Speedo Accuracy = Not

[55 Fargo]

Hi all, brought home the GPS from work, installed on the window of my coupe, then went to see how accurate I am registering on my speedo.

At 56 on my speedo, I am actually going 50 mph, at 62 mph, I am actually doing 55 mph.

Now I have a 3 spd trans, and engine from a 1951 Plym, my rear end is a 3.73, my tires are P205 75 15= 27.1 inches, the speedo gear is a 19 t, I think, as I have a trans with a 17 t also., so it is either a 17 or 19 t, it is the original that came with the trans

Now my engine and trans must have been mated with a 3.73 or a 3.90 rear end , I would think my speedo should be fairly close, given my ratios should be in sinc with a 1951 Plym, witha 3.73 rear end.

What other factors would or could cause speedo inaccuracy?................Thanx Fred

[greg g]

205's are too short especially for the Chrysler. 225's are closer but still not as tall as the original bias plies.

[Dennis Hemingway]

Tire Size. A lot of people have said that the 205 is too short, That is the tire I am running also and my speedo is off about the same MPH.

Dennis:D

[55 Fargo]

That possibly could be the problem, tire size, but a friend of mine has a 48 Chrysler, with the same size tires, and all else factory correct drive train and his speedo is acurate to about a mile or 2.

I don't mind this being out a bit, just was wondering what may be the cause, also was wondering if any of you Guys experienced the same deal......Fred

[randroid]

Fred,

On the bright side, think of the money you'll save in speeding tickets.

-Randy

(When I rebuilt my speedo back in the late eighties I applied WD-40 like holy water, and it hasn't run accurately since. I pulled it and cleaned as much of everything as I could, but it won't ever be close to accurate until I follow Dennis and Greg's advice on tire size.)

[55 Fargo]

Here is a guide for tire sizes and comparisons

http://www.turbinecar.com/tires.htm