Technical Speedo problem

I am going to buy gauges . How do you calibrate a speedo? Do I have to get some kind of adapter? I am using a 39 ford trans and a 37 rear axle with 3.70 gears. Is it legal to just run a tach with marks on it to indicate speed in high gear? Thanks Dave

 

They have electronic ones now that you calibrate in a measured mile.

 

Yep, even GPS speedometers that don't need any calibration, although I'm not sure what they look like.

We calibrate the electronic speedometers by having a p***enger with a hand held GPS set it (leave the wires long enough for the guage to hang out of the dash).

 

or....you could just use math.

# Drive Teeth X Axle Ratio X Tire Rev. per Mile = # Driven Teeth

Tire Rev. per Mile = 20168 divided by Tire Diameter (inches)

 

I dont want to use a digital speedo as I am trying ti build a pre 1960 style car Dave

 

They're not digital, just electronic instead of a cable. They still pick up the signal from where the cable would attach.

 

It would be interesting to know how many of you actually have a working speedometer in your hot rods! I don't think I ever have.

 

There is a chart in the Green Bible showing which speedometer gear cap to use with which rearend ratio and which size tires. Look it up, try and find the gear cap you need. Done.

 

always ck with a GPS then change speedo gears accordingly--have done this is 11 40 fords with stock type speedos--a couple I had to get a little gear box adapter to get it right--very easy

 

I drove my '40 coupe for about 16 years with only water temperature and oil pressure guages. Finally put a full set of Cl***ic Instrument in there nine years ago.

 

I dont know what a green bible is. Where do you get gear box adapters?

 

gear box/speedo adapters are available on ebay--either straight hookup or 90 degree--with ratios-find out how far you are off and go from there

 

If you can run the car on a measured mile and let me know how many 1/10's you are turning in a mile I can build you a gear ratio adapter to get the calibration correct.

 

If you are using a 1939 transmission with a closed drive shaft you can adapt the "turtle" speedometer drive to an electronic speedometer. Dakota digital has the parts. I have a GPS speedometer. It works OK but takes a few minutes to lock on to the satellites. If you want to use a cable drive you will have to figure which gear you need to be accurate.

 

I have gear driven speedos in both my hot rods. First I checked each one versus GPS at every speed from 5mph to 85mph at 5mph increments. I then provided the data to a speedometer outfit that provided a gear box that I attached to the transmission for the cable output. Both cars now have very accurate speedometers which I periodically check per GPS. No sense having instruments that are not accurate.