Edited on 2015-02-06 9:31:04
Edward,

The 3.31:1 ratio was standard for GB and Europe, but the 3.54:1 rear end was standard for North American imports, supposedly because Jaguar wanted the E-type to compete with the Corvette and needed more speed off the line. If a tag on the rear diff housing show the #'s "46/13" (the number of teeth on each of the pinion gears), then the final drive gear ratio works out to "3.54" (46 divided by 13).

Test drive your car at a given engine speed and record the vehicle speed. Use a GPS to confirm accuracy (example below):

observed engine speed rpm = 5000
observed vehicle speed mph = 96
G = 1 (4th gear ratio for E-type is 1:1)
W = 765 = (5280 ft/mile) ?À (6.9 ft/rev measured circum 205/70R15)
or look up revs/mile on Tire Rack website if you are using a different size tire

Solve the following calculation for determining the final drive gear ratio in your differential:

final gear drive = (rpm x min/hr) / G x mph x W = (5000 x 60) / 1 x 96 x 765 = 4.08 : 1

Some Gear Ratios Available for Dana Salisbury 4.HU
Teeth rpm@70mph
2.88:1
2.93:1 2500
3.07:1 43x14 2650
3.31:1 (Euro) 43x13 2875
3.54:1 (US) 46x13 3100
3.77:1 3275

Hope this helps.