Speedometer Question on GTX

[1969VAGTX]

Seeking some feedback on an issue with my speedometer. The speedo on my GTX reads much higher than the speed that I am traveling at. For instance it reads 90 when the car is only running approximately 60. Curious what might cause that issue given that the car is rolling on stock size tires.

 

[rbkt65]

usually speedo gear but have seen speedo itself bad. did you change the trans or rear gears?

 

[1969VAGTX]

usually speedo gear but have seen speedo itself bad. did you change the trans or rear gears?

The rear gears are stock. The trans has a little higher stall converter.

 

[rbkt65]

find the speedo gear chart on the internet and start there. john cope at cope racing trans has the formula on his web site and there is a chart on fabo under speedo gears for sale.

 

[kiwigtx]

You need to use a calculator to determine the correct size speedometer drive gear for you tire size and rear ratio.

Online calculator.....this is one of many available online - there are dozens of them


Speedometer Gear Calculator"

Tire size needs to be 'height/diameter in inches'.

 

[kiwigtx]

Seeking some feedback on an issue with my speedometer. The speedo on my GTX reads much higher than the speed that I am traveling at. For instance it reads 90 when the car is only running approximately 60. Curious what might cause that issue given that the car is rolling on stock size tires.

Sounds like someone ran a 45 tooth with 4.10:1 gears and now you have 3.23 or 3.54 gears.

 

[1969VAGTX]

Sounds like someone ran a 45 tooth with 4.10:1 gears and now you have 3.23 or 3.54 gears.

The car is supposed to have the stock 8 3/4" rear with the 3.54/3.55 rear gears that came from the factory in the car. Maybe need to verify what the rear gears are for sure.

 

[kiwigtx]

The car is supposed to have the stock 8 3/4" rear with the 3.54/3.55 rear gears that came from the factory in the car. Maybe need to verify what the rear gears are for sure.

If your speedometer is reading as high as you say....someone has swapped out the gears, and not told you about it......maybe a previous owner.

90 at 60 is way too big a difference to be solely down to tire size differences.

 

[1969VAGTX]

If your speedometer is reading as high as you say....someone has swapped out the gears, and not told you about it......maybe a previous owner.

90 at 60 is way too big a difference to be solely down to tire size differences.

I will have to do a little researching. The car doesn't launch like it has a particularly tall gear - like a 4.10 or something. It does in fact have the 8 3/4 rear though.

 

[lemondana]

Look up your transmission part number-the left side pan rail, and verify that it has the correct trans. Most likely trans got swapped out years ago for a lesser performance 727 that was in front of a 2.76 rearend.

 

[eldubb440]

Most likely trans got swapped out years ago for a lesser performance 727 that was in front of a 2.76 rearend.

440 cars got a "high performance" 727?

I know next to nothing about automatic transmissions....... what would the differences be?

I ask because I remember my GTX having a similar issue, just not as severe........ I assumed the trans got swapped at some point, but was unaware of "levels of performance"

 

[lemondana]

Yes-there were several different 727's used in any particular year. Let say 1969, 383-2 barrel, 383-4 barrel, 440 standard, like a 440 New Yorker, 440-HiPerf, 440-6Pac, and Hemi all used different part numbers. When automatic cars get messed with, 9 out of 10 times. having improperly adjusted or removed kickdown linkage destroyed many transmissions. Smallblocks are the same way. Post up the PK part numbers on the pan rail, and we can tell you if you have the right trans for your car.

 

[moparedtn]

Here ya go - Ma's own chart, complete with speedo gear colors even:
Speedometer Gear Selection Guide – MyMopar

 

[68gtxman]

 

[pnora]

30 mph is a big difference. Thats like 50 percent more 60 vs 90. I would go on the highway and see what the odometer says. If the highway has mile markers. In your case if you went a mike your odometer should of changed to almost 1.5 miles in a marked one mile span. If it is close to the one mile than the drive for the needle has a problem. Head would need to be rebuilt. GPS can help compare whats going on.

 

[pnora]

Yes-there were several different 727's used in any particular year. Let say 1969, 383-2 barrel, 383-4 barrel, 440 standard, like a 440 New Yorker, 440-HiPerf, 440-6Pac, and Hemi all used different part numbers. When automatic cars get messed with, 9 out of 10 times. having improperly adjusted or removed kickdown linkage destroyed many transmissions. Smallblocks are the same way. Post up the PK part numbers on the pan rail, and we can tell you if you have the right trans for your car.

What he has for a transmission has nothing to do with the speedometer. They all accept the same gear's.

 

[haywire 440]

On my ‘68 Sport Satellite the previous installed 150 mph speedo read the mph correctly. It was changed out to a 120 mph and that Speedo/gauges unit reads extremely high like the op. I actually have a different 150 mph speedo/gauges unit that is planned to installed but haven’t gotten to it yet.
I’m not sure why the 120 reads so high they all start at zero.

 

[lemondana]

True-but most of the time when transmissions got changed, the speedo gears did not. An example story-I had a buddy years ago, that had a nice 71 GTX. He was hard on cars, he was moving to Springfield MO from Lincoln NE. His transmission went out in Springfield and didn't have time for the trans shop to rebuild his. He had them install a rebuild they had on the shelf. When he came back to Lincoln, he was telling us this story, and thought his speedo was way off. I crawled under the car and decoded the trans part number. It was a 72 or 73 400 C body trans that more than likely had 2.76 rear gears behind it. Back in the day kids didn't have access to the speedo gear charts that they do today. No internet no smart phones.

What he has for a transmission has nothing to do with the speedometer. They all accept the same gear's.

 

[1969VAGTX]

As a follow up, I did the garage verification method to try to determine gear ratio. I am pretty certain that it has the factory correct 3.55. Not that it makes any difference, but the rear end housing does have the 3.55 tag and the metal "tag" for Sure Grip on the outside of the housing.

 

[pnora]

On my ‘68 Sport Satellite the previous installed 150 mph speedo read the mph correctly. It was changed out to a 120 mph and that Speedo/gauges unit reads extremely high like the op. I actually have a different 150 mph speedo/gauges unit that is planned to installed but haven’t gotten to it yet.
I’m not sure why the 120 reads so high they all start at zero.

150 or 120 they are all driven the same. If the gear is correct and the speedometer is off it is in the calibration of the head or the head has worn parts.