1966 Plymouth Valiant Signet Transmission
 

Hello. I'm new to the forum as I've just recently purchased a 1966 Plymouth Valiant Signet with the 273 V8 and Automatic Transmission option. Being a new proud Mopar owner I have a few question with regards to the automatic transmission option and the proper shifting.

Were the automatics for this year a 3-speed or a 4-speed auto transmission? Mine seems to be a 3-speed auto, which is fine for around the city, but when I get it out on the highway I feel as though the revs are too high and that I'm missing a gear. Are these cars meant to rev high at highway speed? Is there a way to make it more highway friendly?

This may or may not have any direct relation to my question, but my speedometer also appears to be 15-20 mph off. On the highway in a 70mph zone, with the motor screaming, the speedometer reads 80 while school buses blow by me. I can live with the speedometer being off, but I need help with the high revs.

Thanks for your input, help, and/or suggestions.

No Overdrive on the 904 3 speed auto. Someone may have changed rear end gears or the transmision which would make the speedo off. If you are not looking for performance you could go with a freeway rear gearset like 2.76 but you will need the correct speedo gears to match if you want you speedo to read correctly. Your other option is a later model trans with overdrive.

Only 3 speeds autos were made for that car
Most speedometers are off, but they do tend to get worse with age.

Transmission / Master Cylinder Swap
 

Thanks for your help. Something to live with I suppose. If I were to put another transmission in it with overdrive what would you suggest?

Also, I'm looking to upgrade the "SUICIDE" single piston brake master cylinder to a dual piston brake master cylinder, any suggestions in this department?

Welcome!

While most speedos do have a tendency of misreading as they get older, it is not something that you have to "just deal/live with". There is a speedo gear that is usually in the tailhousing of the transmission. Look at that and make sure that it is not worn out. They usually have 4 positions as these transimissions were used in a variaty of vehicles with different motors and rear axle with different gear ratios. You may need to replace it and/or rotate it to find out what reads best for you.

While you are at it, take a look at the speedo cable and make sure that it is not worn out on either end. Then take a look at both connection points (at tha tranny and at the speedometer) and make sure that they are good. If your speedometer needle "bounces" as you are driving, then you may need to replace the one, two or all three componants.

For an OD transmission, you'll want to look for a A500. This transmission is an 904 with OD basically. You'll be able to find them in vans (B150/B250) and some trucks (D150) from 89-93. This was the first 4 speed automatic that Chrysler came out with for RWD vehicles. You may have better luck finding a 518A though (same type of tranny but it replaced the 727).

You should also see what you have for a gear ration in the rear axle and install a tach.