NonTraditional Speedometer for Used / Homebuilt Car





Introduction: NonTraditional Speedometer for Used / Homebuilt Car

Ever buy a used car and the speedometer died or wasn't working but you realy wanted that particular car?

Here is one method to make it driveable and be safe doing so.

For the cost of only $20.00 one can buy one of these Radar Guns , disassemble it and Route the reader facing down towards the ground ( In a location that is not prone to getting grimed up) and connect the readout on the dash. Accurancy wise they are same (if not better ) in MPH. Another benifit is that it tells true Vehicle speed and not transmission speed.

I will add later the more illustrated procedure of doing so.

1. Wired method of data transfer from sensor to readout
a. "Quickie version" for the "damn thing just died" times.
b. "Clean version" for Homebuilt / Full time useage.

2. RF method of data transfer from sensor to readout.
a. "Mount and Go" for the habitual used car buyer ;)
b. "Remote Mounted" for use with independant monitoring
(example: Your own vehicle "black box")

NOTE: For those concerned with accuracy and this type of speed sensing method, this type of measurement method is used for both company road tests as well as there are industrial models used for speedometer readings for agricultural equipment.

ADDED: If you desire a more Robust version for this application
For about $86 USD you can get a Radar Gun made be "Bushnell" on

The Bushnell one is what I used originaly for this project. Mainly because at the time it was on sale for $75 as well as the "toy" one was not on the market to test with.



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    When my speedo quit I learnt the engine rpms for a certain speed in whatever gear. Rpms will vary up and down hills but you get a rough idea how fast your going and that's really all you need. I never got pulled over with out it and it wasn't due to conservative driving if you know what I'm saying

    seems like it would be easier just to fix the faactory speedo

    2 replies

    But not nearly as much fun or creative...

    Thats assuming you can diagnose the problem quickly and correctly. Bad speedo can be either , bad speedo cable, bad gear from cable to transmission, bad gear in transmission, bad seedometer itself. Or All the above. It's like that old phrase "Takes 5 hours to diagnose the problem, and only 5 minutes to fix " BTW speedo cables are on average $35-$50, Gears are usualy only available from the Dealer (assuming its still available) Those go for about $10-$20 plus a possible shipping wait. As for Speedo itself can go for $50 to swap out from salvage yard. Or about hourly fee at shop plus parts (est. on average $100 +)

    Personally if a vehicle I had to have, had a non functioning speedo, I'd repair it if possible/ affordable or install an after market speedo. That way you would still have an odometer.

    "NOTE: For those concerned with accuracy and this type of speed sensing method, this type of measurement method is used for both company road tests as well as there are industrial models" I'll have to find the forum -- but they determined the calibration was pretty off from the factory. Not to mention range was poor. I'll dig it up later if someone doesn't find it first (I'm not on my home computer at the moment) ;)

    6 replies

    I remember seeing it on the Make: Blog -- that's not the exact forum I was talking about, but there's more information there ;)

    Wait a second -- you said point it down at the ground.... Did you even try this? <-- pretty sure that's not going to work - just based on the physics behind this ;)

    Yes, I've tried this already both with this 'toy' and one I purchased from Radio Shack for $60. You stand still, you measure the speed of object moving. You move it measures your speed. These are not like police radar guns which compensate for their speed with the trackee's speed.

    OK I've never had the opportunity of playing around with one of these, but surely if you are pointing it directly down at the road, i.e. vertically down, the distance between you and the road is not changing, so it would read zero speed. I guess if you mounted it looking forward at a shallow angle to the ground, you'd get a reading, albeit not your true speed, but it would be fooled if you were in traffic and other cars got in the way of the beam. Than again I might be totally missing the wway this works

    Were you to actually try this in the application you offer, i'm betting it would be pretty much useless. Anyone know if this model measures the change in frequency of the beam or times several shots to calculate speed?

    Premise for this for low price quick repair. So that you can get on the road and driveable. Although the higher priced version I just added the link to would make for a more long term solution.

    Calibration for long distance maybe, but for this application the sensor is only about a foot or so away from the street.

    Hey guys, I'm not sure if directly pointing the radar detector at the ground would work, but I know it works if placed on an angle. I work for a county mosquito control dept. and we use radar units mounted on the rear of our trucks to monitor our speed and automatically vary the pesticide output... but in short, yea, it will work, theoretically, it can be done.. not sure if this fisher price unit would be too accurate, or last very long mounted under a car.. but still...

    kind of off topic, but this reminded me of one of my favorite things to do. hold a blowdryer out at passing cars and see if they visibly slow down!

    i saw a hotwheels one in Target, my friends wanted to mount one on a front of a car and watch how many cars with radars slowed down

    1. GPS needs to be out in the open to function properly. Ie: Can't be used in tunnels. 2. Some GPS systems have a slow startup time to calibrate with the GPS signals when you first turn them on. Some taking up to a few minutes.