Bob -
I hate to break it to you, but the standard ratio TD diff. is 5.125:1. Even
worse than you suspected.
Nonetheless, I've been running happily of this ratio for almost 15 years
with no real adverse effects. I admit it revs more than I would like, but it
does make for quick starts. I've often said that I think I could climb trees
with that car.
You can put in a lower ratio gear set. The MGA used a 43/10 ratio and is a
popular conversion. Carl Cedarstrand wrote an extensive book on the subject
of conversion. It's available from Moss. It requires an MGA ring and pinion
and flange. It's not a difficult conversion, but takes a lot of fiddly
effort in setting the thing up correctly. I've been planning on doing this
for years, but have never found the occasion to take the car off the road.
Meanwhile, I run about 4250 RPM at 60 MPH. I used to run 4250 at 55 MPH, but
changing to radials bumped me up to 60. I find that this is an acceptable
RPM. It used to seem like I was over-revving. The valves and springs used to
bounce at around 4500 RPM, but that had nothing to do with the tires and
diff. I found that the wear in my valve rockers was excessive and suffered
from being ground too many times. I have now had these redone (filled and
ground). Now there is virtually no valve backlash or bounce and the engine
is much quieter at 55-60 MPH. Although I would like to be running at 3700
RPM at 55, I'm not sure I want to do all the work or spend all the money
necessary to change the ring and pinion and recalibrate the speedometer. If
you do the work yourself, the parts alone for the conversion are around $250
(new ring and pinion, used flange) or $600-$700 (exchange) if you buy a
completed axle.
Another $100 to recalibrate or regear the speedo.
I also run on radial tires (Dunlop SP4s - 165SR15). I changed from bias ply
tires about a year ago and until then didn't know what I was missing. The
radials make a huge difference in road-holding and smoothness of ride. I'd
never go back. I did have to put in tubes, however. Without them the old
rims wouldn't reliably hold air. I run 30 lbs in all tires.
Am I wearing out the engine too fast? I don't know, but I get a lot of joy
from the TD and I don't really mind the low ratio.
Regards,
Lew Palmer
lew.palmer@uci.com
----------
From: mgs-owner
To: mgs
Cc: british-cars
Subject: MG-TD Rear Axle Gear Ratio
Date: Sunday, September 24, 1995 7:58PM
I am very much confused about the highway capability of the old TD. I am
currently restoring one and would like to take it on short 50 mi trips where
the speed limit reaches 65 mph. I hear that the high gear ratio (4.875:1)
makes the
engine rev too high. What is the maximum crusing speed without undo engine
wear? Also, regarding tires, I was recently advised that radial ply tires
would make the rear axle ratio even higher, and I should use bias ply tires.
Are 15in bias ply tires bigger than 15in radial ply tires? I measured the
distance from the ground to the axle on my old bias ply tires and found it
to
be 12.5in. Where does the 15 come from? Has anybody tried slightly larger
diameter tires to cut down the RPM? Oh, by the way, what is the correct tire
pressure for radial ply tires?
Please share your wisdom, oh wise sage of the cyberspace.
Bob Donahue " 50s FOREVER "
rdonahue@holli.com
1953 MG-TD
|