LOST JEEPS :: View topic - questions about the transmission and speedo?

So, on the way back from Indiana to North Carolina, I noticed something that I hadn't caught before. I was wondering if anyone could tell me for sure, but coming down the hills with the cruise control set, I noticed that the trans (I think) brakes, as I was able to maintain the exact mph the cruise was set at, no matter what the grade of the decline was. Do the Liberty's have a built in trans brake feature in the PCM?

Second question...best I can tell, I'm running stock tire size, but when I've either driven past radar signs, as well as running my gps, I've noticed the speedometer is around 3-5 mph off...has anyone else ran into this? My check engine lights on, but its only on due to the MAF sensor I unplugged to disconnect the EGR...

Author:	dennhop [ Fri Jun 22, 2012 1:23 am ]
Post subject:	questions about the transmission and speedo?
So, on the way back from Indiana to North Carolina, I noticed something that I hadn't caught before. I was wondering if anyone could tell me for sure, but coming down the hills with the cruise control set, I noticed that the trans (I think) brakes, as I was able to maintain the exact mph the cruise was set at, no matter what the grade of the decline was. Do the Liberty's have a built in trans brake feature in the PCM? Second question...best I can tell, I'm running stock tire size, but when I've either driven past radar signs, as well as running my gps, I've noticed the speedometer is around 3-5 mph off...has anyone else ran into this? My check engine lights on, but its only on due to the MAF sensor I unplugged to disconnect the EGR...

Author:	ChooChooman74 [ Fri Jun 22, 2012 6:13 am ]
Post subject:	Re: questions about the transmission and speedo?
dennhop wrote: So, on the way back from Indiana to North Carolina, I noticed something that I hadn't caught before. I was wondering if anyone could tell me for sure, but coming down the hills with the cruise control set, I noticed that the trans (I think) brakes, as I was able to maintain the exact mph the cruise was set at, no matter what the grade of the decline was. Do the Liberty's have a built in trans brake feature in the PCM? Second question...best I can tell, I'm running stock tire size, but when I've either driven past radar signs, as well as running my gps, I've noticed the speedometer is around 3-5 mph off...has anyone else ran into this? My check engine lights on, but its only on due to the MAF sensor I unplugged to disconnect the EGR... Speedo off is common. My speedo is off compared to what the ECU says I am doing through Torque Software. For example, my speedo may say 65, Torque says 63, and GPS says 61. Not saying that is what mine does, but that is an example. As for the braking, just normal engine braking with the Converter Locked.

Author:	papaindigo [ Fri Jun 22, 2012 8:41 am ]
Post subject:	Re: questions about the transmission and speedo?
X2. You might pickup a slight amount of speed going downhill in cruise but I've never experienced a downshift to reduce speed like a gasser will do from time to time. That's on normal downgrades upto and including short 7%. How things would play out on longer downgrades I don't know. Unless you specifically have your speedometer and/or odometer specifically calibrated (IIRC a dealer can do so within limits) there are exactly zero factory speedos/odometers from any car company that are accurate outside of pure luck. Generally accuracy is held to the 5% range high/low and even that varies from vehicle to vehicle including with stock tires whose diameter changes between tire companies and as tires wear. To further complicate things in many cases, CRD included, the data signal to the speedo and odometer and not from the same source and neither source ties to the EVIC so all 3 may be off by a different amount in a different direction. For example by a GPS check my speedo is almost dead on but stoutdogs is 3-4mph high and both of us have to use different mileage correction factors for mpg and my correction factor changed significantly when I replaced the OEM stock tires with the exact same aftermarket size. Just to add the mix there is no good reason to assume radar signs are particularly accurate and hwy mile markers are definitely not consistently accurate.

Author:	05infernoCRDL [ Fri Jun 22, 2012 11:32 am ]
Post subject:	Re: questions about the transmission and speedo?
the transmission will downshift to maintain a speed within 5 mpg over the set cruise speed my speedo with 245 75 16 tires is dead on with my gps and the road side signs.. dont know why but it is (my speedo should show a slower speed) just proves the above further

Author:	cdubs [ Fri Jun 22, 2012 12:43 pm ]
Post subject:	Re: questions about the transmission and speedo?
I've noticed that as well, my trans will downshift and rev higher on down grades to hold speed. Oh and my speed is high side by about 2 to 3 mph . I haven't calculated to see if my odometer is off as well

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Author:	WolverineFW [ Fri Jun 22, 2012 3:57 pm ]
Post subject:	Re: questions about the transmission and speedo?
It will downshift....if you have not experienced it, find a longer hill with a steep grade.

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