1957/58 Speedometer adjust?

[Brandon Todd]

Hello. I was wondering if there is a way to adjust the speedometer on my 1957 Buick without taking it all apart again? Mainly I'm asking if they have an access hole in it to adjust the resistance. 

 

I've had the speedometer tinkered with a couple times, it used to not work at all. Now it works fine, but it's a little too sensitive, saying I am going 55mph when I'm going say 30. I believe the problems lies in the speedometer rather than the transmission, since I've messed with the speedometer a couple times and I know for a fact it was completely broken before.

 

Thanks for any help!

BT

[NTX5467]

Have you checked the odometer readings?  Against the milemarkers on the Interstate, over about a 10 mile distance?  This will indicate if the drive/driven gears on the trans tailshaft are correct for your rear axle ratio AND tire size.  10 miles?  Easier to compute the percentage of error, plus it might really take that distance to let things settle out, at a constant 60mph road speed.  60mph?  1 mile at a true 60mph takes 60 seconds.  Again, speed consistency can make for better results, I believe.

 

Not sure of the workings of your particular speedometer, but typically:  The gears on the trans tailshaft turn the speedometer cable.  That cable attaches to the rear of the speedometer head.  The cable connects and turns a "speed cut" with a bar magnet in it.  As the cable/speed cup turns, the 2nd speed cup (which attaches to the speed indicator side of things) turns that speed cup with it's magnetic force.  NO mechanical connection between the two, just the magnetic force of the cable-driven speed cup turning the front speed cup.

 

The front speed cup/speedometer needle has a clock spring attached to it which returns it so "0" or lower indicated speeds as the speed of the cable-driven speed cup might decrease.

 

The speedometer shops used to have a "zapper" to adjust the magnetism in the bar magnet AND the indicated speed.  A pure trial-error situation!  Also, the indication specs of the speedometer speeds allow for something like 3mph "fast" to 2mph "slow".  Police units had some some of tight-tolerance temperature compensated clock spring in them, to allow 1mph plus-minus speed accuracy (provided the tire size and rear axle ratio vs trans speedo gears were correct for the vehicle.

 

So, FIRST check the odometer accuracy, then worry about getting the indicated road speed corrected, in that order.

 

Many of the older dealership techs used to worry more about "speed" than "distance", but that can have it's own set of pitfalls.  They used to have some loose rules about how many teeth equaled how many indicated mph, but those could vary as "more teeth" gear combinations came into play.  A factory service manual or GM/Buick parts book might detail the speedometer gear/axle ratio/tire size combinations.  Then if you have modern tire sizes, you'll have to compensate for that, too.

 

So that gets back to my orientation of getting the indicated distance correct, then worry about the indicated speed.

 

NTX5467

[Ttotired]

Its a job for the professionals. But take 3 speed readings, low middle, high to a value, say 10, 30, 50 mph on the speedo and compare them to GPS speed

and take that to an instrument tech and they can calibrate the speedo correctly.

 

If you have done this already and they cant get it right, find a new tech

 

 

[Brandon Todd]

Thank you two so much. I appreciate the input. I'll have to try the odometer "trick." I think it's the speedometer itself, but it's possible it's the transmission gears.

[NTX5467]

Keep forgetting about the GPS use.  I just like using a digital watch as that's what I used to use and wear.  Easier to "work" than the timer on my flip phone.

 

NTX54567

 

 

[Bloo]

NTX5467 nailed it though. Get the gearing (odometer) right first. If the speedometer needs work, it should be calibrated to the odometer.

[60FlatTop]

18 hours ago, Brandon Todd said:

Hello. I was wondering if there is a way to adjust the speedometer on my 1957 Buick without taking it all apart again?

 

"The level of perfection a person is able to achieve is directly proportional to the number of times they are willing to take things apart and do it again."

 

My experience has been that "willing" is the keyword.

Bernie

[EmTee]

On 5/28/2018 at 9:40 AM, 60FlatTop said:

My experience has been that "willing" is the keyword.

 

...assuming that you don't break something irreplaceable in the process.  The speedometer in my GP reads about 5 mph low at 55 mph.  Rather than muck around with the car, trying to correct for the error introduced by the modern radials, I have re-calibrated my brain to drive at an indicated 60 mph for 55 actual mph.  No parts to buy and no risk of damage.  I save my energy for things that matter. 

[Brandon Todd]

On 6/3/2018 at 7:54 AM, EmTee said:

 

...assuming that you don't break something irreplaceable in the process.  The speedometer in my GP reads about 5 mph low at 55 mph.  Rather than muck around with the car, trying to correct for the error introduced by the modern radials, I have re-calibrated my brain to drive at an indicated 60 mph for 55 actual mph.  No parts to buy and no risk of damage.  I save my energy for things that matter. 

I'd like to get it fixed as much as i can, because it does matter to me, silly. ?

[NTX5467]

Starting sometime in the '60s . . . or somewhere else back then . . . GM started to use "ratio adapters" that were screwed into the cable end at the transmission speedometer cable fitting.  This helped "hit the gaps" in the gear ratios of the drive and driven speedometer gears.  Sometimes, they would speed-up the cable rpm, sometimes slow it down.  This also related to the odometer, too!  Many would have been when the driven gears had about 25 teeth on them and the drive gear was about 8 teeth.  In the '80s, the number of teeth on the gears seemed to double, but adapters were still needed in many cases.  The adapter ratio is stamped on the housing, usually rectangular.

 

When I went to GR70-15 (factory Z/28 size) from the stock FR78-14 size on my '77 Camaro, I discovered that I could just remove the 1.05 ratio adapter and put the speedometer cable direct to the transmission fitting and everything worked just right.  Possibly a 2% error?  I always checked the odometer calibration and then converted that to mph at 60mph checking speed.

 

ALWAYS check the odometer calibration first, then the elapsed times to cover 10 miles on a flat Interstate (using the mile markers) for the speed function.  Odometer is also critical for accurate determination of fuel economy, too.

 

NTX5467

[Barney Eaton]

Here is one of those adaptors that Willis mentioned