In an article in our newsletter, Pat McMullen wrote about tire sizes and what
speed you are traveling with different sizes.So he had a formula. This formula
will not work on automatic tranies.You need 3 of these 4 things.
1.RPM
2.TD= tire diameter
3.MPH
4.DR= drive ratio(final drive)
RPM is self explanitory,you have tire size but need to get tire diameter(ex.
205 65 R15 205 is tread width in millimeters.Convert to inches(205 times
.0394=8.077). The 65 represents tire profile, a percentage of the tread
width.So 65% of 8.077 is 5.25 inches. Now rim size is 15 inches in diameter. So
you add that to the tire profile twice to get tire diameter.(5.25+15+5.25=25.5
inches). Now you got MPH. Now to figure DR you use this formula.
DR = RPM x TD divided by MPH x 336 (336 is a constant and
will never change)
Hope this will help. It was a great article by Pat.
Kevin Andrews
68 TR-250(CD7766L)
Siler City, NC
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From: tonyr@dzn.com[SMTP:tonyr@dzn.com]
Sent: Monday, September 30, 1996 10:05 PM
To: triumphs@autox.team.net
Subject: Re[2]: What differential?
JO>>
JO>>As a warning, if you follow this process with that car jacked up and with o
JO>>wheel touching the ground and only on wheel turning, you will have to multi
JO>>your answer by two to get the correct answer
JO>>
JO>> ratio =(length of string x 2)/diameter of drive shaft.
JO>>
JO>>Zenon
JO>(snip)
JO>yes, perhaps i didn't read the original procedure description closely
JO>enough, but it might be easiest to have it on the ground and push the whole
JO>car one revolution of the wheel so that both wheels would be forced to turn
JO>the same distance if you could find a way to do this and still retrieve and
JO>measure the string, otherwise, jack up one rear wheel only with other on
JO>floor as is often done for balancing rear wheels and follow above
JO>directions
JO>joe
Forgive me guys, I am not picking on you, but this is the most assinine
theory I have ever heard.
The ratio cannot be determined by the circumfrence of the tire in
relation to the diameter of the drive shaft.
The ratio is determined by the number of teeth in the pinion gear and
how many teeth it has to travel on the similiarly beveled ring gear to
make one complete revolution and then how many times the pinion gear
has to turn to make the ring gear make one complete revolution to
complete the final drive.
Sorry, I just don't see anyway you can determine the ratio considering
the variation in tire sizes and the variation of non-stock drive shafts,
like mine.
Anybody care to elaborate. Maybe I am wrong, but I don't think so.
Regards,
Tony
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