Per Techline, the stock trans sending unit in the 89 should have been putting out 3960 pulses per mile.
You can look at your setup in the 59 and see how your pulse per mile compares.
For a point of reference, I put a 3.73 rear end with 29.5" tires under my 58. Using the SM420 and the Dakota Digital electronic sender (screws into the speedo cable outlet), I am seeing just under 12k pulses per mile. If you have a similar setup, 25mph actual could be read at the ECU as ballpark 100mph.
