Speedo reading

[daddyo23]

Has anyone had the speedo read about 10-15 mph fast? When I got my P15, the cable wasn't hooked up. I pulled the inner cable out and lubed it up good. Put it back in and it still reads fast. Could it have something to do with the trans hook-up? I'm not the brightest bulb on the tree when it comes to some of this, I'm learning every day. Any help would be appreciated. Thanks,

Wayne P.

[Normspeed]

If the rearend has been changed over the years to a different ratio, the speedo would be off unless the speedo drive gear were also changed. Tire size can affect it too.

[Brendan D25]

Possibly, someone might of replaced the speedometer cable with one from a different model and used the incorrect gear that goes in the transmission. I think they are colored coded for different ratios. Maybe someone else could enlighten us on this. Brendan.

[Young Ed]

The mopar ones have a number stamped on them like 18T. You have to match that up to your rear end ratio.

[Plymouthy Adams]

our own member Jim Yergin posted a chart last month I think on what gear goes with what ratio rear end etc...the chart was a bit fuzzy so I did not save it to my files..perhaps Jim can scan it again and place on this forum..

[Jim Yergin]

Tim,

Here it is. Sorry it is so blurry.

Jim Yergin

[Plymouthy Adams]

Thanks Jim..I found I had save that image after all and blew it up some and can make out the ratio, tire size and gear count..part number..not so good...also pay close attention to the note on what trannies had a 5 or 6 tooth drive gear on the tailshaft of the tranny..this is the gear that ultimately drives the speedo cable...through the 16-17-18 tooth gears..

If someone here has the 1938-48 book and can go to the transmission section and scan a clearer copy..would be an asset for our technical section. Would really like to see thaem part number snad tranny notes a bit clearer...

[Jim Yergin]

Tim

I got the chart from Chet's website where he has posted the parts book. It is a pdf image so I had to print it off and then scan it in order to post it. That is why it is so blurry. But if you go directly to Chet's website (http://chetscoins.com/1948p15/DocumentWeb/) and type in the password: "password" you will find a clear copy of the chart at page 314 of the book.

Jim Yergin

[55 Fargo]

Hey Daddyo, 1st determine what rearend you have, what size of tires do you currently have. If the rearend has been swapped out, or your trans is got a different speedo driven gear, you can alter and change your speed reading.

Shorter tires, lower gears in diff i.e. if the car was equipped with a 3.90 diff, and it now has a 4.11 diff, you will have a speedo reading higher than the actual speed your travelling. Add shorter tires, and you even widen that margin of error.

Let us know what diff gearing you have, the size of your tires, and see what the rest of us can come up with...........Fred

[55 Fargo]

Unless the speedo driven gear was pulled out of the trans for some reason, yes it would alter the speedo, but just putting in a new cable shouldn't do anything.

I had this very problem, most of was caused by, a lower geared diff was swapped in, it was a 4.31, I now have a 3.73 set of gears, but the speedo is still out a bit, my tires are tall enough, but the speedo is a 17 tooth dirven gear, not sure if that might still be my trouble or not....................Fred

[Young Ed]

Rumor has it Wayne is looking for a 3.73 or 3.54 to swap in. Then his speedo will have to be corrected for sure!

[Plymouthy Adams]

here is the deal...I have a very nice scan now of this page...I can transmit e-mail but will not be able to host to my photobucket till later..if someone here would like me to mail it to them so they can host it..and then paste the url...send me your private e-mail via PM

[Young Ed]

[daddyo23]

Thanks for the response! Thanks Tim and Ed for the chart. I'll climb down there tonight and check the rear end, but it looks original. They may have changed the tranny, the guy gave an "extra" he had when I picked up the car. I'll check that,also. The rumor is true, I would like to find a gear ratio a little more highway friendly. I am planing on talking to Olddaddy about splitting my manifolds and getting a shock kit. I'm also on the hunt for a cam, that way I can have it ground and throw it in. I figure with the hop-ups it should be able to handl the taller gears. Any comments, advice or rants will be avidly read. Thanks again,

Wayne P.

[daddyo23]

Well, I got out there and raised her up abit and scraped all the gunk off and found the numbers, 39f. I kinda figured it was stock. I'll have to pull the speedo drive I guess. Should I just count the teeth? I'm running p215/75/r15 tires. Anything else I should look for? Thanks, Wayne P.

[Normspeed]

The speedo gear is really easy to take out. On later transmissions like my 53 the gear is nylon. Yours is probably good old American steel. They all interchange though.

When it comes to Old Mopar speedometers and fuel guages, it's easier to live with their quirks than get them to work right:D

[daddyo23]

The speedo gear is really easy to take out. On later transmissions like my 53 the gear is nylon. Yours is probably good old American steel. They all interchange though.

When it comes to Old Mopar speedometers and fuel guages, it's easier to live with their quirks than get them to work right:D

That's the way it seems sometimes. Maybe I'll just have someone clock me once and see where it's at. If I know roughly how fast I'm going, that would work.

[Normspeed]

I watch for those portable trailer radar checks where it tells you the speed limit and how fast you are going as you approach. When you find one, you can make several passes at different speeds.

Remember when you could get your speedometer checked and they would give you a little calibration card to stick on your instrument panel?

[Plymouthy Adams]

Nowadays those are light blue and about 4 x 7 and has a date to appear written on it somewhere..