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Old 07-09-2020, 02:31 PM   #1
Bob/AZ
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Default Speedometer error

My speedometer is off a little; it reads about 50 when the gps shows 46 or 47.
The rear end is a 3.78 and the wheels are 19". Is it the wheel size or just a bad speedometer?
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Old 07-09-2020, 03:03 PM   #2
Werner
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Default Re: Speedometer error

Hi Bob,

correct. With the wear of the tire rubber the outer diameter is minimally reduced. As a result, because of the smaller diameter the wheel turns faster than a new wheel for true spped. Your speedometer showes you a higher speed.
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Old 07-09-2020, 03:07 PM   #3
Kurt in NJ
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Default Re: Speedometer error

If it is tire size or gearing issues the odometer will be off, if the odometer is correct it is in the unit itself
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Old 07-09-2020, 04:25 PM   #4
Bob/AZ
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Default Re: Speedometer error

Would my 30 town sedan have come from the factory with a 3.78 and 19 inch wheels? The tires have very little wear since new and are the Universal brand (4.75/5.00-19). I understand Universal uses original molds for their tires?
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Old 07-09-2020, 04:42 PM   #5
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Default Re: Speedometer error

Hi, If the tires are new the problem may lie in either the speedometer itself or in the speedometer gear assembly. Removing the assembly is easy, just two small bolts. Check the number on the housing; for a 3.78 rear end it should have 19 stamped on it (19 gear teeth). Since you are reading fast more gear teeth are needed to slow down the reading. Your reading is just a little bit fast so I think a 20 would put you spot on. Good luck! Gary
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Old 07-09-2020, 04:51 PM   #6
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Originally Posted by gary678* View Post
Hi, If the tires are new the problem may lie in either the speedometer itself or in the speedometer gear assembly. Removing the assembly is easy, just two small bolts. Check the number on the housing; for a 3.78 rear end it should have 19 stamped on it (19 gear teeth). Since you are reading fast more gear teeth are needed to slow down the reading. Your reading is just a little bit fast so I think a 20 would put you spot on. Good luck! Gary
Good info Gary. Thanks, Bob
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Old 07-10-2020, 04:09 AM   #7
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Default Re: Speedometer error

I think speedo error is normal, most folks don't realize its there. They didn't seem to have much accuracy till the 70s.

Mine [ A ] reads low. 5mph low when GPS says 45 and 8 mph low when GPS says 58
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Old 07-10-2020, 07:01 AM   #8
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Default Re: Speedometer error

Mine is off about 5 mph at 30 miles per hour. I guess it could be fixed, but for the price of about $100 I'll just keep the difference in mind.
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Old 07-10-2020, 08:55 AM   #9
Bob Bidonde
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Default Re: Speedometer error

Go to a school speed zone with a radar speed sign and check your speed. GPS has a margin of error.
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Old 07-10-2020, 08:56 AM   #10
Patrick L.
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Default Re: Speedometer error

There is a fella in upstate NY [Govuerneur [sp] ] that does speedo work. Although I'm not really sure if he still does it. He used to advertise on the sites. Maybe he still does.
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Old 07-10-2020, 09:27 AM   #11
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Default Re: Speedometer error

You can calibrate them on the work bench, use a variable speed motor and a laser RPM meter on the cable. They are very accurate when calibrated properly. Am I the only one that still does my own repairs, all I hear is send it to someone??
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Old 07-10-2020, 11:31 AM   #12
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Default Re: Speedometer error

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Originally Posted by Bob/AZ View Post
My speedometer is off a little; it reads about 50 when the gps shows 46 or 47.
The rear end is a 3.78 and the wheels are 19". Is it the wheel size or just a bad speedometer?
Our 1929 with a rebuilt speedometer reads almost the same as yours.

It is off 2 to 3 mph at 50 mph. New tires on car.

Enjoy.
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Old 07-10-2020, 12:20 PM   #13
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Default Re: Speedometer error

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You can calibrate them on the work bench, use a variable speed motor and a laser RPM meter on the cable. They are very accurate when calibrated properly. Am I the only one that still does my own repairs, all I hear is send it to someone??


Thats nice, a good idea. Thats sounds interesting and like fun.

Do you pour and machine your own babbitt and grind your cranks?
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Old 07-10-2020, 12:40 PM   #14
1930 coupe
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Thats nice, a good idea. Thats sounds interesting and like fun.

Do you pour and machine your own babbitt and grind your cranks?
Yes I pour and machine my own Babbitt, I do not have a crank grinder
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Old 07-10-2020, 12:44 PM   #15
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Default Re: Speedometer error

My new Toyota truck was 2-3 mph off. Toyota said it was within specification.
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Old 07-11-2020, 08:02 AM   #16
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Default Re: Speedometer error

Readjust the speedometer. Easy peas.
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Old 07-11-2020, 08:03 AM   #17
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Default Re: Speedometer error

Dumb spell check.
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Old 07-11-2020, 08:17 AM   #18
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Default Re: Speedometer error

Who says your GPS is accurate, reading a few mph fast may just keep you from getting a speeding ticket! I got a ticket once 27 mph in a 25 mph speed limit? It was in a residential area, I should have went to court and fought the ticket.
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Old 07-11-2020, 09:54 AM   #19
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Readjust the speedometer. Easy peas.
Just how does one go about doing that?
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Old 07-11-2020, 10:01 AM   #20
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Default Re: Speedometer error

Thats as close as my '06 F150 is!
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