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TexasTodd
Dodge Dakota
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1/11/2007
16:58:45
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 Subject: Tire size, speedo, and Super Chips tuner
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Cut to the chase. I have an 01, with the tire and handling pack.
It came with 255/65-16's. ~ 29in diameter
I recently put 255/70-16's on it. ~ 30 in. diameter.
The SC's tuner said the stock tire ht was something like 27.75, so I added an inch to it and reprogrammed.
I'm only asking those that have this tuner, have done the tire diameter change, and know something about it.
Do you first have to give it the stock diameter, or does it read it? And if it reads it, why was mine wrong?
TIA for your informative replies.
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well
Dodge Dakota
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1/11/2007
17:52:38
|  RE: Tire size, speedo, and Super Chips tuner
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I have one and tried the biggest tire size to see if it would change the shift points,it didn`t(going like 25 showed 75 on the speedo lol). I had to manually select the stock size(27.25 I think)to get it to work right again. I measured the tire height and it worked. Is your speedo ok now? If not put in 30 inches or whatever the height of the tire is,no?
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modain
GenIII
1/11/2007
19:04:07
|  RE: Tire size, speedo, and Super Chips tuner
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Todd, my SC experience on my QC is the same you have encountered. It seems to me that the 27.75" measurement is for the standard 235/75/15 tire size used on the QC (others I have "spoken" with concur). The PCM is not updated depending upon which tire package is installed onto the truck.
The calculated tire sizes are 255/65/16 = 29", 255/70/16 = 30'. However, mounted and loaded (with the truck's weight) the tire loses about 1" in height. That's why SC says to measure the tire from top to bottom while on the truck. You should have measured 29", as I did.
The tire size difference between the 15" tire and our OEM 16s (measured size is 28") is .25" - not really enough to make a worthwhile difference in the speedo/odo. This is probably why they don't flash it to the correct size. But when you add another inch with the the 255/70/16s *that* makes a worthwhile difference. Now it needs the PCM's tire size corrected.
I set mine to 29" and it is right on the money.
OK, now let me answer your questions... :-)
1) Do you first have to give it the stock diameter, or does it read it? - It is read from the PCM.
2) And if it reads it, why was mine wrong? - Stated above: factory base tire size.
I like how the increased tire size fills the wheelwell much better than the OEM tires. Do you like the new look?
What's your left foot doing?
=========================================
2002 Graphite QC SLT Plus, 4.7L, NV3500, Superchips 3715, 3.92 LSD, loads o'fun...
2001 Patriot Blue Durango SLT, 4.7L, 45RFE, Superchips 3715, 3.55 Open (for now)
1999 Forest Green Dakota RC Sport, 2.5L, NV1500, 3.55 Open
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N56629
Dodge Dakota
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1/11/2007
19:33:10
| RE: Tire size, speedo, and Super Chips tuner
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The most accurate way to measure the height is not from top to bottom but from bottom to center and double it. This has proven both accurate and logical since you never drive on the top of the tire. Most manufacturers give the unloaded tire diameter.
I'm not sure but I think Dodge uses the tire size on the driver door for calibrating the speedometer. My sticker says 215/75/15. I changed may tires back to 235's for the winter and removed the SC tuning and tire size for the time being. My trip to work is now noticably shorter. LOL
I wish that I could only the speedo calibration and not the SC tuner so that I could get a better comparison in gas mileage.
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TexasTodd
Dodge Dakota
JOIN HERE
1/12/2007
03:13:59
|  RE: Tire size, speedo, and Super Chips tuner
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Thanks for the excellent replies.
Mr. Well, I now put in 30inches.
Modain, My 16's (255/65-16's), ordered on my truck, seemed to haver the speedo right on, although it has a sticker for the 15's in the door frame. True the difference isn't much, but I want it right on.
I do like the new look. I went with Dominators Sport A/T- they had great ratings from consumers on Discount Tire's web site.
MOre agressive feel, look, and definitely better all around traction, wet of dry, sand, etc.
I have the manual tranny, and had 3.55's. I went to 3.92's, and with the new tire size, put me almost back to the 3.55 final ratio. the manual has short enough gears anyway.
N56629, I'll try measuring the way you said, and may recalibrate once more.
Thanks to all for the excellent replies!!
Todd Bouton
Houston/Galveston
01 QC SLT, yadda, yadda
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TexasTodd
Dodge Dakota
JOIN HERE
1/12/2007
22:34:00
|  RE: Tire size, speedo, and Super Chips tuner
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Well, I measured my wheel from bottom to center.
My tires are 44lbs max, they had 34lbs when I measured them.
The bottom half measured 14 and 1/8 inch!
So the rolling diameter mow underinflated a bit is only 28 1/4 inches! I have the SC's set @ 30 inches now. I'll be re adjusting after tire inflation and re measure.
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Super bee
GenI
1/13/2007
10:09:38
| RE: Tire size, speedo, and Super Chips tuner
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if im not mistaken shouldn't it be measured in circumference??
thats the way my Edge Juice is on my diesel
it makes more sense as well
1990 sport RC SB, 1995 318drivetrain, 3.92 gears (came with truck) Powertrax "traction system", 95 dash/steering wheel, hearthrob exhaust, headers, cutout after y-pipe, necessary electric fan add-a-leafs, 30x9.5 mud tires, cranked T-bars, AR 39 15x8 rims, grill guard, roll bars
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N56629
Dodge Dakota
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1/13/2007
11:07:40
| RE: Tire size, speedo, and Super Chips tuner
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You are right, circumference is the correct way to measure the tire. The SC calculates that for you when you input the diameter. The problem comes when you try to measure the diameter. When unloaded the tire is round but loaded the tire is anything but a true circle. The radius measured from center to bottom is much smaller than measured from the center to the top.
Imagine putting an adjustable metal band around the tire and squeezing it so that you have a load equal to the weight placed on it by your truck. The diameter would be smaller and the sidewalls would expand to accomodate the displaced air. That diameter should be equal to twice the loaded radius that I told Todd to measure.
It would be possible prove it by measuring the distance a vehicle moves with one revolution of a wheel. Inflate the rear wheels to 45 pounds, mark one of the tires on the side with chalk and also put a mark on the ground. Now push the vehicle forward until it makes one revolution and place another mark on the ground and measure the distance between the two marks. That will give the distance traveled with one revolution.
Next reduce the pressure to 10-15 pounds and repeat the procedure. You should find that the deflated tire does not travel as far on one revolution.
This is the reason that it is so important to keep both tires equally inflated with a posi rearend. Unequally inflated tires has the same effect as unequal tire diameters or different sizes. Same problem with 4x4's.
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Steven Roark
Dodge Dakota
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12/21/2008
13:58:06
| RE: Tire size, speedo, and Super Chips tuner
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Message:
Stupid
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