At 06:31 PM 3/25/97 EST, dmeadow@juno.com wrote:
>Most of you are probably aware of the T-Series (especially TD) "rear
>differential problem". The problem being that the stock TD came with a
>5.125 ratio rear end. This high ratio means that the motor will thrash
>itself to death at highway speeds. Many people have put MGA rear
>differentials (4.55, among other options) into their TD's to make them
>more drivable at highway speeds.
>
>My question is this: Does placing radial tires on a TD exacerbate this
>problem? Some time ago a friend of mine measured a 5.60/15 bias ply
>(original TD size) and found that the circumference of the tire was a
>good inch more than the circumference of the equivalent (155/15, I think)
>radial tire. This makes sense given that radials are supposed to be
>"lower profile". It seems that if you put a radial tire on the TD, it
>will make the engine RPM even higher for the same speed than if you use
>bias plies. I looked at the Coker tire web page that someone mentioned
>to compare statistics on radials vs. bias ply. A certain 155/15 radial
>had about a 9/10ths of an inch smaller *diameter* than a 5.60/15 bias
>ply, which would translate into a 1.41" difference in circumference
>(thank Pythagoras for good old pi), which seems to more than confirm my
>friend's observation. What would be the math to figure out the
>difference in RPM? If I use the increase in circumference as a
>percentage of total circumference, I get an increase of about 3.5%, or
>175 RPM at 5,000 RPM. Is that a valid calculation?
>
>y
>
>
>I'm trying to decide which tire to eventually put on the TD, and, if
>possible, avoid the considerable expense of putting in an MGA diff.
>
>Thanks in advance for your help.
>
>David Littlefield
>Houston, TX
>--------- End forwarded message ----------
>
David, I might be wrong, but I think that the original size was 550 -15,
not 560. In any event I run radials on my TF with original ratios. I would
not drive on bias after riding on radials. As for changing the ratio, while
I do scream at highway speeds, I rather slow a bit on the few times I am on
the interstates, and have the acceleration and zip on the local and back
roads where I drive the most. One mans opinion, most of my friends have put
in MGA rears, and love it.
Mike
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