# Need help with Dip switches on back of speedometer

#### gkmissingca

Nov 20, 2017
2
I'm bought and modified a firetruck to use over the road. I changed the gear ratio from 4.11 to 3.55. On my test drive, my speed app read 60mph, truck speedo was 50 mph for a difference of 17%.

The only info I was able to find on a Prestolite speedometer was:

I took electronics in the Navy but that was over 20 years ago. Anyhow this pic is the original positions and (after lots of trial and error) this pic is what I stumbled on as correct.I would like to note the embossed legend to the left, 0 and 1 seem opposite of what a normal dip setting should be. Since you can't really see the dip numbers, #1 is on the left.

Anyhow- I will be changing the gear ratio again to 3.25 for a difference of 8%, and will need to change the speedo again. If someone could help me figure out how to decipher this I would appreciate it. I have tried standard binary decoding and have come up empty handed with anything that makes sense, or I'm doing something completely wrong. Thankyou

#### kellys_eye

Jun 25, 2010
5,095
You could apply a (known) signal and adjust ONE switch at a time and note the difference in reading it makes. When you've done all the switches individually - in theory, each individual switch will increase the 'change' by 100% - you should have sufficient information to allow you to trim the reading no matter what input you apply.

#### gkmissingca

Nov 20, 2017
2
Ok, I think I got it figured out. Visually it doesn't make sense, but running the numbers it does. Remember my speed was off 17%. So it started with

0011111100 =672 pulses per mile (ppm)

I stumbled on a correct setting of 0001101100 = 288 ppm on the chart. But this doesn't make sense as that's less than 50%.

Not sure why, but I inverted the binary code to make 0011011000=576

576/672= .857 or 86% i.e. 14% difference - close enough for me.

So ideally, when I change to 3.25 gears for a difference of 8%, I should be able to switch to 0110001100 (00110001100 inverted) for 527ppm.

Hopefully this might help any one else that searches for this same problem.

J
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