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Old 12-19-2012, 12:00 PM   #1
jeeperjkj
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Superchips TrailDash

Got my TrailDash installed. Tried to change the tire size to 39.75". Went for a drive and my speedo, TrailDash, and GPS all showed the same speed. I thought everything was good until I looked at the tire size setting and it's still on the stock size.

My question: How can the speedo be right if the tire size never really changed?
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Old 12-19-2012, 12:05 PM   #2
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Funny thing ..

I am swapping a PROCAL for a SuperCHips. I set my JK back to stock today so I can sell the PROCAL.

I know it reset because my TPMS warning came on immediately .. I have no sensors and had them turned off with the PROCAL.

I had my JK programmed for 35" tires and 4.88 gears.

It is now set back to stock ... 3.21 gears and 31" tires ... Guess what.

My Speedometer is only 4 MPH slow ... I guess the ratio's of gears and tires are very close in either setup. I'm sure there is a math equation to prove it.
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Old 12-19-2012, 12:41 PM   #3
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Originally Posted by jeeperjkj View Post
My question: How can the speedo be right if the tire size never really changed?
It can be a bit finicky. I'd change it back, then change it again. Make sure the key is on and the engine is not running. I've run into problems changing the tire size with the engine running.
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Old 12-20-2012, 06:37 AM   #4
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It can be a bit finicky. I'd change it back, then change it again. Make sure the key is on and the engine is not running. I've run into problems changing the tire size with the engine running.
I did all my changes with the engine off and key in the "on" position. Are you saying that it'll let you change the size while the engine is running?

As far as "change it back, then change it again", it never changed. Are you just saying to try it again or change it to 39.5", then to 30.75", then to 39.5" again?
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Old 12-20-2012, 07:02 AM   #5
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Originally Posted by jeeperjkj View Post
I did all my changes with the engine off and key in the "on" position. Are you saying that it'll let you change the size while the engine is running?

As far as "change it back, then change it again", it never changed. Are you just saying to try it again or change it to 39.5", then to 30.75", then to 39.5" again?
Don't know if the trail dash will "let" you, but I know the Jeep wont bawk at it, but it doesn't always take. And there is no error spit out to know it doesn't take.

It's been a long time since I have looked at it but I believe the tire size ends up stored in more than one module, I might be mistaken. I'd have to go back and dig through my notes.

I'm saying change it to anything else then try 39.5 again. I've had to do that before.

Also I'd key off for a while first then key on, engine off.
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Old 12-20-2012, 12:20 PM   #6
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Don't know if the trail dash will "let" you, but I know the Jeep wont bawk at it, but it doesn't always take. And there is no error spit out to know it doesn't take.

It's been a long time since I have looked at it but I believe the tire size ends up stored in more than one module, I might be mistaken. I'd have to go back and dig through my notes.

I'm saying change it to anything else then try 39.5 again. I've had to do that before.

Also I'd key off for a while first then key on, engine off.
Gotcha. Will do.

I'm still trying to figure out how my speedo can be correct, though, if the tire size isn't correct... and if I change it to 39.5", will that make my speedo be wrong?
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Old 12-20-2012, 12:31 PM   #7
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Originally Posted by jeeperjkj View Post
Gotcha. Will do.

I'm still trying to figure out how my speedo can be correct, though, if the tire size isn't correct... and if I change it to 39.5", will that make my speedo be wrong?
IF the speedo is dead on with the GPS .. why mess with it?
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Old 12-20-2012, 12:45 PM   #8
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IF the speedo is dead on with the GPS .. why mess with it?
Because there's a reason the unit has a place to change the tire size. I'm just wondering if I inadvertently changed it somewhere else and later, if I change it again my speedo will be f'd up.

My OCD is probably playing a part in it as well. It irks me to see the 30.75" on the screen.
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Old 12-20-2012, 04:27 PM   #9
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your speedo was off from the factory, ALL cars from the factory are off 2 to 3 MPH. thats why most time you have to use a different tire size than what you have to get it right on the money
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Old 12-20-2012, 07:10 PM   #10
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your speedo was off from the factory, ALL cars from the factory are off 2 to 3 MPH. thats why most time you have to use a different tire size than what you have to get it right on the money
He's not saying at which speed he's at, but speedos tend to be off that 3-4 mph at 55-65... running 39.75 tires coming from 32's would be more like 15% off. (did too much math today don't feel like figuring out the exact same number, but it's WAAAAY beyond that 5% off that a stock vehicle tends to be, so at least it would be showing 10% under or better.
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Old 12-21-2012, 03:43 AM   #11
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Yeah, between programmers (I had the FlashPaq on it until ~Aug) I was about 25% off. My speedo showed 40mph, but I was really going 50mph.

Now, it's dead balls accurate.
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Old 12-21-2012, 06:32 AM   #12
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My OCD is probably playing a part in it as well. It irks me to see the 30.75" on the screen.
Thats what would get me too
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