A fellow lister has enquired about the lock up speed and engine's rpm range for the torque converter's locking up.
Browsing through the pages of the workshop manual of Discovery Series 1 (1989 - 1994) model year 3.9 vehicles I found that they quote a speed of
51 - 54 mph and 1875 -2000 rpm .
Unlock at 49 - 52 mph and 1825 - 1930 rpm.
Always with gear selector in D
There is a note quoting that the above numbers are approximate and only intended as a guide though.
Above figures for the 200Tdi engine Discos is
Lock 46 - 49 mpg at 1600 - 1850 rpm Unlock 45 - 48 mpg at 1700 - 1800 rpm.
HTH
Take care Pantelis