86 Jetta speedometer help needed.

[30dodgeboy]

Working on a 86 Jetta with a AGS transmission.  Speedometer reads 10-15% faster than car is going.  Tires are P195/60R14, plenty tall.  Checked speedometer gear, it has the white gear in it like it should.  Is there any other things to check?  Could speedo head of the cluster be the problem?

[TylerDurden]

Is the odo accurate?

[30dodgeboy]

No, the ODO reads too fast too. 

Once I got the speed working (previous owner has cable off behind cluster) I noticed the speedo seemed to read faster than my seat-of-the-pants velocity meter... so I then compared the ODO mileage against mile markers on the expressway, yeah it reads too fast as well.

Is there any gearing in the speedo head or does the cable drive it directly?

[TylerDurden]

Any chance you have the gasser AGB? That box runs the engine/speedo 15% faster.
http://www.brokevw.com/020ratios.html

You could put in a green gear and get closer, and/or tweak the speedo by moving the needle on its shaft. Odo would still be off.

I'm wondering if the cluster is original, or from elsewhere.

[30dodgeboy]

I confirmed that AGS is stamped on the trans.  Wrong trans would be way too easy right???   The AGB would make the engine spin slower / drive further for a same RPM and make the speedo read slow anyways.  Now the ECO AVX trans at 4.25:1 could cause this, but again the trans is clearly stamped AGS.  Could the final drive gears have been swapped in the case saw at some rebuild previously?  I have no tach to record RPM and compare calculated / actual MPH.

Cluster appears to be original, with about 182K on it.  Has part numbers 191 919035BJ and 191 919059A and (a date code?) of 008/ 298/19 on it.  Not sure how to read it if its a date code.

I did just recently try the green gear - it is a little more accurate but still reads too fast.

I really don't want to move the needle as then the speedo will only be correct at a certain MPH.  And as you said the odo will still be off.

Thanks for the help.  Any other ideas???

[Dakotakid]

Purchase and install a different diesel cluster.

[30dodgeboy]

Purchase and install a different diesel cluster.

Do you know where I can find one (reasonably priced)?

I have a 1.8 8V cluster from a 1991.  Would the speedo head be the same?  I suppose I could just install the entire cluster to test the speedo accuracy without hooking up the electrical...  just thinking out loud now.  Maybe once this blizzard is over.

[TylerDurden]

I would first verify the total drive ratio... Put the car in 5th and push it back one tire revolution, counting the times the crank pulley turns.

[30dodgeboy]

I would first verify the total drive ratio... Put the car in 5th and push it back one tire revolution, counting the times the crank pulley turns.

Good idea.

[vanbcguy]

Hey, doesn't the speedo drive off the final output side of the diff?  Meaning that the ratios in the transmission are meaningless? 

[30dodgeboy]

Hey, doesn't the speedo drive off the final output side of the diff?  Meaning that the ratios in the transmission are meaningless?

Speedo is driven off the pinion, so final drive ratio is a factor.

[30dodgeboy]

I confirmed the final drive is 3.94:1  I marked a tire and the crank pulley.  Put the trans in 5th and pushed the car exactly one tire revolution while counting crankshaft revolutions.  Divided the count by .75 for 5th gear ratio and my calculator said 3.9:1 ...

Odometer seems like it may be close but speedo reads much too fast.