I am GUESSING at a 195/70 X 14, which is 630 revs per mile.
So 60 X Engine RPM = 60,000
630(wheel revs/mile) X diff ratio 4.63 X top gear 0.83
630 X 4.63 = 2916.9 X 0.83 = 2421.027

60,000/2421.027 = 24.782871 MPH/1000rpm.
Hope this is near enough for you!!
tabetha