Speedo accuracy on a 62

[Glaswegian]

Two issues to ask about, first the speedo is off, lower, by about 7-8 mph and the counter seems to be showing I am travelling about double the distance I actually drive. Would both issues be linked ?  Thought slower speed would have lower miles travelled than more if you catch my drift?

Maybe cable needs replacing, as original, and car was unused for a long time. The speedo was very jerky when I got the car but gradually smoothed out. The trip counter was how I noticed first after resetting, but it seems to be the main counter too. The new cables are not expensive via rock auto but I'm guessing a PITA to fit ? hope speedo does not need a rebuild as that would be a nightmare with me being in UK.

Thank you as always for the replies in advance 

[russ austin]

Are you running radial tires?

[Glaswegian]

I am Russ, 235 75R 15  WW types

Looked up google for this effect and that would seem to be the case, but the size fitted is what seems to be recommended for this car, on this site too ? I'd have thought this effect would have been reported by other owners. Can't find a site which lets you compare the Bias Ply's to radials only radial to radial difference when changing tyre sizes.

[Eric DeVirgilis CLC# 8621]

Two issues to ask about, first the speedo is off, lower, by about 7-8 mph and the counter seems to be showing I am travelling about double the distance I actually drive. Would both issues be linked ?

The speedometer and odometer operate off of two different principles. Something would have to be radically off for the odometer not to be ticking off the correct mileage.

"Seeming" is not the same as verifying. Have you tested between mile markers on the open highway to see whether any mileage discrepancies indeed exist in the odometer reading? Otherwise the speedometer error in vintage cars is well known - even when they were new.

[Glaswegian]

Thanks for the reply Eric, I reset the trip a few drives ago, no reason other than to see it worked. I then went a journey I usually drive in my normal car and it showed on the trip about double what I know it is. After a little drive yesterday( sun was shining for first time in weeks) I noticed the odometer was doing same, had not paid any real attention to it before as was just enjoying the driving experience.

Speedo I can live with as I know roughly what speed its out by.

[bcroe]

If things are not working smoothly, that needs to be tracked down first. 
Might need lube in the cable or head, or damaged gears.  Once that is
done, carefully measure your odometer reading against a calibrated
distance.  If off, speedo gears can changed in the trans can be changed
to correct it, or an external speedo cable gearbox added at the trans. 

If this does not bring the speed needle in line, the speedometer head
will need to be serviced so that the speed and mileage track.  Bruce Roe

[Eric DeVirgilis CLC# 8621]

Thanks for the reply Eric, I reset the trip a few drives ago, no reason other than to see it worked. I then went a journey I usually drive in my normal car and it showed on the trip about double what I know it is. After a little drive yesterday( sun was shining for first time in weeks) I noticed the odometer was doing same, had not paid any real attention to it before as was just enjoying the driving experience.

Speedo I can live with as I know roughly what speed its out by.

I assume you're making the proper conversion from Km to miles since you're in Scotland - correct?  If that's the case, I have no idea what could be causing such a major discrepancy. An incorrect gear somewhere? That's very strange.

[Glaswegian]

UK is miles, rest of Europe is Km's 

I thought it strange, perhaps due to cable needing lube as car was listed with sticker on door in 1991 having  38059 miles. and when I got it 38561. So had been sitting around for a long time unused. The needle jumped about quite a bit at first but has now settled down and seems acurate but same 7-8mph lower than it should. It def shows more miles than what I'm travelling 100% sure ! New cable is only about $15 then postage, but guessing a pain to change ?

Thanks Bruce

[Caddyholic]

At what speed is it off 7 to 8 mph? is it fast or slow? Was the rear gear ever changed or the speedo gear in the tail of the transmission? The difference would roughly be .09 

[fishnjim]

Doesn't make any sense 2 me that it's just the cable. 
The gear* in the trans turns the cable based on revolutions of the wheels.   That determines miles traveled and the speed is the rate of that.   So they are related, per se.   Only one cable for both but the interpretation happens in the speedometer/odometer.  Trip feeds off the odometer.    So all are faulty.
Anything that old probably needs a lot of help to be anywhere near accurate.   You can remove "pod" and send out to be fixed, but a chore and cost.   Not sure, ex USA, if anyone in your area does this. 
This is one reason why they stopped the "odometer" statement laws on sales after ten years.
* - has to be correct size for tire diameter.

[Glaswegian]

Jim speedo shows about 22 -23 when doing 30 and this difference seems to stay the same up to around 65 which as fast as I've gone. No idea what has been done in the past, but given the cars condition I expect nothing with gears.

Tyre size I guess making speed indication issue but distance, I would have thought, would be the opposite of what is happening ?

Mystery to be solved...

Thanks all for input 

[Cadman-iac]

Typically a speedometer error that's caused by an incorrect gear or the wrong tire size is a consistent percentage from the start up to the maximum speed.  In other words,  if it's  10 % off at 10 MPH, then it's still 10% off at 80. The reading at 10 would just be 1 MPH off, but at 80, it would be 8 MPH off.
The cable turns a small magnet that sits just under a "drum" if you will, and the magnetic field is what causes the drum to move. It's attached to the needle. There is a spring that has a specific pressure on the drum to 1, return it to zero when the cable stops turning/vehicle stops,  and 2, it calibrates the needle.
This problem is based upon the needle and drum not having any other issues, such as a blockage of the needle that would prevent it from going through the full sweep, or a bushing that is clogged with old lubricant. There are two bushings,  one is located in the drive "head", the part with the magnet, and the other one is located higher on the shaft that the needle is attached to.
The lower bushing in the drive head allows the needle shaft to remain centered, but if it clogs up with old lubricant,  or if the end of the shaft has worn off enough to allow the shaft to dislodge, then the drive head will contact the drum and can cause it to move erratically, which you mentioned it having done initially.
The odometer issue, is usually only caused by an incorrect drive/driven gear in the transmission.
Have you checked the cable or cables to see if someone has installed an adapter to change the gear ratio? This would make a difference in your odometer and the speedometer.  But again,  unless there is another issue inside the speedometer itself, an adapter will only cause a constant ratio difference the entire range of the speedometer.

Maybe this will help some.

Rick

[cadillacmike68]

That's too  much of a percentage difference with the 235-75-15s compared to the 9 x 8.20 series 80 or 90 that was originally on the car. The diameters are only 1-3% different. And you are citing a 20-30% difference.  and it should progress with the speed of the car.

I would first have the speedo cable(s) cleaned and or replaced as needed. Do you have cruise control? If so you have two cables.

The trip being an even higher percentage off is troubling, but have the cable checked first.

[Daryl Chesterman]

The website below shows charts for interchanging the old tire sizes over to the radial tire sizes.  There are also charts for dimensions of the various tire sizes and profiles.  Some very good information for car owners who are wanting to put a radial tire in place of the old bias ply tires.

     https://www.turbinecar.com/tires.htm

Daryl Chesterman

[Glaswegian]

Thank you for all the input, will order up a new cable ! car has no cruise. RA gives 2 types and cost is only about $8, plus tax and shipping for non cruise type.

Won't be changing the tyres as almost like new condition, but if buying again will pay more attention

[russ austin]

Radial tires are throwing the speedo off.  In my 63, I am running the AC gears and radial tires 23575R15.  I put a non AC gear into the trans and my speedo is spot o by 1 MPH at 60.

[Eric DeVirgilis CLC# 8621]

Radial tires will not result in anything close to the discrepancy the OP is reporting. 

[Glaswegian]

Radial tires are throwing the speedo off.  In my 63, I am running the AC gears and radial tires 23575R15.  I put a non AC gear into the trans and my speedo is spot o by 1 MPH at 60.

Whats this gear you talk of and where is it fitted? excuse my ignorance  My car has AC.

Radial tires will not result in anything close to the discrepancy the OP is reporting. 

When I looked up changing radial tyres to different sizes it made a big difference to speedo, but could not check between BP and radials

For instance this one https://tiresize.com/speedometer-calibration/

Still cant fathom how it would change distance to more, thought it would be less miles travelled 

[Eric DeVirgilis CLC# 8621]

Whats this gear you talk of and where is it fitted? excuse my ignorance  My car has AC.

When I looked up changing radial tyres to different sizes it made a big difference to speedo, but could not check between BP and radials

For instance this one https://tiresize.com/speedometer-calibration/

Still cant fathom how it would change distance to more, thought it would be less miles travelled 

The smaller the tire, the greater the mileage will be registered. It will be directly proportional to the circumference between the two tires. Even with the smallest possible tire that can be used on the wheels,  it still wouldn't  be anywhere on the scale you've reported (ie, double +/-).

[cadillacmike68]

Thank you for all the input, will order up a new cable ! car has no cruise. RA gives 2 types and cost is only about $8, plus tax and shipping for non cruise type.

Won't be changing the tyres as almost like new condition, but if buying again will pay more attention

Unless you want to go to on original size bias ply, then 235-75-R15 is the closest to the original size you can get.