SO I got pulled over yesterday for speeding, but i didnt think I was. Officer clocked me at 52 and my speedometer said 40.

I put the car on a road load dyno and have come to find out that in 1st and 2nd, the speedometer reads 12-15 fast on acceleration, but once you cruise at a steady speed it comes down to within 2-3mph.

Its also off differently in each gear. 6th for example, at 75 im doing 83.

I already have a certified statement that my speedometer is in fact, wrong, which means the solicitor and judge will lower the speed by that many mph, and when they do its below GA's 10mph so it gets dropped.. but im not sure what is causing my speedometer to have a heart attack

 

Bad speed sensor - only other would be the speedo itself.
Gotta be a sensor.

 

Well what was really weird is while on the dyno. Accelerating through 1st and 2nd up to 35mph the speedo read 0 until I held it at 35 for a few seconds but the dyno was reading speed off of the obd connector. Too weird

 

Now that sounds like the speedo or electrical connection to it at least.

 

Well I'm gonna start with all four speed sensors and go from there. It's never done what it did on the dyno before. But I know that the speedo reads slow. Can't figure out why unless my tire sizing is making the PCM go nutty. It's a manual so I dont the PCM is as dependant on tire sizing but I could be wrong

 

I might just try wiring the front speed sensors inline with the rears so they get the same speed signal as the read and see what that does. That should tell me if it's the tire sizing. If it still does the same thing that would tell me one of the rears is bad. If not it's one of the fronts. Weird thing is it's not throwing any codes to indicate that a sensor is bad

 

Yeah that is strange - I would have at least expected an ABS light or code with a bad sensor.

 

The abs light came on when it was on the dyno. A quick cycle of the ignition and it went away. Hasn't been on ever except for that one time

 

Wait, I thought you didn't get speeding tickets because all of the cops in your area were afraid of your Mom?

 

Not sure but are there any sensors in the transmission? such as input shaft sensor/ output shaft sensor? just a thought since it changes with different gears, and I also would think if it was a bad speed sensor it should affect the ABS, and set some kind of code such as whell speed out of range, or iradic signal, may not set a hard code if its intermintent, can you read any history cdes? hope this helps some.

 

Nope no trans sensor (at least for the speedo)
Speedo works off a "combination" of output from the wheel speed sensors.

 

REALLY? These cars are weird - or is this normal these days?

 

Still haven't really figured it out. Speedometer is off at random intervals. No codes or anything still.

 

The Crossfire's just prepping your diagnostic skills in advance of your attempted turbo install.....