You can also adjust the zoom level at which various roads appear which would be more useful for cycling. For instance, I don't care much about Highway 17 over the Santa Cruz mountains, although there are plenty of smaller roads going through the redwood forests which are quite nice for riding.

There's a mkgmap java program which combines a bunch of things to produce .img files for Garmins, including a .TYP file which associates various map object type / attribute combinations with their on-screen representations. Both associations and representations can be changed.

The .TYP files are created from style files in a friendlier human-readable form.

This thread on the openstreetmap forum has a magic incantations for the style files mapping various road type/attribute combinations into line objects

How to use different way/line for unpaved / Garmin maps / OpenStreetMap Forum

The line types roads are mapped to can be modified with color arguments
DayCustomColor=#color
NightCustomColor=#color

The openfietsmap (one of the sites baking the OSM data into Garmin-friendly .img files) style files are available online
Map making procedure - OpenFietsMap

I've only owned a compatible Garmin for three days and run mkgmap a couple times so I've yet to personally apply this, although it shouldn't be too painful.

Now it's an 810 running the 5.0 firmware, although that's already powered-down once and corrupted a .fit file so I'm replacing it with a refurbished 800 that shipped today. It does boot and acquire the satellites quickly....

Pro tip: it is insanely faster to fill your micro-SD card when it's not installed in your garmin. I used this $6 USB to SD/micro-SD adapter
http://www.amazon.com/Sabrent-SuperS.../dp/B00OJ5WBUE