So just to clarify what's been said and not to confuse others, here are the FACTS and why:

Rear end gearing DEFINITELY DOES NOT affect the speedometer accuracy.

Changing overall tire diameter from the stock DOES affect the speedometer reading. Those with 20+ rims or 19" fat tire set ups will be driving faster than what their speedometer indicates.

The reason why this is so, is because the ABS sensors are used to electronically measure and calculate the speed, unlike the traditional mechanical speedometers that utilized a gear driven cable attached to the transmission.

 

of course redgs4 would recommend the 3.76 over the stock geared limited slip because it is faster from 0 - 60 & 60 to 135mph. Our two cars have very similar mods and were dynoed very close in HP & TQ and yet I have run faster for atleast 15 out of 15 times that we have raced in the 1/4 mile. We attribute the difference to the 3.76. That's why he switched to the 3.76, but I'll bet he's not selling his old gear set up, just in case he goes FI down the road.

In any case a challenge between redgs4 and myself is coming to a NorCal dragstrip in the near future. Guess that'll force me to go FI just to stay ahead.

I can see this situation escalating.

 

Well then, since I'll never do FI I guess you will just have the faster car.

I'm just saying my famous last words now to get them out of the way.

 

Well, slightly OT here maybe, but I would say that NAV systems use the GPS (Global Positioning Satellites) and the process of triangulation to determine location, bearing etc. Dead reckoning is primarily a nautical term and has been around for centuries. Still useful in some situations...here is a brief definition (if anyone really cares )

"Dead Reckoning is the process of estimating your position by advancing a known position using course, speed, time and distance to be traveled. In other words figuring out where you will be at a certain time if you hold the speed, time and course you plan to travel."

Thanks RMM for summarizing the effect of a gearing change on the GS . Nice to be able to do that without the speedo/odometer issues being a factor!
Let us know the results of redgs and your next runs-wish I could be there to run mine as well!

 

Well then I am definitely mistaken then, I thought the GS also had a VSS on the transmision for the speedo function. If it only uses the ABS sensors, then you are correct....

Carl C