Hello all,
Yesterday I sat down to try to figure out a conversion to use for my speedo.
The speed is very much innaccurate due to taller tires and an unknown axle
ratio. What I was after was a conversion equation I could use for any
indicated speed. I had previously had a friend drive in front of me to tell
me what speed we were going at 10 mph increments up to 65. Later I timed my
truck over several miles in Idaho to find the indicated speed at 75. So
then I basically knew the actual speed at certain indicated speeds.
I wrote all these down to find the relationship between them. It seems that
the speedo variance increases on a logarithm. For any particular indicated
speed, actual speed seems to be about 1.5 times indicated speed. For
example 20 mph incidated is actually 30 mph, 30 mph indicated is actually 45
mph, etc. I figured the logarithm to be Log(x)= 1.5x where x=indicated
speed and Log(x)=actual speed. Correct me if I got this messed up. Now
with this conversion I can theoretically convert any indicated speed to
actual speed quite easily, as long as the variance is constant.
When I first got back to Montana 4 months ago, I took the old '50 to a
friend's house for the weekend. This was only a few weeks since I had
gotten all of my updating and conversions done to drivetrain and chassis so
I decided to stretch her out a little and see what she would do. Over Elk
Park the highway has a long straight stretch that usually has very little
traffic and I've never seen Johnny Law there. This became the test grounds.
The truck accelerated very nicely up to an indicated 75 mph. I could tell
I was moving pretty good and the truck actually handled fairly well. At
this speed, though, with a rutted highway and a cross wind the truck started
to choose its own line so I slowed back down to a more sedate rate of speed.
That was brief but quite fun and gave me a sample of what my truck was now
capable of.
So I plugged 75 mph indicated into my new equation yesterday and was very
surprised to turn up with an actual speed of 112.5 mph! Holy cow! If I had
known this while I was doing it I would have gotten pretty nervous. If my
calculations are correct I've got a pretty fast AD now. Whew! That's
pretty cool, even though I'm not much of a speed freak. I don't expect to
do much further speed testing of this nature as I don't know if everything
is up to going that fast, especially on a public highway.
By the way, does anyone know why the speedo variance increases by logarithm
instead of a constant variance?
Dave Riffel
oletrucks is devoted to Chevy and GM trucks built between 1941 and 1959
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