How to correct speedo after changing to 3.73 on '88 Cougar

Topic: How to correct speedo after changing to 3.73 on '88 Cougar (Read 9334 times) previous topic - next topic

How to correct speedo after changing to 3.73 on '88 Cougar

Reply #30 – May 01, 2016, 09:39:50 PM

Quote from: AT8 Cat;455008

**UPDATE** Finally got nice enough weather here to get the car out of hibernation and have the 3.73's installed. Job went well. Car now has a bit more "get up and go". On the highway, car used to rev around 2000 rpms at 115 kph, now it's 2000 rpms at about 100 kph.

I ordered a 23 tooth speedo gear from the local Jeep dealer as suggested by fast ed. It was an exact fit and since it's from another OEM dealer I'm hoping it's better quality than the aftermarket 23 tooth that was also available. However, according to my GPS with the 23 tooth gear I'm off by about 7 kph less than what the speedo reads. Do I need another gear or is that as good as it's going to get with the limited amount of gears available? I did the math, and like many suggested on this thread we come up with 23 tooth. How can we be off by 7 kph? This is my first gear swap so I'm a little stumped.

Meaning you need a gear with more teeth... Speedo shops can install a external drive that would correct reading... Used to be OEM on vehicles that had like 4.11 & 4.30 cogs from factory...

How to correct speedo after changing to 3.73 on '88 Cougar

Reply #31 – May 02, 2016, 10:07:45 AM

When I changed from an T-5 to the Tremec TKO in my Bird and upped the exhaust to 3-1/2" to the lers the area where the speedo cable connected to the sensor assembly (http://www.americanmuscle.com/pyi-speedsensor-8793.html) got really small and the speedo cable would not turn a 90 quick enough to get it to connect to the sensor assembly and clear the exhaust, cross memeber, and sheet metal. I ended up buying a 90 degree adapter which has a built in ratio adapter to solve the problem. Mine is like the one this page http://www.partshp.com/speedometer_gears.htm so scroll down until you see #43. The only problem is that I also had to get a new speedo cable as the speedo cable attachment point is threaded which I believe was like P/N 6800 on the previously mentioned website. This is not a cheap solution but in your case most likely cheaper that pulling the AOD and swapping the out put shaft if that will even correct the issue.

Mine will be coming out shortly as I am swapping over to AutoMeter gauges and the speedo is electric so I can go back to the original sensor assembly and then calibrate the speedo. This is months down the road or I would offer my setup to you.

How to correct speedo after changing to 3.73 on '88 Cougar

Reply #32 – May 02, 2016, 04:36:45 PM

This page explains how you figure out what driven gear you need for the VSS.
http://www.accutach.com/calculators/ford-vss-calculator

Basically, the ECU is looking for 8000 pulses per mile from the VSS.

To calculate pulses per mile:

Tire rev/mile * Rear End Ratio * VSS gear ratio * VSS reluctor pulses per revolution = VSS (pulses per mile)

For your stock setup...
Tire rev/mile, P215/70/R14 = 811 (See http://www.1010tires.com/Tools/Tire-Size-Calculator)
Rear End Ratio = 3.08
VSS Gear Ratio (drive/driven) = 8/20 = 0.4
VSS reluctor pulses per revolution = 8 (Constant for all Ford VSS)

So,

811 * 3.08 * .4 * 8 = 7993 pulses/mile. The ECU is expecting 8000, so you have an error of 0.08% Completely negligible.

For your new setup:
Tire rev/mile, P215/70/R14 = 810 (See http://www.1010tires.com/Tools/Tire-Size-Calculator)
Rear End Ratio = 3.73
VSS Gear Ratio (drive/driven) = 8/23 = 0.3478
VSS reluctor pulses per revolution = 8 (Constant for all Ford VSS)

810 * 3.73 * 0.3478 * 8 = 8407 pulses/mile. That's an error of 5.08% - which is what you should see with your present setup.

You'd need a 24.17 tooth driven gear to get a 'perfect' speedo output with your tires and rear end.

If you had a 7 tooth drive gear, you'd want a 21 tooth. (0.7% error)
6 tooth drive: you'd want a 18 tooth (0.7% error)

I think either the Speed-Cal or Ford Racing speed-dial will convert your 8407 pulses/min to the expected 8000 pulses/min. They run ~$100-130.
They specify 1994+ Mustangs because 1993 and previous Mustangs had mechanical cables.

How to correct speedo after changing to 3.73 on '88 Cougar

Reply #33 – May 02, 2016, 04:43:07 PM

Yep, 94-98. Midyear 98 they swapped to an oss which uses 32000ppm.

Google speedo calibrator. For $80+ id deal with the 4mph difference.

How to correct speedo after changing to 3.73 on '88 Cougar

Reply #34 – May 02, 2016, 04:55:41 PM

I completely missed the part about no speedo cable in the OP's first post. Sounds like the OP has a definite and reasonably priced solution now. Great job guys!!

How to correct speedo after changing to 3.73 on '88 Cougar

Reply #35 – May 02, 2016, 07:23:40 PM

Quote from: JeremyB;455119

I think either the Speed-Cal or Ford Racing speed-dial will convert your 8407 pulses/min to the expected 8000 pulses/min. They run ~$100-130.
They specify 1994+ Mustangs because 1993 and previous Mustangs had mechanical cables.

I was going to say the same thing. It should fix his issue.

I've been thinking about getting one to correct the error in my speedometer (1% so at an indicated 70mph the car is actually going 69.3mph) and get rid of the 23 tooth gear so I can run a 21 tooth or lower gear. The thing is I can live with a less than 1mph error when the fix costs over $100.

How to correct speedo after changing to 3.73 on '88 Cougar

Reply #36 – May 02, 2016, 11:31:53 PM

I lived with a 400% error in speedo for two years in my tbird. 1st gear was over 85mph.

How to correct speedo after changing to 3.73 on '88 Cougar

Reply #37 – May 11, 2016, 11:16:41 PM

Wow, sounds like I've got a few options! You guys are awesome. Thanks for the good information, I'll have to sort through it all and figure out what's best for me. 7 kph isn't too bad, at least I'm going slower than what speedo says. I like the idea of swapping over to a t5 but that's probably a few years down the road. I appreciate the tips!

Cheers!

How to correct speedo after changing to 3.73 on '88 Cougar

Reply #38 – May 17, 2016, 07:41:50 PM

According to Late Model Restoration, your AOD speedo drive gear is 7 tooth. Mine is 7 tooth. Just scored a free 3.73 gearset today, so tomorrow I'm ordering a 20 tooth driven gear from LMR.

How to correct speedo after changing to 3.73 on '88 Cougar

Reply #39 – May 18, 2016, 03:00:56 PM

Quote from: TheFoeYouKnow;455358

According to Late Model Restoration, your AOD speedo drive gear is 7 tooth. Mine is 7 tooth. Just scored a free 3.73 gearset today, so tomorrow I'm ordering a 20 tooth driven gear from LMR.

That applies to Mustangs. For some reason the Thunderbirds/Cougars got the 8 tooth drive gear in the AOD. Was your AOD originally from a Mustang? The factory AOD (which is still in my Thunderbird, just rebuilt) has an 8 tooth drive gear.

How to correct speedo after changing to 3.73 on '88 Cougar

Reply #40 – May 18, 2016, 03:06:53 PM

Mine is originally from an 87 Mark VII. Add far as I'm aware from the research I did before my wide ratio mod, all AOD are 7 tooth until AODE and 4R70W. I had to retain my original output shaft for this reason.

How to correct speedo after changing to 3.73 on '88 Cougar

Reply #41 – May 19, 2016, 03:18:13 PM

Quote from: TheFoeYouKnow;455365

Mine is originally from an 87 Mark VII. Add far as I'm aware from the research I did before my wide ratio mod, all AOD are 7 tooth until AODE and 4R70W. I had to retain my original output shaft for this reason.

HO AODs had the 7 tooth drive gear up till 89. After 1990 they have an 8 tooth drive gear (in Mustang applications). The standard output AODs (behind Thunderbirds, Cougars, Crown Victorias, Grand Marquis, and Town Cars) have an 8 tooth drive gear. I have no idea why Ford did this.

How to correct speedo after changing to 3.73 on '88 Cougar

Reply #42 – May 19, 2016, 06:12:14 PM

That could explain it.