gear and tire revs are set in. Attaching the files makes it easier and quicker for him than having to dig them up.
 

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Once files are written I don't think another SCT tuner can view any details about the settings inside that file.
 

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did the roll out like u said it was 114 1/2 " so it was 550 rpm
 

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its 553.xx so I would set it on 555, but your original 550 was almost spot on! I guess your tire manufacturer doesnt BS their sizes huh!
 

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guess so . but it good to have done it that way two
 

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Discussion Starter #14
I am just going to have to measure the tire on the ground by rolling it........ anybody have any Idea of the revs per mile on the 40 (39.8) MT ATZ............
 

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well, 40 inches times Pi(3.14) equals 125.6inch circumference.

Then 63360(inches in mile) / 125.6 equals 504.45 inches. So it should come out around 505 revs per mile...
 
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