Which gear to use to correct speedometer reading?

[histoy]

My '68 is running F70-15 tires with a 3:23:1 sure grip.  When the speedometer reads 60 mph, the car is actually running at 55 mph, so the speedometer is reading 109% of actual speed.  In looking at the speedometer drive gears available, it seems to me that each tooth added will change the speed by 3%.   I'm thinking that I have to add a gear with 3 more teeth to slow the speedometer from 60 down to 55.  Am I correct?

[71superbee]

According to the chart in the Year One catalog, you will need a 32 tooth speedometer pinion, their part # is FD032

[hemigeno]

According to the chart in the Year One catalog, you will need a 32 tooth speedometer pinion, their part # is FD032

That's what the chart in the Parts Book says too... 

[histoy]

That's the one that was installed, however the speedometer reads 5mph faster than the car actually is running.  That's why I'm trying to figure out which gear to use to correct the speed.  The last time I had to do this, I think 1 tooth on a gear would change the speed roughly 2 mph.  Have others had similar experiences?

[Nacho-RT74]

speedo gears are not exactly.. frequently they are just aprox

http://www.dodgecharger.com/forum/index.php/topic,16033.0.html

[almerkel]

The gear can be installed in several positions.  On my 4 speed there are marks to line up on the trans and the gear holder.  If the incorrect mark is elected, the reading is incorrect.  Find someone that knows about old mopar transmissions.

[Nacho-RT74]

the gear housing bad positioned will translate on no reading speedo.

[John_Kunkel]

That's the one that was installed, however the speedometer reads 5mph faster than the car actually is running.  That's why I'm trying to figure out which gear to use to correct the speed. 

Keep in mind that the speedo error might be in the head itself, a more accurate indicator of correct pinion gear is the odometer reading. The odometer is directly geared to the transmission output shaft and won't have any mechanical error; the speedo needle is driven by magnetism and can be way off when the odometer is spot on.

Check the odometer over a known distance, it shouldn't be over 5% off, if the odometer error differs from the speedo error the head needs repair/calibration.

[Nacho-RT74]

the speedo needle is driven by magnetism and can be way off when the odometer is spot on.

magnetism ? are you sure ? I sweared it is a counterweight and spring or something like that. well that my 3rd gen, not sure earliers.

[John_Kunkel]

The spring loaded drum in the speedometer head is what drives the needle, the steel drum is driven by a spinning magnet inside. The faster the magnet spins the harder the drum tugs against the spring.

Since there is no mechanical contact between the speedo cable and the needle, the springs can weaken and/or the magnets can lose their power and this affects the accuracy of the speedo. The odometer, however, being direct geared will be as accurate as the pinion driving it.

[smokinrt]

the gear housing bad positioned will translate on no reading speedo.

Nacho-rt74
I read your response on the speedo problem and thought you might be able to guide me.  I just replaced my restored guages and a speedometer cable at the same time.  The sppeedometer dial stays on 0, where do you think I should start
looking for the problem???

Thanks
smokinrt
Bryan 

[John_Kunkel]

You need to disconnect the speedo cable from the speedo head and determine if the speedo cable is actually turning when the car is moving.

If the cable is turning, the fault is in the speedo head.

If the cable isn't turning, the adapter in the transmission tailhousing might be improperly clocked for the installed pinion; there are three positions the adapter can be installed in, the number range on the adapter must match the teeth on the pinion and the appropriate hash mark aligned with the raised dot on the tailhousing.

[histoy]

I removed the 32 tooth gear and installed a 34 tooth gear, and now when the speedometer reads 60 mph, the cars actual spped is 59 and not 55, which is close enough for me....

[smokinrt]

You need to disconnect the speedo cable from the speedo head and determine if the speedo cable is actually turning when the car is moving.

If the cable is turning, the fault is in the speedo head.

If the cable isn't turning, the adapter in the transmission tailhousing might be improperly clocked for the installed pinion; there are three positions the adapter can be installed in, the number range on the adapter must match the teeth on the pinion and the appropriate hash mark aligned with the raised dot on the tailhousing.

John,  Thanks for the info.  Here's what has happened, The speedo was restored in Feb,06 and recalibrated. Prior to the restore, it seemed to work fine, but don't know if it was calibrated correctly.  It has taken this much time to get the cluster back and have this time to install.  I bought a new cable  because the old one broke at the plastic clip. So,  I'll assume the picture you sent was of the transmission connection because this is my first speedometer cable and I can't see the transmission under the car. Anyway, assuming the speedometer is restored correctly, I'll pull the guages and  look at the cable to see if it spins.  It'll be some time before I get around to working on it, but I'll keep in touch and let you know how things go.

Thanks,
Bryan

[73chgrSE]

According to the chart in the Year One catalog, you will need a 32 tooth speedometer pinion, their part # is FD032

Any pics of this chart?

[Ghoste]

Here's one.

[73chgrSE]

thnx