Programer
07-23-2009 | 07:30 AM
#11
Thread Starter
JK Enthusiast
Joined: Jan 2009
Posts: 263
Likes: 0
From: some place crappy
I'd suggest that you concentrate on what the programmer was designed for: Setting the proper tire size and gearing. How did you determine to set 35 inches? If you just went by the fact that you've just installed 35s, then your speedo may still be off (as is everything else tied to it, including odometer, trip computer, etc). If that's what you did, I suggest that you first check out your speedo against a known good source (a vehicle you trust, GPS, cooperative cop, etc). Most of us have found that we must set the tire size about 1 to 2 inches below the expected size to get proper readings. For example, I've got my Hypertech set to 31.25" with 33" tires, and it has checked accurate against three standards (my son's truck, GPS and radar gun). 
07-23-2009 | 08:26 AM
#12
JK Jedi
Joined: Apr 2008
Posts: 5,168
Likes: 1
From: texarkana,tx
While slowing your speedometer down will raise shift rpm, it will also promote gear hunting at highway speeds under load. For most that seems to a bigger issue than where the transmissions shifting under roll out acceleration. A programmer will assist in tuning shift points to achieve better drivability while a calibrator will only return to stock shift points for new tire and gear changes.
Shift points are determined by three things, throttle position, manifold pressure and speedometer. Messing around with just one of the three isn't likely to deliver the desired effect and would be best addressed by a programmer.
Shift points are determined by three things, throttle position, manifold pressure and speedometer. Messing around with just one of the three isn't likely to deliver the desired effect and would be best addressed by a programmer.
I am not trying to argue with this... but I have to. At least until someone points out a VSS under their JK. The TJ had a VSS located in the transfer case. That is why on the TJ you could adjust for gear changes by just wapping out a speedo gear in the t-case, and not having to reprogram the computer. The JK has no such gear, or reading at the T-case. That is why tire size alone effects speedo.
I can tell you this for a fact. How you ask? I have 5.13 gears and 35s. My Jeep is programmed for 4.10s and 32" tires. Everyone knows that the final drive ratio from 5.13s and 35s is lower than 4.10s and 32s.... right? To have the same ratio as 4.10s and 32s, I would have to have 5.13s and 37s. But I don't, so my RPM is higher than it used to be when going the same same speed. My speedo registers slower than actual speed according to GPS and the state trooper that pulled me over.
It has to be read at the tone ring. Not at the transfer case. Re-gearing WILL NOT change speedo reading. Changing tire size will change the speedo reading. If there was a VSS at the transfer case, like on my TJ... Then I would actually be traveling slower than my speedo says. Because of the lower ratio of 5.13s and 35s than 4.10s and 32s.
07-23-2009 | 08:55 AM
#13
JK Jedi Master
Joined: Mar 2007
Posts: 14,796
Likes: 366
From: Midwest City, OK
07-23-2009 | 09:05 AM
#14
JK Jedi
Joined: Apr 2008
Posts: 5,168
Likes: 1
From: texarkana,tx
This gets to the heart of the earlier discussions. Unfortunately, the service and parts manuals are terribly deficient in describing how the JK's speedometer works. The best I've ever found is. "The PCM continually monitors the Vehicle Speed Sensor (VSS) to determine the vehicle road speed. The PCM then sends the proper vehicle speed messages to the ElectroMechanical Instrument Cluster (EMIC)." Looking at the parts breakdown, the VSS is located at the wheels. Which made me think that you were right. Except they're at all four wheels, and they're used for the ABS, etc. That's what initiated a call to the local dealer (not me, BTW). And that's where the testimony that it's located on the transfer case came from. Regardless of your experience with your programmer, I've read similar testimony from folks both directions: Gear ratio affected speedometer; gear ratio didn't affect it. I'm perfectly neutral on this, but I would like to know the correct answer.
When I went from 4.10s to 5.13s my RPM at speeds changed, but my speedo reading versus GPS speed reading did not change. Maybe my Jeep is odd and has a different package than yours, but there is no way my speedo gets its reading before the axles.