Dave, it's some kinda math thing. It's a speedo gear ratio/percent kinda
thing. My speedo was way off when I put the car 4-speed in. This mismatch is
probably due to 30 inch tall tires with a 3.08 rear gear ratio. I didn't do
any calculating but the guy at the trans shop gave me a gear for the output
shaft on the trans with more teeth on it. This would slow down the
speedometer by whatever percent teeth(don't remember now) I added. Since, I
have driven by the portable radar thing. if my memory is correct, the speedo
said 40 and the radar said 50. Using my math, I figured I needed to reduce
the speed of the speedometer by about 20% (10 mph from 50 mph). I took out
the cable speedo gear, it had 22 teeth. 22 teeth minus 20%(4.4 teeth) equals
about a 17 or 18 tooth gear. The Chevy dealer had an 18 tooth gear, close
enough. Just last week I drove by the portable radar at 50 mph and again at
30. (can't go too fast, its a 30mph zone near a school zone)As close as I
can tell its near perfect.
My old trans had an adapter on the speedo connection. It broke and I put
back together without it. I think your speed calculations are about what
mine was with the old traans.
I hadn't thought about it much, but once I drove 80 mph with the old speedo
gears (20% off). Was I doing 96?!
If any of the math is wrong, sorry, but I somehow ended up with an acurate
speedometer
----- Original Message -----
From: dave riffel <cafe_dave@hotmail.com>
To: <oletrucks@autox.team.net>
Sent: Sunday, August 29, 1999 3:15 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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