It might be a projection issue. GPSapp is currently using a transverse mercator projection in the WGS84 datum, but the difference with a real UTM projection is that I center around the middle of the rectangle that fits around the route instead of the using the actual UTM zone in which the route falls.

The Tiger data is probably in the NAD27 datum, although they might have switched to WGS84 in the recent versions. There is approximately 200 meter difference between the two. I'm also guessing that roadmap doesn't do a cartographic projection to a rectangular grid (i.e. 1 pixel in either direction == 1 meter), but is using something like 1 pixel == 1 degree (or 100th of a degree), I forgot the name of this projection, but it is commonly used for world maps (which is why many people think that europe and north america are larger than they really are.

Basically you can use a ruler on the gpsapp route display and the distance between two points in any direction is always correct for the scale. When there is no cartographic projection 100 meters north might be 10 pixels, but 100 meters east is only 7 pixels. The difference is even more near the polar regions where the latitudes are getting closer to each other while the longtitude lines always have the same distance.

I'm leaning towards dropping the transverse mercator projection. Typically people won't be able to tell the distortion, and it makes it easier to draw areas that span several UTM zones, like the whole US when we zoom out a lot.