Mechanical speedos ?

[kiwi mopar]

Hi guys

i know how a Mechanical speedo works with the rotating magnetic field, and being balances with the hairspring to give you your speed.

but here my ?

if you take out your speedo gauge and put in a later model one or even a ford one or one from a U.K. car.

as long as the fitting on the back of the speedo gauge all fit wood it still read the same speed ?

or are the magnetic field all differant.

[Plymouthy Adams]

the Porsche guages I installed in my 41 Dodge pickup with the big block mopar and 727 was accurate..converting the tach to register correctly for the V8 was also just a matter of installing a trim resistor and I placed it external for tweaking if needed..it was adjusted on the bench with known standard and worked excellent int he truck also...

[james curl]

My understanding is that all U.S. speedometers will read 60 MPH at 1,000 RPMs. RPM= (mph x gear ratio x 336) divided by tire diameter. In my case 22 mph x 3.55 x 336 divided by 26.22" = 1000.8237 rpm at the speedometer so what the speedometer people told me so that I could calibrate my speedometer was wrong. I have always worked on the theory of 20 mph per 1000 rpm which is true in direct drive in most vehicles. From old Road and Track info.

[TodFitch]

Echoing Jim Curl, I was told a long time ago (1974) by an old (at the time) speedometer repair man that all US speedometers were setup for 1000 turns/mile on the cable. So 60 MPH = 1 Mile/minute would be 1000 turns/minute (RPM).

Also, working backward from the tire size, rear end ratios and speedometer gear numbers in my 1933 Owners Manual indicate that my car is pretty close to 1000 turns/mile for the various configurations of the car that were available.

[martybose]

When I had my car at the speedo shop a long time ago, they drove it over a measured course with a counter in place of my speedo, then put in a gearbox to correct the count to be 1000/mile. Then they used a 1000 RPM bench source to fine tune my speedo to be accurate.

Marty