I always assumed it was "around" Jersey City or something.
In an early archive, Tom mentioned that it was south of East Brunswick which would place it in Central Jersey. However, I think he is trying to create a twisted version of his hometown (Woodbridge,NJ?).
Well, if one looked at the PO Box on an old 18 Wheeler release one would find that Tonto's home town is Dunellen, NJ, which is actually a bit north of East Brunswick and a bit east of Woodbridge. Old Bridge, however, is a bit south of East Brunswick so we might assume Newbridge is near there as well. Ironically, South Old Bridge is actually north of Old Bridge and that, in a nutshell, is why so many people get lost in New Jersey. I also recall hearing somewhere that Giovanni da Verrazzano dicovered Newbridge so it may be a bit closer to Staten Island that either of the above two locations.
The why I see it, it's got to be somewhere near Interstate 287, Route 1 or Route 18, perhaps near where they all converge in the Piscataway/Edison/Highland Park area.