Speedometer error

Bob/AZ

Bob/AZ

Senior Member
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?
 
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.
 
If it is tire size or gearing issues the odometer will be off, if the odometer is correct it is in the unit itself
 
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?
 
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
 
gary678* said:
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
Click to expand...

Good info Gary. Thanks, Bob
 
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
 
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.
Marty
 
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.
 
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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Bob/AZ said:
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?
Click to expand...

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.
 
1930 coupe said:
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??
Click to expand...



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

Do you pour and machine your own babbitt and grind your cranks?
 
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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