I took it for granted while I'm rather used to see it shown and tracked while within a tunnel and speed has no influence... and I don't stay or make silly changes in a congested tunnel thus, I rev up if necessary.
Since my recent acquisition of my Nuvi 3790 from brudder SB, I've swung past, under and through several tunnels in Singapore last weekend (I'm now in Thailand btw) and like I said, I had no concerns or doubts to raise except for the fact about taking it for granted that it simply works and indeed, it worked!
Nothing of the sort mentioning the loss of satellite signals. The unit is superb and I'm glad for deciding this latest model.
Btw, I ordered the ZAGG screen shield and since there isn't one currently shaped out for the 3790 I ordered the 1690 model instead. That will suit me fine and I almost have it for FREE after purchasing the FULL protective shield (front, back and all around) for my Samsung Galaxy S2 phone in the same order.

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