RE: speedo and tach calculation

[farna@xxxxxxx]

1. Go to www.4lo.com. There are several useful calculators there. Just put in "1" (for 1:1) for the transfer case ratio. What you need to do is put in speed, tire size, and axle ratio. It will give you rpm. I know what you're doing, I had a speedo problem for the longest time myself!

2. Did the speedo read correct (or very near) before? Older TSMs (pre 1969) give speedo gear tooth count, newer ones don't. It's in the parts catalog for 1969 and later models. The last date for the speedo gear (and front spring specs as well) might be 1967 -- I don't have a 68 TSM to see if they are in there. You would need the tire size as well as gear ratio.

I have a 67-72 parts book. The 2.87 used a 30 tooth speedo gear w/D-70, E-70, E-78 14 inch tires or E-60 15 inch. The 3.54 used a 37 tooth speedo gear, same tires. You'll need to find a tire size equivalency chart to figure out what size tire you have. If it's not one of the sizes above (or close), don't worry! Those are the only sizes listed for the Javelin and those gear ratios, but there are other sizes listed for other Model 20 rear axle cars. Car series won't matter. But I need the LETTER SIZE to look it up -- that's the only thing listed. If you need to contact me again on this, don't forget to mention that it's on page 370 in the parts book so I can find it relatively fast! E-mail me directly if you need the tooth count with a different tire size. A conversion chart is here: http://www.tirerack.com/tires/tiretech/45_conversionchart.html

--
Frank Swygert
Publisher, "American Independent 
Magazine" (AIM)
For all AMC enthusiasts
http://farna.home.att.net/AIM.html
(free download available!)

original message----------------------------------------------

Date: Sat, 3 Dec 2005 23:02:37 -0800 (PST)
From: Marty Bricker <mbrickerusn@xxxxxxxxx>
Subject: speedo and tach calculation

I changed the rear end of my 74 Jav from a 2.87 to a
3.54 and my speedo is now showing much faster than
actual.  I believe I remember a site being mentioned
that had the formula for calculating speed from tach
reading based on various parameters.  Anyone have the
site bookmarked?  Thanks.
Marty
Is there a way to determine what speedometer gear I
need to order for my 998 tranny (I assume the MOPAR
guys would not be much help on  figuring it out but
they probably carry the gear)