I finally got my car to stop eating speedometer cables (and reconnected it to the back of the gauge after it promptly fell off there the last time, because of course it did), and while driving it around on the freeway (driving to/from Woodward this past weekend), at around 75 mph it said it was turning close to 4,000 RPM. Based on my calculation sheet, 75 mph and 4k RPM do not correlate. While I'm not sure the speedo is accurate yet (I believe it was, based on my brother driving behind me on surface streets), I can certainly say the car was not at 4,000 RPM - because that would mean I was doing about 95 mph (and I definitely was not). Is this a pretty common thing? I figure the speedo should be pretty accurate given it's an entirely mechanical setup (with new upper and lower cables, and a new, correct, conversion box, along with new correctly-sized tires), so it can't really be wrong. The tach, however, is electrical, so it's entirely possible. Thoughts?