i have my scangauge flush mounted in the ecu cover of the vx.. still can pull it out fairly easily, i ordered a second cable for it (scangauge separates from its cord via an RJ-45 connector) so i leave the one behind the dash and the other in the glove box so that i can pull it out and test other cars

as far as i know the speedometer reading is a mechanical thing you need to change the gears in the gauge cluster to correct it.. bbut you can get an accurate reading on the scangauge of your speed by putting in your tire size change factor