A very simple formula, which can quickly be entered into a simple
spreadsheet, that covers everything except tire expansion at high speed, is:
Speed, in miles per hour, equals (RPM x C)/(88 x Gt x Gd)
RPM is engine RPM
C = circumference of YOUR properly inflated tire, in feet
88 = is the constant that takes care of the feet/min to mph bit.
Gt = the transmission gear ratio - for OD gears, simply multiply the box
ratio times the OD ratio (for 4th OD, "Gt" would BE the OD ratio, such as 0.822)
Gd = the differential gear ratio
By this formula, at 3000 in 4th OD, I get 71 for a 3.9 and 79 for the 3.55.
At 5000, it gives 113(4.1) 119(3.9) and 131(3.55). IMHO, for a stock Healey,
you will run out of power (to say nothing of good sense) before you run out of
revs, even with a 4.1.
I HEARTILY concur with David's 80MPH comment, for most/many of our "daily
street drivers" - which is another reason why I'm still waffling about changing
pumpkins. If it were not for highway cruising (which I personally do not do a
lot of, right now) I'm not sure the 3.9 (which I have) or even the 4.1 would be
so bad - I'm a LOT more interested in the fun of "the push in the back" than
seeing how bloody fast I can go in a 45 year old car.
Obviously, Chris Dimmock is in an ENTIRELY different situation, driving a
heavily modified, WELL-maintained race car, where he can feel confident that it
is going to hold together, and stay on the road.
Dick Hosmer
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