North American Turbocoupe Organization



Speedo Gear Selection
Robert Camp Offline
Posting Freak
#11
I'll make some assumptions here.
1. You started with a 6T output shaft gear (black) and a 17T VSS gear (white). But I'm not sure as your posts are confusing.
2. The transmission you installed has a 7T output shaft gear (yellow).

Original ratio: 6/17 = 0.3529
Solution: 7/20 = 0.350
20T (black)

That's as close as you're going to get. If the assumptions are incorrect you'll need to post exactly what you started with and what you have.

Working with the same assumptions, a 7/17 gear ratio would result in a 16.6% increase in VSS output, 60mph real speed indicated as 70mph.

Further note: The 6/17 original gearing is based on having a 3.55 rear and 225/60-16 tires. Changes there will affect the accuracy of the speedometer also.
Robert Camp
'86 Medium Regatta Blue TC, 5-speed, original owner.
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Jeff K Offline
Administrator
#12
Robert, check your math. % difference = 100*(original value - new value)/original value. Putting in the numbers: 100*(.3529 - .350)/.350 = .83% difference between 6/16 and 7/20 ratios. .83% wont alter speedo readings or any PCM calculations made with VSS by much of anything.
Jeff Korn

88 Turbo Coupe: Intake and exhaust mods, T3 turbo at 24 psi, forced air IC, water injection, BPV, Ranger cam, subframes, etc., etc.
86 Tbird 5.0 (original owner): intake, exhaust, valvetrain mods, 100 HP N2O, ignition, gears, suspension, etc., etc.
11 Crown Vic Interceptor
14 Toyota Camry (wifes car)
95 Taurus GL Vulcan winter beater
67 Honda 450 Super Sport - completely customized
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Robert Camp Offline
Posting Freak
#13
Jeff, read my post carefully. The math is correct for what I state. If you're referring to the line

"Working with the same assumptions, a 7/17 gear ratio would result in a 16.6% increase in VSS output, 60mph real speed indicated as 70mph."

That's a comparison of what he was getting when he used his original VSS with new transmission gear.
6/17 vs 7/17. Sorry for the confusion.

I agree .35 and .3529 are miniscule and as good it gets.
Robert Camp
'86 Medium Regatta Blue TC, 5-speed, original owner.
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T-BirdX3 Offline
Senior Member
#14
Robert to be honest I'm not sure what color or tooth either were. (Yes I wish I had checked.) But I feel almost certain my TC was all stock and that the 'stang trans was still in stock form.

I took the complete TC trans out of my car and installed the complete 'stang trans.

So as of right now my car has a complete '88 'stang trans (input shaft to tailshaft) in it. The gear on the VSS that bolts to the tailshaft housing is the original one from my TC.

I hope that clears up my posts?
'88 T-Coupe 5 speed/'88 T-Coupe Auto
'87 T-Coupe 5 speed/'87 T-coupe 5 speed(76k orig mile)
..... and there was light!
http://i1224.photobucket.com/albums/ee36...tled-2.jpg
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Robert Camp Offline
Posting Freak
#15
What speed does the 8mph error occur? And is indicated higher or lower than actual? How did you determine the error?
Robert Camp
'86 Medium Regatta Blue TC, 5-speed, original owner.
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T-BirdX3 Offline
Senior Member
#16
The 8mph error was at about 55 (speedo reads about 63 gps shows 55). I used a speedometer app on my smartphone (uses gps to determine speed, so should be fairly accurate). This may not be the best method but it is what I used.
'88 T-Coupe 5 speed/'88 T-Coupe Auto
'87 T-Coupe 5 speed/'87 T-coupe 5 speed(76k orig mile)
..... and there was light!
http://i1224.photobucket.com/albums/ee36...tled-2.jpg
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Jeff K Offline
Administrator
#17
Robert...... of course, you are correct. I didnt read your post correctly!

X3..... What Robert said... how did you determine the mph error. I have checked calibration on several TC speedos using calibrated signal generators substituted for the VSS input, and many are pretty far off. I have seen the calibrations off by as much as 6 mph @ 60 mph, and much worse at higher speeds. In every one I checked, they always read high. I have verified this inaccuracy in the stock speedos with GPS.
Jeff Korn

88 Turbo Coupe: Intake and exhaust mods, T3 turbo at 24 psi, forced air IC, water injection, BPV, Ranger cam, subframes, etc., etc.
86 Tbird 5.0 (original owner): intake, exhaust, valvetrain mods, 100 HP N2O, ignition, gears, suspension, etc., etc.
11 Crown Vic Interceptor
14 Toyota Camry (wifes car)
95 Taurus GL Vulcan winter beater
67 Honda 450 Super Sport - completely customized
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Robert Camp Offline
Posting Freak
#18
Since day one the speedometer on my 86 has erred 6.5-7% high based on 1 and 2 mile measured markers. I even took it to Ford and complained and their answer was that it was in spec.

The following from
http://fordfuelinjection.com/?p=33

"The Vehicle Speed Sensor produces 8 pulse per rotation which a stock computer assumes 8000 pulses per mile."

is in agreement with the following from Ford Racing Parts Catalogue pg 129;

"Driven Gear Teeth = (Drive Gear Teeth x Axle Ratio x Tire Rev. Per Mile) / 1000
= (7 x 3.73 x 815) / 1000 = 21.3 (driven gear teeth)"
This assumes a .9655 tire correction factor for the change in a tires height due to the weight on the tire.

Keeping these in mind, the OEM setup for the 86 agrees within 99%. Except my speedometer is still 6.5-7% high.

As far as knowing what was in the Mustang 2.3 trans, it all depends on the OEM tires and rear end. I would suspect a 6T like the TC but I don't know, it could have been a 7/20 as calculated above.

T-BirdX3, your reading is 63/55 = 14.5% high. That's in range to the 16.6% error that would occur if you did indeed go from a 6/17 ratio to a 7/17 ratio. You may indeed have a 7T in the trans. In that case the 20t VSS gear is as close as you're going to get. It probably will even swing the other way, your speedo will indicate a little low.
Robert Camp
'86 Medium Regatta Blue TC, 5-speed, original owner.
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T-BirdX3 Offline
Senior Member
#19
Robert thanks for your help! It is greatly appreciated! I'll locate a 20 and give it a try.

A special thanks to everyone else that chimed in too! I will keep everyone updated.
'88 T-Coupe 5 speed/'88 T-Coupe Auto
'87 T-Coupe 5 speed/'87 T-coupe 5 speed(76k orig mile)
..... and there was light!
http://i1224.photobucket.com/albums/ee36...tled-2.jpg
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Paulie Offline
Banned
#20
Quote:Originally posted by T-BirdX3:
Quote:Originally posted by Robert Camp:
[b] if you used the output shaft gear and VSS from the original T5.
This is what I SHOULD have done, but I didn't even give a thought to changing it until I had it all back together and was giving it a test run.

I am hoping for a real simple fix that does not include removing the tailshaft housing. I believe that it ought to be as simple as just changing the gear on the VSS. I am just not sure which tooth gear to try on the VSS. [/b]
Sir their are a fiew gears that can accomplish this from the ford store. They range from 17T-21T i use them all the time when swapping rear gear sets. thank You
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