Matt Beazer <teseract@moparhowto.com> wrote:
[...]
> Someone has dinked around with
> this thing since if you assume a stock tire size of 235/75R15 and
> compare it to a 31x10.5R15 tire, it SHOULD read 56mph when I'm actually
> doing 60mph rather than what it does now, which is read 64mph when I'm
> doing 60.
Hmmm... Any chance the truck wore 33s in a previous life? If
someone were to install 33s and correct the speedometer to that tire,
and then down the road the tires got swapped out to 31s without
correcting the speedo, 64mph is exactly what your speedometer would
indicate at 60.
If my calculations are correct, the same basic result could be had
if the speedo were calibrated correctly for the 31s and the rearend
gears were swapped from 3.23s to 3.55s or from 3.55s to 3.91s. (64mph
would be indicated at an actual speed of just over 58mph)
Another (highly unlikely) possibility might be to have the
speedometer left stock, switch to 31s and swap the stock 3.91 gears
out for 3.23s. I think this would result in about 67mph indicated at
an actual 60 though.
Something to keep in mind also is that 31s aren't always 31s. There
is a huge range of variability out there. You can't actually be sure
the tires are 31s unless you measure them on the truck, and if you are
calculating based on 31s when the reality is somwhat different,
obviously that can throw things off too.
Let us know if you find out what is going on there! Sure seems like
something got changed somewhere along the line - either the speedo
gear or the rearend gears, or maybe both. :-)
--
-Jon-
.- Jon Steiger -- jon@dakota-truck.net or jon@jonsteiger.com -.
| '96 Kolb Firefly, '96 Suzuki Intruder, Miscellaneous Mopars |
`-------------------------------- http://www.jonsteiger.com --'
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