My 2008 Fit has 103,000 miles on it. Recently, the tachometer went haywire, then the dash lights went off. It then started started jerking. It has become, essentially, undriveable. I had it towed to a Honda dealer, but so far, the dealer has been unable to diagnose or fix the problem. What could it be?

 

While I cannot help you, sorry to hear about this issue happening to your car.
I'll be watching this thread hoping to see what the cause is.
Have you looked at the wiring to see if any rodents got under the hood and chewed it?

 

alternator ./battery fuse box will make good starting points