Speedometer showing I am going faster than I am

[Bill Stoneberg]

My new 1960 Buick shows me to be going about 20 MPH over what I really am at 60 MPH.

 

The car is running p225 X 75 BF Goodrich Silvertown Radials. This shows a tire diameter of 28.30 inches  which is close to the 8.00 X 15 Bias ply tires it had to begin with at 28 inches.

 

Could this difference make that much of a speedometer difference ?

 

Granted I am not used to reading the rolling speedometer yet but it sure looks like what I see.

[old-tank]

Is the odometer accurate?

On mine the switch to radials caused 4% change in speedometer reading and odometer reading.  Yours is 20%...it ain't the tires .

[bhclark]

That explains all those slow-moving Buicks people are always complaining about!

 

Speedo says 80, but you're only doing 60?  That's too much for tires, I agree. I swiched from a 3.07 to a 3.42 rear end in my Widcat and it accounts for about 10% difference in mileage/speedo.

[RivNut]

My new 1960 Buick shows me to be going about 20 MPH over what I really am at 60 MPH.

 

The car is running p225 X 75 BF Goodrich Silvertown Radials. This shows a tire diameter of 28.30 inches  which is close to the 8.00 X 15 Bias ply tires it had to begin with at 28 inches.

 

Could this difference make that much of a speedometer difference ?

 

Granted I am not used to reading the rolling speedometer yet but it sure looks like what I see.

If your radials are larger than your original tires the speedometer would show that you're going slower than you actually are.  It would take fewer revolutions per mile for a 28.30" tire than it would take for a 28" tire so the speedometer would make fewer revs also.  Sumpin' else is outa' whack.

[Guest shorttimer]

The issue is likely with the speedometer drum. The rotating magnet pulls the drum closer to it over time, causing it to register a higher speed. It will eventually make contact and bounce the drum around.

It is a fairly simple fix to get it back within range. I documented my workaround over on the B59 forum several years ago. I will look it up and post a link when I get a chance.

[JamesBulldogMiller55Buick]

My new 1960 Buick shows me to be going about 20 MPH over what I really am at 60 MPH.

 

The car is running p225 X 75 BF Goodrich Silvertown Radials. This shows a tire diameter of 28.30 inches  which is close to the 8.00 X 15 Bias ply tires it had to begin with at 28 inches.

 

Could this difference make that much of a speedometer difference ?

 

Granted I am not used to reading the rolling speedometer yet but it sure looks like what I see.

 

It's not the tires.

The magnet's magnetism has weakened and is not providing the needed resistance.

 

Check your odometer against a few mile markers and/or a GPS unit.

 

The speedometer in my Buick shows 30% - 33% faster than actual speed 

The odometer is dead nuts accurate, though

 

A speedometer rebuild is in order

[JamesBulldogMiller55Buick]

The issue is likely with the speedometer drum. The rotating magnet pulls the drum closer to it over time, causing it to register a higher speed. It will eventually make contact and bounce the drum around.

It is a fairly simple fix to get it back within range. I documented my workaround over on the B59 forum several years ago. I will look it up and post a link when I get a chance.

What he said^

 

near simultaneous posts...

[37_Roadmaster_C]

I put 225 75 radials on my 60 Electra 20+ years ago and the speedo was close enough to ignor the error.  The only problem was that the tires would rub in tight turns, basicly lock to lock tight.  Your tires are NOT the problem.

 

R

Edited November 24, 2015 by 37_Roadmaster_C (see edit history)

[Guest shorttimer]

Here is the link to the B59 thread. You will need to log in to the forum and gallery to view the pictures:

http://www.buick-59.com/phpBB3/viewtopic.php?f=22&t=7027&p=59793&hilit=Speedometer#p59793

Here is the text from one of my posts to that thread:

Doing a little google searching this week, I came across a speedometer shop about 10 minutes from my house that sounded like they knew their way around older cars (Gail's Speedometer in Costa Mesa), so I figured I should stop by with my parts and see what they had to say.

Mike at Gail's couldn't have been more helpful and proceeded to give me a crash course on the GM ribbon/drum speedometer. Mike immediately identified the previous repair on my drum and explained how the problems with these units arise. According to Mike, the magnets used in these units were probably stronger than they needed to be, and over time they have a tendency to pull the drum toward the cable end of the speedo head (the right side of the image below).

Image

The movement of the drum can occur two ways: 1) the drum moves along the spindle at its center, or 2) heat and friction over time physically erode the end of the spindle (shown below) which rests in the spinning magnet assembly .

Image

With enough movement, the magnet will contact the drum, resulting in the groaning noise and inaccurate readings many of us have experienced.

Mike explained he has two ways to solve this problem: 1) mill down the edge of the assembly which contains the magnet (visible in my first post), creating more space between it and the drum and/or 2) realigning the drum on its central spindle. Gail's will do both of these and recalibrate the unit for $100. Unfortunately, Mike said he had a several week backlog before he could get to my unit. Of course there is also a third option, which is to get a new drum.

Since my drum had already been repaired, Mike suggested I try and realign it myself and gave me a few pointers. For anyone still following along at home, here is what I did (note - my car is a '60, so keep that in mind if things look a little different):

1: Remove the two screws shown below (not the best picture...) which secure the drum to the head assembly, then remove the drum and bracket.

Image

2): Carefully detach the tension spring from the bracket and separate the bracket from the drum. Make sure your hands are clean and not oily before touching the drum, as you don't want fingerprints on the painted surface of the drum.

3): Stand the drum up on a hard and level surface, with the spring end (shown below) pointing upwards.

Image

4): Here's the trial and error part - use a small hammer or other object and tap the end of the drum spindle, pushing it towards the opposite end of the drum. You only need to move the spindle a very small amount (likely less than 0.1"), so go SLOWLY and make sure you contact the rod squarely, as you don't want to bend the spindle or strike the drum.

5): Reassemble the drum and spin the magnet assembly to check for interference. Make sure you put the bump stop on the correct side of the end bracket when you reassemble, or your speedo won't spin. If the magnet assembly hits the drum, repeat step 4. I probably did this 5-6 times.

6): Once there is no interference, you would ideally check the accuracy of the speedometer on a bench tester. I don't have one, so I skipped this steep. (You could also put the speedo in the car and road check the accuracy against a speedometer app on your phone - what I have done since)

7): There is a jewel in the center of the magnet assembly where the drum spindle rests. Apply lubricant here (I applied MMO using a small awl) as shown below.

Image

8 ): Re-assemble and install. Don't forget to reconnect the tension spring and to get the bump stop in the correct place. I also polished up the plastic lense and cleaned up the rest of the cluster's components.

Image

It is probably worth it to pull the speedo cable and lube it while you can easily get to it. Mine was really noisy and the reason I started this project in the first place. Don't forget to check for kinks while you have it out.

9) Put it back in the car and take it for a test drive.

Image

I am happy to report that not only is my cable now quiet, but the speedo works smoothly in silence and is much more accurate than it was before. Once I had the cluster disassembled, the spindle realignment probably took me no more than a half hour. You could also save yourself the time and send your unit and $100 to Mike at Gail's or the shop in Jeff's thread (I think it was Valley Speedo in El Monte).

Unfortunately, it is only a matter of time before the magnet pulls the drum out of alignment again and I need to repeat the process, but I hope to have picked up a NOS drum by that point. If this turns out to be just a short-lived fix, I will post a follow on update.

Shorttimer

Heading for the Highway

Posts: 150

Joined: Mon Jan 01, 2007 5:03 pm

Location: OC, CA

[NTX5467]

FYI, 60mph = 60 seconds/mile.  Checking the odometer on the mile markers (5 or 10 miles worth at a constant speed) will verify the distance, which will verify if the gears in the transmission are correct.  Seems like I recall the speed specs in the AC Speedometer Catalog to be -2 slow to +4mph at 60mph?

 

When I watched a speedometer shop do their calibrations after a rebuild, they had a "zapper" gun to vary the magnetism level in the speed cup's magnet.  A "hit-or-miss try again" activity at best, as one zap could increase the speed and another one would decrease it . . . until the best/closest speed was showing vs what the machine's speedometer showed.

 

The clock spring which returns the needle/drum to "0" is the resistance against which the magnet's force work against.  It's strength, or lack thereof, can affect the ultimate speed reading.

 

NTX5467

[Bill Stoneberg]

Great Information and I now know more about these speedometers.

 

Anybody have the steps to pull the speedometer out ?

[60FlatTop]

Be careful about the speedo on those cars. It's all done with mirrors. Just don't pass too many and don't let too many pass you.

 

 

Bernie

[RivNut]

To accurately calculate your speed over a measured mile (mile marker to mile marker on the highway) divide the number of seconds it takes to cover the measured mile into 3,600 (the number seconds in an hour.)  The "quotient" will be you speed in mph.  ex.  3600 / 60 = 60.  If you're time is exactly 60 seconds, your speed is exactly 60 mph.  If your time is 58 seconds, your speed is 62.+ mph.  62 seconds is 58 + mph. if your time is 45 seconds, your speed is 80.   My little Garmin GPS reads mph very accurately.  Some of the newer aftermarket digital speedometers simply use GPS, no need for a cable to run to the transmission.

 

Ed

Edited November 24, 2015 by RivNut (see edit history)

[Barney Eaton]

Barney Eaton, on 25 Nov 2015 - 09:18 AM, said:

There should be a plastic gear located at the speedometer connection at your transmission.    Different colored gears have different teeth,  fewer teeth will speed up the reading and more teeth will slow the speedometer reading.

 

GM also used this little gear box with different gears....they are probably still available from GM but are expensive...you can find them on Ebay

 

I have pictures of the gears and box but the system does not want to let me attach them today

 

[Smartin]

Bill, I did this just before the BCA national meet this year to my 60.  The top of the dash needs to be removed.  To access some of the bolts, you need to remove the glove box and lower valance of the dash.  The instrument cluster is removed as a unit (including the white/grey surround).  There is a main harness plug on the back of it, as well as the speedo cable and clock power plug.  Set aside a few hours for R&R.  It's not that bad of a job...compared to a 58.