speedometer error...

So after MUCH testing and examination, I've determined that my speedometer is consistently off by almost exactly 5mph at speeds above 30mph. I.E.

indicated... actual...
30 => 25
40 => 35
50 => 45
60 => 55
70 => 65

speeds below 30mph indicated result in a variation of either 3-4mph difference according to the GPS.

my odometer however is reading correctly. a 5 mile drive at 60 mph (65 indicated, actual verified by GPS) too exactly 5 minutes to traverse (+/- 1 second error). This was tested using cruise control and on as flat of roadways as I could find, with 5 tests being done.

Question is... HOW do I fix my speedo for this error? is this a take apart dash and cluster and remove/replace the needle fix?

Thanks!

Do you have larger tires on it? I think you can take it to the dealer to have it calibrated or whatever it is they do.

stock sized tires on it.
I had considered a plus zero tire change back when I thought he error was a consistent percentage, but it's not so such a tire change wouldn't help me... I'd just be wrong in a different way.

GPS isn't always accurate... just my advice.

Were you using the trip odometer that has tenths of a mile to give an accurate reading, or just the regular odometer which will give you the wrong results?

my primary measurement method is a stopwatch and mile post signs, as well as the speed trap lines on some of the local highways (which are 1/4 mile apart.) I actually no longer trust the odometer either.

I also did speed pacing with 3 other vehicles (none of which were Chrysler products) and those spot checks at 60mph came up with the same error of indicated 65.

I was going to try and get more data related to the odometer, but my travels were in far too much traffic to perform it safely.

So what you're saying is you won't be speeding and when you go to sell, it will legally show less miles then it really has. And this is a bad thing? LOL I reprogrammed after going 265 75 and it's more accurate now then when it was new, go figure.

The dealer can flash the computer. Find out from the tire manufacturer what the rpm of your tire is and take it to the dealer. The older jeeps you could change a gear but in the newer ones it's electronics

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Pretty standard on the 07's from mine and what i've seen on this forum. 245 75 16 put it +- 1 mph.

my GPS does the same and i dont trust it worth shit.
i just follow the road speed. i can check tomorrow and post because i havent tested it at a exact speed so ill drag my girlfriend out with her brand new suv and see if 50kmh is actually 50 or if my gps is doing what yours is

My GPS (Garmin) says my speedometer is reading 3 mph fast. If the cop I blew by an hour ago was awake, I'm trusting the GPS. Speed limit was 70, Speedometer said 77 and GPS said 74. 74 mph in a 70 zone and he might have cut me some slack. 77 and I'd have probably been paying up. :nutkick:

I added larger tires to my rig, and went to a dealer to have the speedo reset. regardless of whether you have larger tires, you may want to just get the speedo re calibrated. Cost me 40.00 bucks. fixed the speedo to correct.