I have just recently discovered, and proven, that, at least on the 430N, the "GPS" speed indicated on the 430N is NOT actually from the GPS but gets it's data from the speedometer. (yeah, I know it sounds crazy)

My 430N "GPS" and speedometer readings agreed with each other and other GPS devices when I was running stock tires. I Just recently mounted 37" tires. After doing so, I pulled up my speed on the 430N display to see how much my actual speed would be different from the speedometer. To my surprise, I noticed that it still agreed EXACTLY with my speedometer even after mounting the 37" tires. (WTH???)

I have onboard Topo maps and handheld GPS. I also have a GPS dashcam that displays the speed on the display. The Topo, handheld GPS and GPS dashcam speeds agree EXACTLY with each other while the 430N "GPS" speed still agrees EXACTLY with the (yet to be updated) speedometer which still reads 15% slow because it still thinks I'm running stock tires.

Needless to say, I would not have believed it if I hadn't proven it... and it raises some interesting questions. I'm going to be doing more experiments.

FWIW

KG6SLC aka Eugene

 

For some reason that doesn't surprise me. I guess I've never really thought of GPS/NAV unit as being the same as a true GPS. I can see it regurgitating the same data the computer is calculating.

 

That info is accurate. I've got a friend in the same boat- 37's without a corrected speedo and it pulls from the speedo as opposed to the GPS signal. I plugged his nav unit into my jeep (no GPS antenna, just Sirius) and it wouldn't tell me a speed at all.