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Hello again fellow Packard lovers
I have estimated the turns per mile the speedo cable makes is 1000 . I used a cordless drill torqued on to the cable with a white chalk mark on the chuck and it was easy to count the turns at low speed , I did this for the turnover of 6/10ths of a mile and for each of the 10ths the revs were 100 . I had read somewhere that someone thought the turns per mile was 1000 and it looks like thats correct . With that number , the tyre size and the diff ratio the correct gears can be worked out for the new R700 box . Thanks for your help and interest .

Chris.

MPH = rpm x C (tire circumference in feet), divided by the result of F (final drive ratio) x 88.

Piston speed: stroke (in inches) x rpm, divided by 6

Or for our Euro Packard owners: stroke in mm x rpm divided by 30,000

Into the 1950s, maximum safe piston speed was 2,500 ft. per minute, tho' metallurgy, bearing design factors.