Speedometer reads slow.

[jaed_43725]

My speedometer is approximately 10mph off slow. So when I am doing 40 it reads 34. The gap widens the faster you go. The speedometer also does not start until 10mph, which is a stupid design. I havent checked anything yet, I figure its best to ask before I dig in where I dont need to. If anyone knows what to do at all I welcome the suggestion. The car is a 85' Jetta TD with 1.6 and MT.

[Kazuki]

Cable needs to be replaced

[hillfolk'r]

What size are your tires?
Stock tires usually read like 4-5 mph fast. Are you runnin truck tires or somethin?
Maybe someone changed a speedo gear or somethin. 
Mine reads 10mph fast but i have 175-50-13 tires.

[ORCoaster]

Why replace the cable?  I have one cluster that reads slow and the other reads fast running off the same cable.  I think someone messed with the needle placement. 

[mystery3]

If the needle doesn't start moving until 10mph the cable may well be bad.

OP does the needle bounce around at all?

[jaed_43725]

My first thought was the cable. But the Speedo does not bounce. I am running factory GLI rims and tires. The cluster may not be original, but I don't think anyone messed with it internally.

[vwnut84]

What speedo do you have in the car?  Factory for that car should be a 120mph unit. There are three different colored gears on the end of the speedo calble (red, white & green) that go with the different clusters (85, 100, 120, 140 & 160). If the gear doesn't match the cluster it will throw the speedo off.

If the cable was culprit, I'd think the speedo would not work at all, or be bouncy down low.  Also, something else to think about, there are plastic gears for the odometer that require repair sometimes, so someone could have been in there at some point in the cars past repairing the gears, and didn't index the needle correctly.

[ORCoaster]

Anyone have the process by which one would index the needle correctly?  All sealed up so does a drill at some magic RPM become the index standard? 

[jaed_43725]

It is a 120mph speedo. Which speedo cable do I need? How would I get the speedo fixed to index correctly?

[R.O.R-2.0]

FWIW, ive never seen an old VW that had an ACCURATE speedo..

my 92 Jetta is 3 fast @ 55

my 86 GTI is 4 slow @ 55..

thats with the same speedo drive gear, same tires & wheels, same trans, same everything..

the faster you go, the worse it is..

i KNOW my gauge clusters have never been messed with either..

ive chalked it up to being an expected gremlin on these old cars..

like i say, ive NEVER seen a mk1/2 with an accurate speedo..

mk3 they went to electronic, and they became much more accurate..

[R.O.R-2.0]

It is a 120mph speedo. Which speedo cable do I need? How would I get the speedo fixed to index correctly?

the cables are all the same

the speedo doesnt index to anything..

most of the time, the tach doesnt read accurately either, so you cant adjust either based on the other one..

i just remember my speedo is off, and do some quick math in my head when i look at my speedo whenever i go faster than 60mph..

if your speedo has ANY READING AT ALL, then the cable is fine..

the speedo cable will either cause the speedo needle to bounce, or not work at all..

ive never once seen a speedo cable make a speedo read SLOW..

[vwnut84]

Actually, the needle does in fact index to the speedo face. When the speedometer is out of the housing, you gently pry the needle up and over the rest pin, and the needle should come to rest at a small white mark about 8mm below the stop pin on the face. The mark is very small and can only be seen with the speedo out of its housing. Search around vwvortex for mk2 speedometer repair and you should come up with a write up on the procedure to remove the speedo and disassemble it as far as you need to at least check/fix the needle placement.

What are you checking the speedo against?  GPS?  Another car?

[ORCoaster]

Thanks for the SECRET SHTZ. I have had the speedo out of the case to do gear repairs on it.  Also to Ahmm, adjust the mileage to "proper" age of vehicle.  I got a second speedo from a Caddy and it was like 120 K over mine, so I had to roll it back.  Done with a drill a rubber stopper and a good battery. 

I will look at the index spot you mention on the other speedo just for giggles and grins.  Next time I have it out I will adjust it accordingly.  Again thanks for the tip.

I know my speed is off because I run speed and distance off the GPS.  Or was for 250 a trip twice a week.  Not so much anymore.  I did notice that the fast I went the more it was off. 

[hillfolk'r]

I've usually had the speedos read a touch fast. Like indicated 60 is 56 actual.
If you are using a gps check it on the level road.  Im pretty sure most view you in 2 d and doesn't notice when you climb a hill.
And i thought the different speedo gears in the trans were for the different final drive ratios used. I didnt think they were different for different speedo heads.

[mystery3]

The speedo on my truck was perfect according to various radar guns at various speeds. I had 195/60r14 tires on an '81 truck. With the stock wheel/tire combo it read fast about 5mph at 60.