How many RPMs are you turning at 35, 45, 55 & 65 mph?

My wife parked her '94 E320 outside last Saturday night while in PA and the overnight temperature dropped to 9 degrees. There must have been moisture in the speedo cable that froze. The cable snapped as soon as the car moved the next morning. The tach is working fine and we'd like to use it to give us an idea of what speed she is doing. The tire is the standard 195 65 R 15. I'm not going to mess with a new cable until it warms up some.

Thanks