Speedo Accuracy

[55 Fargo]

Hey all, 2 weeks ago I installed new speedo glass, itno my dash, I also swapped in a different speedo.

This speedo works well, the other would bounce more, and the odometer was stuck.

Now the donor speedo, the needle does not bounce much, both the trip and regular odometer work.

I do find this speedo seems to register actual speed better. Is this possible, because the odometer is working on this speedo?

I have not verified it with a GPS yet.

The old speedo was registering about 8 to 10 mph too fast, this speedo seems closer to actual speed.............Fred

[BobT-47P15]

I suppose the working odometer could be related to why this one

does better. But, am no expert on the inner working of speedos.

My odometer works, and the needle doesn't bounce much, it's

just off by about 15 mph. Hard to tell. Have been considering

sending one of mine to a repair place - may cost up around $100

or so from what I can gather to have it gone thru.

What one repair place said when I called them - you send it to

us, we look at it and give you an estimate. Then you can either

have us go ahead and fix it, or we can send it back to you (you

pay shipping again) unrepaired if you don't like our price.

[greg g]

The cable does a couple of things inside the speedo unit. It spins the magnet which spins inside a drive cup attached to the speed indicator needle, and it turns the gears which drive the Odo. The odo drive is strictly mechanical, while the speedo needle depends on the strength and consistancy of the magnetic field against friction and the hair spring which returns the needle to 0.

T

he gear ratios between the transmission and the drive cable are important to initial accuracy,

But even though they are driven by the main cable, they can be quite different in accuracy. It one of those mechanical mysteries

[greg g]

The cable does a couple of things inside the speedo unit. It spins the magnet which spins inside a drive cup attached to the speed indicator needle, and it turns the gears which drive the Odo. The odo drive is strictly mechanical, while the speedo needle depends on the strength and consistancy of the magnetic field against friction and the hair spring which returns the needle to 0.

The gear ratios between the transmission and the drive cable, and the tire diameter as compared to original are important to initial accuracy.

Even though they are driven by the main cable, they can be quite different in accuracy. It one of those mechanical mysteries

[55 Fargo]

Hi Greg, just looking at a 47 Chrysler speedo, could the speed be adjusted by making adjustments to the speedo needle shaft, and the needle stops, not sure if that is what is done to correct these.......Fred

[greg g]

Don't know enough to go there. You can probably make small changes by varyig the resistance of the hair spring. To make larger adjustments you need to change either the drive gears at the transmission, or increase or decrease the magnet's strenght.

[DonaldSmith]

Scott the speedo man rebuilt and adjusted my speedometer. He spun the speedometer a set number of revolutions per minute, I think its 1000, with a special device. Then he adjusted the hair spring so the speed registered at 60 mph. So the odometer and speedometer needle were in sinc.

The speedo still read (past tense, pronounced "red") about 10 percent too fast, so I bought a ratio changer on line, for a ten percent reduction. (I had to cobble the tip-toe transmission cover plate to make room for the ratio changer and end of the cable, but it does not show under the carpet.)

So now my speedo reads (present tense, pronounced "reeds") right on, as verified by my Garmin.

(I've been thinking of changing spellings, to clarify the pronounciation words like of "read" and "lead". Present tense will be "reead" and "leead" for the long "e" sound, not to be confused with the aquatic plant "reed" or the verb "to lead". Past tense for "read" will be "redd" not to be confused with the color "red". The metal "lead" will be "ledd", not to be confused with the past tense of "lead", which is "led".)

Stranger things have happened to the language.

[RobertKB]

Hi Greg, just looking at a 47 Chrysler speedo, could the speed be adjusted by making adjustments to the speedo needle shaft, and the needle stops, not sure if that is what is done to correct these.......Fred

Fred, my advice for what it's worth and it may not be worth much. Check out the accuracy using your GPS as mentioned. Know the difference. Live with it. Old speedometers are notorious for being inaccurate, usually airing on the optomistic side.

[55 Fargo]

Fred, my advice for what it's worth and it may not be worth much. Check out the accuracy using your GPS as mentioned. Know the difference. Live with it. Old speedometers are notorious for being inaccurate, usually airing on the optomistic side.

Hi Rob, I agree, and that's exactly what I will be doings.

Hey this newer speedo does the colr change thing at night, very cool, my Son noticed it before me, on a night ride last night.

Hey I have put on a 150 miles on the car the past 3 or 4 days........Fred

[BobT-47P15]

One seller of speedometer items on ebay has included this chart in his

ad. I thought it might be helpful here.

Link to the ad: http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=360245031321&viewitem=&sspagename=STRK%3AMEWAX%3AIT

If the tire size or gear ratio is changed, the speedometer will no longer read accurately. By changing the speedometer gear,

you can accommodate for such changes. Generally, one tooth change will equal 3 miles per hour.

(Use the chart below to help you determine if this is the correct Speedo Pinion that you need.)

Mopar Speedometer Pinions

Tire Overall Diameter

AXLE RATIO 24" 26" 28" 30"

2.76 30 Teeth 28 Teeth 26 Teeth 24 Teeth

2.94 32 Teeth 30 Teeth 28 Teeth 26 Teeth

3.21 35 Teeth 33 Teeth 31 Teeth 29 Teeth

3.55 39 Teeth 36 Teeth 33 Teeth 31 Teeth

3.91 43 Teeth 40 Teeth 37 Teeth 35 Teeth

4.10 45 Teeth 42 Teeth 39 Teeth 36 Teeth

4.30 44 Teeth 41 Teeth 38 Teeth

4.56 45 Teeth 43 Teeth 40 Teeth

(shipping is $5 in the 50 U.S. States)

MAKE SURE YOU TAKE A LOOK AT OUR LARGE INVENTORY OF NEW MOPAR PARTS IN OUR E-BAY STORE UNDER THE NAME OF

BILL KAY CHRYSLER

BILLKAYPARTS

Edited March 22, 2010 by BobT-47P15

[BobT-47P15]

Then there is a gear ratio adapter to change the speed......

Link to seller's ad: http://cgi.ebay.com/ebaymotors/Gear-Ratio-Adapter-to-slow-your-Speedometer-down-27_W0QQcmdZViewItemQQhashZitem3a58f8350dQQitemZ250600764685QQptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories

There is a whole section on ebay about speedometer gears and adapters. Just need to know what you need.

I had never looked there before. Just putting this out for info for anyone interested.

Edited March 22, 2010 by BobT-47P15

[Plymouthy Adams]

Bob T, is that for the older mopars or the later model? this is the chart for older mopar application..sorry for the size of the file..I copied this long ago when it was first posted here on this forum, I do not recall who submitted it..sorry...

[P-12 Tommy]

Even my reading glasses don't help on this one.

[Plymouthy Adams]

maybe someone with the book will go to page 314 and scan a better version..

[P-12 Tommy]

BTW.... What does a speedo gear look like in our 3 speeds? I've never seen one.

Tom

[Jim Yergin]

BTW.... What does a speedo gear look like in our 3 speeds? I've never seen one.

Tom

Here you go.

Jim Yergin

[1949P17BC]

Just got my 47 speedo back from Scotts Speedometer repair. Speedo worked not the odometer (had run a long time with damaged gears) $300.00 to fix, reset at 0 miles

[Jim Yergin]

Bob T, is that for the older mopars or the later model? this is the chart for older mopar application..sorry for the size of the file..I copied this long ago when it was first posted here on this forum, I do not recall who submitted it..sorry...

I think I am the one who submitted it. I found it in the parts book located on Chet's site.

http://chetscoins.com/1948p15/DocumentWeb/Parts_Reference/4/314.pdf

Jim Yergin

[Plymouthy Adams]

I can't get there from here....though I did try....thanks for the repost...

[P-12 Tommy]

Here you go.

Jim Yergin

Thanks Jim.

Tom

[BobT-47P15]

Tim, he must have been talking about something newer as there are

about twice as many teeth listed than our books say for a P15.

The pinion looked like this........

I think I'll try sending a message to the seller describing the model of

car, rear end ratio of mine (4.2-1) (from a 39 Dodge, maybe truck),

and 15" tires which measure approx 27" tall. Will see what reply I

get.

Chet's page came up OK for me and is definitely readable.....found the

part number mentioned in my manual.

[P-12 Tommy]

Tim, he must have been talking about something newer as there are

about twice as many teeth listed than our books say for a P15.

The pinion looked like this........

I think I'll try sending a message to the seller describing the model of

car, rear end ratio of mine (4.2-1) (from a 39 Dodge, maybe truck),

and 15" tires which measure approx 27" tall. Will see what reply I

get.

Chet's page came up OK for me and is definitely readable.....found the

part number mentioned in my manual.

That's exactly the same type of speedo gear I put in my '66 Charger.

Tom

[Plymouthy Adams]

thanks..I was behind a firewall thus could not see the photos at the time..but I remember one time seeing this guy on the bay..and remembered they were later model stuff..I also sad to say only had the poor copy of the gear chart on that particular computer..but... I did try to help...lol

Edited March 23, 2010 by Tim Adams

[BobT-47P15]

"I think I'll try sending a message to the seller describing the model of

car, rear end ratio of mine (4.2-1) (from a 39 Dodge, maybe truck),

and 15" tires which measure approx 27" tall. Will see what reply I

get. " QUOTE

THEIR REPLY ----

Thank you for visiting BILLKAYPARTS.COM The part you are looking for has been CANCELLED from Chrysler. Our only suggestion would be to call up your local auto salvage yard and see if they have a used one for sale.

If you have any other questions, feel free to contact us at:

BILL KAY CHRYSLER

attn: Parts Dept.

2100 Ogden Ave.

Downers Grove, IL 60515

(630) 968-9270

[BobT-47P15]

So, where does one buy a new speedo gear for our cars??