You can have one of two possible speedo errors. Either the gear is
incorrect, causing a ratio problem (such as you described), or a dial
calibration error. A dial calibration error (imagine your speedo needle
being bent) would cause a constant error across all speed ranges.
-----Original Message-----
From: dave riffel <cafe_dave@hotmail.com>
To: oletrucks@autox.team.net <oletrucks@autox.team.net>
Date: Sunday, August 29, 1999 3:38 PM
Subject: [oletrucks] Ton-Up AD or Speedo Calibration Mahem
>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
oletrucks is devoted to Chevy and GM trucks built between 1941 and 1959
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