Speedometer reads 5 more than actual at 60

[MrEarl]

Rita calls it a built in  Speed Ticket Reducer but I like the rush of  knowing when I'm breakin the law. At 60 mph actual (as determined by GPS and also mile marker and stopwatch)  the speedometer in the '79 Estate Wagon shows 65 + -. As the speed is decreased the gap grows smaller, faster bigger. I wondered if the new one size bigger than OE tires would make a difference but in checking that found that that would actually work in the opposite direction.

 

 

 

 

 

Also wondered if perhaps when the PO replaced the rear differential a few years back  that maybe a higher or lower ratio was used.  It appears a 3.42 could closely match the example but I'm doubting that is the case. Really don't want to have to pull the back plate off to check. 

 

 

 

\

 

 

Any thoughts as to what else might be causing this ?   99% sure it existed before the transmission was rebuilt a few months ago also.

[NC-car-guy]

Federal regs only require +/- 5mph accuracy at 50mph..

[Ben Bruce aka First Born]

I think mechanical speedos have always varied. Electronic is better. GPS is best.  I THINK!

 

  Ben

[MrEarl]

48 minutes ago, NC-car-guy said:

Federal regs only require +/- 5mph accuracy at 50mph..

 

30 minutes ago, Ben Bruce aka First Born said:

I think mechanical speedos have always varied. Electronic is better. GPS is best.  I THINK!

 

  Ben

 

So y'all think it's likely just the speedo itself , nothing to worry about?  

The concern came about when was considering the relationship between speed, rpm's, a tachometer, optimal shift points, cams, torque converters etc etc.

[NC-car-guy]

13 minutes ago, MrEarl said:

 

 

So y'all think it's likely just the speedo itself , nothing to worry about?  

The concern came about when was considering the relationship between speed, rpm's, a tachometer, optimal shift points, cams, torque converters etc etc.

If you're super serious why not put it on a dyno?

[MrEarl]

5 minutes ago, NC-car-guy said:

If you're super serious why not put it on a dyno?

 

$$$

 

but not totally out of the question

Edited September 10, 2019 by MrEarl (see edit history)

[Kosage Chavis]

Can mechanical speedometers be calibrated?

[JohnD1956]

You should be able to pull the speedometer drive gear out without so much as a drip of leakage.  Note the color and number of teeth.  Then I believe if you can find a gear with one tooth more,  you will be closer to matching the speedometer reading and actual mph.  

[Ben Bruce aka First Born]

5 hours ago, MrEarl said:

 

 

So y'all think it's likely just the speedo itself , nothing to worry about?  

The concern came about when was considering the relationship between speed, rpm's, a tachometer, optimal shift points, cams, torque converters etc etc.

 

 Yep.  Me thinks you worry too much. Must be the engineer in you.

 

  Ben

[NTX5467]

BEFORE, note "BEFORE" you start worrying about the indicated SPEED, check the ODOMETER for accuracy FIRST!  The odometer runs directly off of the speedo cable as it's turned by the speedo gears in the trans tail shaft.  A direct link!  

 

The speedometer speed needle is run by a spinning bar magnet, whose magnetism turns the other speed cup as the magnet is turned by the speedometer cable.  A VARIABLE interface at best.  Use the mile markers on the Interstate for a 10 mile constant speed run.  or the same distance with a GPS.  THEN see how far the odometer is off and adjust the related gearing accordingly.  Might need a "ratio box" installed at the base of the speedometer cable, as GM did with MANY vehicles in the '70s and '80s.  The ratio is stamped on the side of the adapter.

 

The "traditional" way to do the deal is just to worry about the speed reading.  As that change will also affect the odometer reading, you might suddenly see a change in your mpg, with no engine changes.

 

Enjoy!

NTX5467

 

Edited September 11, 2019 by NTX5467 (see edit history)

[Bloo]

NTX5467 nailed it. Fix the odometer first, and then see what you have. If it's still wrong (it probably won't be), send it to the speedo shop.

[packick]

My '55 Century is also off by 5 mph too.  65 indicated is 60 true.  i think all of the old speedometers were hit-and-miss and mostly inaccurate.

 

Of course then there is the backwoods solution.  Tear a strip of duct tape, attach it to the face of the speedometer, then write the actual speeds on the tape to coincide with the needle.  Problem solved.

[dmfconsult]

I’ve not checked the 78 EW but the 67 Electra is only accurate at 60 mph. All other speeds are off. Odometer on the other hand is dead on correct. 

[Barney Eaton]

There is a speedometer gear in the transmission where the speedometer cable attaches......they are different colors with a different number of teeth.

I think I am thinking correctly this early in the day but you could try a gear with one more tooth that is in there now.   See photo with several different plastic gears.

 

There was also a small gear box that would attach at the transmission output that had interchangeable gears in an attempt to get the speedometer correct.   They were mainly used on trucks

because of the wide choice of tires on trucks.  I have seen the gear boxes on Ebay and they were asking about $50 but I don't know if or where you can get the different gears.

Edited September 11, 2019 by Barney Eaton (see edit history)

[Bloo]

Those "conversion" gearboxes are available for about anything, and cost about $100-$120. I doubt there is any situation you would need that on a late 70s GM car. Just get the right pinion gear for the transmission.

 

You may have to remove the tailshaft and change the drive gear too, but that is almost never necessary in reasonably stock 70s cars..

 

Drive the car 10 miles and watch the odometer. See how much it is wrong, and in which direction. Change the pinion gear that percentage (or as close as you can get). More teeth turn the cable slower (to fix an odometer that reads high). Gears are all over ebay, or at your local speedo shop if you have one. Pay attention to the way the teeth lean on the gear, and make sure the new one has the teeth leaning the same way.

[Ben Bruce aka First Born]

 

 I think your first post, indicating the differential changes with speed ,  confirms Willis's post.  Changing the driven gear will change and make the speed "right"  somewhere, not everywhere. Nature of the beast.  Save money and time . Know 65 is 60, 70 is 64, etc,  and enjoy the darn think..

 

  Ben

Edited September 11, 2019 by Ben Bruce aka First Born (see edit history)

[Kestrel]

Along what 'pac kick" said, , speedometers in state police cars, at least in NYS,  were calibrated using radar guns. They were all off slightly from what the needle said. It was this calibration card that was used In court to win trials.

So you could make up your own card and paste on the panel somewhere.  👮‍♂️

 

 

[gdmn852]

Hello I think that seems to be normal on older cars the 49 Buick is actually 50 mph when indicating 55  the 71 MGB about the same and 69 Porsche at 62 mph at 70 on speedometer. I know police cars are checked and corrected for accuracy,had a 96 Ford interceptor had inspection stickers on  speedo and it also went to 150 mph. Probably can only get so close on gear driven older types plus age and wear added to the equation.Gary

[Barney Eaton]

I too have owned many cars that the speedometer was incorrect.....usually reading higher than you were going.

When I purchased my 39 Century,  I went for a drive with my Garmin and the old '39 showed 50 and the Garmin agreed.

[NTX5467]

GM used the "ratio boxes" until the electronic clusters started to come online in the later '80s, cars included.

 

There were some of the '80s years where the number of teeth on the drive and driven gear pretty much doubled from earlier years.  Which should have increased accuracy, I suspect.

 

When I put factory '78 Z/28 15" tires/wheels on my '77 Camaro Type LT, after much research in the speedo gear charts in the parts book, I determined that taking the 1.05 ratio box off of the 14" set-up would make the speed/distance correct for the 15" factory set-up.  And it worked just as suspected!

 

When our Delco Radio Warranty service station started to do speedometer work, I would look at the Delco Speedometer manual.  I was surprised at the +3/-2 accuracy spec, but when I discovered that varying the magnetism in the bar magnet was a hit/miss trial/error situation, that wide of a spec made a bit of sense to me.  The variance would allow for the lower speeds (in town) and the moderate speeds (highway driving), with the greatest accuracy desired on the highway, it seemed.

 

Genuine police car speedometers were marked "Calibrated".  I ordered two new ones for my '80 Chrysler Newport, while I could still get them.  Just wanted them for the higher speed numbers on them, rather than the normal 85mph deal.  They came with a "Calibration Card", which listed the indicated speeds would be = or - 1mph from actual velocity.  I asked our speedometer repair people about that and he said the added accuracy was from a better clock spring that the speedo needle turned against, which returned it to ":0" when the vehicle stopped.  There was also a note on the card that mentioned a particular "vehicle interior ambient temperature range", for the accuracy, too.

 

Just remember, 1 measured mile in 1 minute is 60mph.

 

Enjoy!

NTX5467

[Frank DuVal]

Usually error is a percentage, not a fixed number of MPH. Usually the factory had them reading high, so not to get tickets. Very normal!

 

I have a 70 Estate Wagon that was 13% off in the ticket getting direction. No matter how I stressed to people who wanted to borrow the car "drive at 55 MPH do NOT exceed 55 or you will get a ticket" , they still complained they got a ticket doing only 58 MPH!😊  Stopped loaning the car, as they would usually bring it back with a fried clutch....😡

[Marty Roth]

14 hours ago, Barney Eaton said:

There is a speedometer gear in the transmission where the speedometer cable attaches......they are different colors with a different number of teeth.

I think I am thinking correctly this early in the day but you could try a gear with one more tooth that is in there now.   See photo with several different plastic gears.

 

There was also a small gear box that would attach at the transmission output that had interchangeable gears in an attempt to get the speedometer correct.   They were mainly used on trucks

because of the wide choice of tires on trucks.  I have seen the gear boxes on Ebay and they were asking about $50 but I don't know if or where you can get the different gears.

 

I done what Barney suggests years ago with one of our Suburbans when I replaced one of the rear ends and had a different ratio. Used both approaches - changed gear one time, then next time with other ssssssSuburba, used the box

[Marty Roth]

Lamar,

 

Another possibility could be the difference between the originally supplied tire size VS the modern radial 75, or 70 aspect ratio.

Older Michelin guides used to give revolutions per mile for varied sizes, but in lieu of that, tire height multiplied by Pi (3.1416 is close enoough) = Circumference; take that number in inches and divide into 63,360, the nimber of inches in a mile to get tire revs per mile. Compare original size to modern replacement, which are not a true replacement - usually just equivalent in weight carrying capacity

[NTX5467]

In many cases, the "Revs/Mile" info is in the sizing information for particular tires in the "Specs" section of the particular tire on the www.Tirerack.com website.

 

There is a comparison device on the Miata website, too, where you put in info on the current tire size and then the info for the new tire size.  A nice graphic to show you what's generally going on, too.  I've seen a similar tool in other places, too, but the first place was on the Mazda Miata ehthusiasts' website.

 

In general, a P215/75R-15 or P225/70R-15 tire size (replaced the old "G" size designation . . . G78-15 and G70-15 respectively) is in the 750-754 revs/mile range.  Check the GM service manual for the OEM-supplied tire sizes for the car.  W.hich should have been P-Metric sizing.

 

NTX5467

 

 

 

[dmfconsult]

Ok @MrEarl I finally got out in the 78 EW today for a 100 mile round trip up the coast. Checked my speedo against a GPS Speedo on my phone. The actual speed seems to be consistently 3-4 mph faster than indicated on the cars speedo across the board. 
 

I think we have the same tranny and rear end ratio in our cars.  Tire size is 225 75R15 so pretty close to stock size. 
 

Edited September 30, 2019 by dmfconsult (see edit history)

[MrEarl]

Interesting, tha's just the opposite of my case. I'm fine with mine reading faster than I'm going, like I said  " built in  Speed Ticket Reducer "  lol    Thanks for sharing. Looks like it was a nice drive.