That's interesting - What are the chances that the NAV system actually recalibrates your speedometer for you? This would certainly null the problem of changing wheel diameters.

 

That would be neat, but I have a feeling that Navtech was contracted for NAV only and the speedo software was developed elsewhere. Just guessing.

 

The following data is used normally on in factory installed nav's; Sat data, heading detection (angle, rate of turn, direction), speedometer, some units will also have a digital internal compass.

On aftermarket units the GPS sat data is the SOLE data input which is why aftermarket units are slower to react to "fixing" lose of signal. In built units will normally still continue to guide even if the sat is lost based on heading direction, speed, rate of turn, etc however at a more unreliable rate of error.

 

Just something to think about. Hand held GPS's can still tell what direction you are traveling and your speed. I don't think it has anything to do with tire or speedometer.

Just my two cents

 

The GPS computer is connected to the impulsemeter of the speedometer..
These impulses need to be adjusted once they're outside a certain margin, just like the tachometers in european trucks...

 

I used handheld gps to calibrate spedometer and learned there was constant over estimate by spedometer by about 2 mph. (I have done this many times before in other cars.) I then tested the nav data to the odometer data via trip computer. Odometer data tracked with nav trip computer. So the distance displayed in the trip computer is generated by the car which I already prooved was reading constantly high. Same deal with fuel consumption. Now the big mystery to me was the major difference between reported consumption in the trip computer and the BC computer display on the dash. As I am a scientist, I know enough to set the starting conditions to a known state before reading the instruments. I made the comparisons accumulatively during a long trip and at time did the consumption values match The BC computer value was closest to the real value using odometer and gas pump data. Clearlly, the odometer data is already suspect but what is the excuse for the trip computer which was reset with the odometer and tank filled?
BTW,nav and gps did match exactly on lat and long reading.

 

sg530, I am planning a long term comparison (maybe over the life of the car) of the actual mpg vs the OBC. So far, the average is closer than my E39, but time will tell. I will check the speedo with a cheap hand held I bought for walking. Texas and most states allow 10% above the posted speed before you pick up points. So, if the speed limit is 70 and I have a 2 mph error, I can set the cruise so I don't go above 79 down hill. That's the safe speed. When I get bored, I take my chances and pick up the pace. I don't get much exposure in Alabama because I usually drive through near the coast on 98 and I-10 where it's narrow.