Assuming your feedback is valid on the loss of RPM (which I have no reason to doubt), then that verifies that the electronics do compensate for a change in rear wheel/tire setup. Bigger size would require lower RPM (drive shaft speed) to maintain equivalent gound speed. The fact that you've indicated a change in Speed readout indicates to me that the overall accuracy of the Speed is within an acceptable range, which would explain the small +/- change.

When you say the MPH changed, at what speed did your see that? Most Speedos aren't exactly linear throughout their operating range. Was always a problem back in "the good old days" of mechanical hardware. Most likely improved with today's electronics, but a smaller impact.

And per previously mentioned, there are quite a few sensors providing input to overall speed control of the XFire, which has to be a factor in what the Speedo reads out. I do recall seeing a different notation on the 6-speed manual. Gotta go back & look at it now!

Interesting thread!

 

And if the sensors on the front wheels do drive the Speedo, your experience would still verify that the electronic sensors in the Tranny are "doing their thing" when a bigger geometry is installed at the rear. If the rear wheels are turning a slightly lower RPM to maintain same ground speed, the front wheels would be turning the same RPM as before. Speedo would give the same readout. If the electronics did not compensate for the increased geometry at the rear, then the fronts would be turning at a higher RPM too resulting in the Speedo being off (slightly higher).

 

If you really care, you could raise the whole car (jackstands or lift please) and with key on spin one wheel at a time and see if anything registers on the speedo. Then start engine & let rears turn at idle in third. That would be a good set of indicators.

Might set a code also so be ready to clear.

ps "lost 350 rpm at 80" - maybe the tires grew... Nah going from 26 to 28" diameter only drops 200 rpm (2800 to 2600 in 6th). More likely the programming has a fudge factor and the speedo is non linear. Again, need to compare to GPS.

 

Interesting reading, but I did not find my answer.
The Corpus Christi TX-area Deputy insisted he clocked me at 82 mph today. My cruise control was set to 78. My tire sizes are exactly as called out in the owners manual, so my question is - is the speedo on the 05 Crossfire prone to reading 5 mph off? Need to know to keep my daughter ticket free.
First time in 45 year of driving I have ever told a cop I didn't believe his radar. I've been pulled over more in the last 4 weeks than I have in the last 15 years. Cops sure like pulling it over. Cop said, "It looks fast.", so I showed him when I drove off that it was. He followed me for another 10 miles.

 

See if you can get a GPS unit that gives the MPH and have someone note the readings on the speedo and the GPS.
I find my speedo and GPS pretty much agree with each other from what I see.

 

Using my GPS, I have found that my last two Crossfires read 1-2 mph lower than reality when compared to my Garmin. I thought that was unusually accurate. My Ranger shows 80 when my GPS says 77.

 

Mine is spot on - 60 on speedo = 60 on GPS, 75 = 75

 

Find a long straight road and set cruise control to 60mph. Time one mile, it should read one mile per minute. If you time it for 5 minutes, it will give you a much closer reading of actual speed (the more miles/minutes the more accurate). Make sure you set CC right at 60 and don't do it on a hilly or twisty road. Good luck!

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Thank you once again 180 and Pizzaguy! Downloaded a speedometer app. LIke PizzaGuy's case, it was real close. 75 on the car showed 74 on the app. 78 on the car was 78 on the app. I have to concur that the speedometer is pretty darned accurate.

 

Traffic court is not a place to expect justice. Best to find a lawyer who specializes in this sort of thing (some even guarantee "no points"). Figure $200-$250.