garmin 660

It has an internal speaker AND a headphone jack, as to how LOUD it is would depend on the environment it's used in. I can her my unit in my truck with the radio going.

I can hear mine on the freeway with the radio on and windows open.

I have listened to myine in the rv and it was plenty loud. I use my cell with it and can hear very good.

I've taken some actual measurements of the StreetPilot C510 and Nüvi 310 - but have no figures for the Nüvi 6xx range
http://www.poi-factory.com/node/9741

I suspect that your existing C330 will be similar to the StreetPilot C510 and that the 660 will be similar to the Nüvi 310 (though have absolutely no evidence to support that!).

If that is the case, you're talking about an o/p of about 80dBspl - which is just about adequate. The main difference, will be that you have to concentrate more and listen out for its instructions, rather than having them bellowed at you!.
(On the C510, I turned off the 'attention tone', because it was very disruptive. On the 310, it's more of a necessity).

But to get an actual measurement would be good...

I have both the 660 and the 330. I found the 330 to be much louder and easier to hear above the road noise than the 660. Most people seem satisfied with the 660's volume but to my aging ears it's not loud enough. I purchased a set of fold up speakers with a volume control at radio shack. It improved both the volume and the tone. Forget about playing fm through the car radio: too much interference. You'll need additional equipment to play through the car radio.... see faq's.

gusb

Though I have no problem hearing my 660 with the top up, once I put the top down all bets are off. I have noticed if you set up the 660 to give you a warning tone before each direction, it gets your attention to listen closely.

I've only tried it once in my Saturn Vue, but it seemed loud enough for me.

Yeah, the c530 was louder then the nuvi 350

I've got a 660 and have no trouble hearing it. One of the reasons I got it was the FM feature, which I've tried & it works well, but the internal speaker is good enough. In fact I've turned it down a bit. I use it in a Ford F150 pulling a trailer mostly. My hearing isn't the greatest either.

I have a 650 which is similar to the 660 with out the FM feature. Even with the radio on I have had no problems hearing my 650. I know that my hearing has been in questioned by my wife.

What speakers did you get from RS - do you know the catalog number?

Portable Folding Amplified Speaker System
Model: 40-1441
www.radioshack.com

I found these ones there on sale until Saturday.

http://www.radioshack.com/product/index.jsp?parentPage=searc...