Feets you know i have a 71 imp, with a 493. it came with 3.23s, i've swapped in 2.76s and it much much MUCH better on the highway (but noticeably weaker from a standstill). i live out in the desert and drive alot at 70-80-90mph on the southern california freeways. it could use even LESS gear and be fine, so when i get to it i have a non-lockup 518, a new "tight" converter, and i want to try 3.73s, those * 0.69 od = a 2.574 final drive.

my 65 pontiac tempest ran last with a 455/6-speed/3.23 rear. the trans was 0.76 od, so a 2.45 final drive. 19mpg from Denver to soCal. my new rear end for that car is 3.00:1 so 2.28 final drive.

had a c5 z06 corvette, top gear in that car was 1.90:1 final. i know these are quite different (meaning, lighter!) cars but make interesting examples.

does your imperial also have an 8 3/4 axle? if so... easy to change!!

pics on http://www.flickr.com/photos/hpivonka/sets/72157600005938126/