Having dealt with trucks, where tractibility and gear ratios are are of
the equation, I can tell you that it depends on where you live, what you
want to do. Going to a 4.3, 4.1, or 3.9 rear end ratio is possible, but
there are trade-offs. First of all, it is doubtful that you have a
standard bore engine any more, unless you've resleeved and gone back to
66.5mm bore. So, you have a larger than 1250cc engine. OK.
Now then, using a Pirelli 165 R 15 tire for reference purposes, it has a
diameter of 23 inches; a rolling radius of 11.5 inches. The number of
tire revolutions per mile = 10084 ÷ RR (11.5"), which equals 877. What
final drive rpm would you like to run? As the rear end ratios are not
variable, the choices easy to figure. All calculations will be made at
60 mph.
4.55 rear end = 60 ÷ 877 X 60 ÷ 4.55 = 3990 rpm
4.3 rear end = 60 ÷ 877 X 60 ÷ 4.3 = 3770 rpm
4.1 rear end = 60 ÷ 877 X 60 ÷ 4.1 = 3595 rpm
3.9 rear end = 60 ÷ 877 X 60 ÷ 3.9 = 3420 rpm
The ideal axel ratio if you wanted to go 60 mph at 3000 rpm would be:
3000 ÷ 877 X 60 ÷ 60 = 3.42. Well, you can't have that one. If I were in
a mountain area, I would not go below a 4.55 rear end. Some hills, a lot
of freeways, and flat 4.3 or 4.1. 3.9 is useable in a 1500cc (1466cc)
engine. You will get to use third gear a little more, so tha you will
now ahve two gears to use; third and fourth.
Carl Cederstrand wrote a book on doing the actual work on changing the
gears in your TD/TF differential. You could take the gears out of your
MGA rear end (preferred) or locate MGA solid wheel axles, drums, and
wheels. The MGA rear end has stronger axles. BUT, the MGA wheels are
four bolt.
John Seim
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