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post #1 of 12 (permalink) Old 05-03-2016, 08:13 PM Thread Starter
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Someone help me out here...

WHen I bought my car from Traveler, he told me it had 3.73:1 gears in it, but according to my code reader/diag tool, the rpm's are too high to be 3.73's. I run 2114rpm at 60.3mph (according to my tool), which tells me I have 3.90:1 ratio in the back, as there's just too much difference to be 3.73 or 4.10.

I know they make a 3.90 gear for the 8.8, but kind of an odd size, don't you think? Measurements were taken as I was in OD with TC locked. We don't have much "flat" land so I expect the rpm's to jump around a little, but would y'all agree with me? She just revs too high to be 3.73's. The tach needle is off a couple hundred RPM, but scan gauge tells me the ECU's reading of the RPM.

97 Thunderbird LX (The GT Bird)
2003 Explorer WAP block with Modular Head Shop "street ported" heads and Stage 2 PI NA cams, 75mm Accufab throttle body, C&L upper intake plenum, Kooks 1.75" primary/3" collector headers, 2.5" full exhaust with mid mount Magnaflow dual in/out muffler, 24lb/hr injectors, 80mm MAF, Tuning from Don @ www.lasotaracing.com, CAI that feeds from fenderwell. Jmod, 3.73:1 TL in Mark VIII carrier, Mark VIII aluminum LCAs, 93 Mark VIII driveshaft, PBR brakes (soon to be Cobras), 18x9 wheels with 35mm offset, 255/40/ZR18 Tires, Front and Rear strut/shock bracing, GR-2 shocks, Eibach 1.5" springs, 1989 SC front and rear sway bars.
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post #2 of 12 (permalink) Old 05-03-2016, 08:22 PM
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Verify speed with a GPS first rather than rely on the speedometer or PCM readout since it goes off the VSS. The VSS gear isn't guaranteed to be the right one - in fact, the popular belief that the 21 tooth gear is needed for 3.73s is usually wrong (20 tooth is better in most applications) unless the tire is really short. Determine your tire's revs/mile, then you can determine what gear you have based on the actual vehicle speed and RPM readout.
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post #3 of 12 (permalink) Old 05-03-2016, 08:29 PM
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You can't go by the speed that the computer thinks you are going. That is based on the speedo gear in the trans, which is not exact, and that also assumes stock size tires, which you are not running based on your signature. Use a GPS to determine your actual road speed at a given engine RPM, in 4th gear with the TC locked, then input your engine rpm, transmission gear ratio (.7 in OD), and tire size into this website...
Top Speed Calculator
...and see what gear ratio matches the road speed per the GPS
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post #4 of 12 (permalink) Old 05-03-2016, 08:32 PM
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A little more numbers for you.

With a 225/60/16 tire of 757 revs/mile and a 3.73, you need a 20 tooth speedo gear (well, in exact terms you need 19.76527 teeth). At 2114 RPM with that tire size and a 3.73, you're actually going 64 MPH.

If you put a 21 tooth speedo gear in there, you would be going the same speed but your speedo will read 6.25% too slow - meaning at an RPM of 2114, your speedo would display about 60.3 MPH.

I wrote this little calc to help me out a few years ago. Feel free to play around in it if you like as well. http://wannerweb.noip.me/Public/TireCalc.htm
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Gone but not forgotten: 96 Mark VIII, 94 Cougar XR7, 93 Mark VIII

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post #5 of 12 (permalink) Old 05-03-2016, 09:26 PM Thread Starter
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Quote:
Originally Posted by theterminator93 View Post
Verify speed with a GPS first rather than rely on the speedometer or PCM readout since it goes off the VSS. The VSS gear isn't guaranteed to be the right one - in fact, the popular belief that the 21 tooth gear is needed for 3.73s is usually wrong (20 tooth is better in most applications) unless the tire is really short. Determine your tire's revs/mile, then you can determine what gear you have based on the actual vehicle speed and RPM readout.
Already done. GPS speed is dead on accurate even with my 255/40/18 (26.0" outside diameter, vs 26.1" with the 255/50/16 that were on when I got it), and they are shorter than the stock 26.6" tire size.

I have my app set up to read GPS speed/ECU speed/ RPM, Coolant temp, etc. GPS and ECU speed are in lock step, and I had a second phone with GPS using a different speedometer app that is pure GPS and it reads in lock step as well. Also when I pass the "your speed is" signs they always match my speedo.

On the online calcs, I have inputted tire Rev per mile, and come up the same. By all the calculators I can find, the car has 3.90 gears in it. Next thing I'll do is jack it up, put it in neutral, and mark the drive shaft and the tire. Since I have a trackloc, I can spin either tire and count how many times it goes around compared to the driveshaft. For every one revolution of the shaft, the tire should go around either 3 and 3/4 (with 3.73) or just shy of 4 (if 3.90).

I don't want to take the diff cover off, but this will bug me till I find out what's up. I do not know what gear is in the VSS, I just know that on the 255/50/16's it measured up perfectly, (within one or two tenths of a mph) so I used that as a baseline when getting new wheels/tires and matched them up. I kept the width because I liked the big contact patch, and the 18's are only .1" smaller in OD than the 16's were, well within acceptable limitations.

97 Thunderbird LX (The GT Bird)
2003 Explorer WAP block with Modular Head Shop "street ported" heads and Stage 2 PI NA cams, 75mm Accufab throttle body, C&L upper intake plenum, Kooks 1.75" primary/3" collector headers, 2.5" full exhaust with mid mount Magnaflow dual in/out muffler, 24lb/hr injectors, 80mm MAF, Tuning from Don @ www.lasotaracing.com, CAI that feeds from fenderwell. Jmod, 3.73:1 TL in Mark VIII carrier, Mark VIII aluminum LCAs, 93 Mark VIII driveshaft, PBR brakes (soon to be Cobras), 18x9 wheels with 35mm offset, 255/40/ZR18 Tires, Front and Rear strut/shock bracing, GR-2 shocks, Eibach 1.5" springs, 1989 SC front and rear sway bars.
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post #6 of 12 (permalink) Old 05-04-2016, 07:29 AM
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With that tire size and having confirmed your PCM speed and GPS speed are correct, it sure does sound like there's a 3.90 and a 21 tooth gear in there. Let us know what you find.
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-Brandon
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post #7 of 12 (permalink) Old 05-04-2016, 08:52 AM Thread Starter
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Originally Posted by theterminator93 View Post
With that tire size and having confirmed your PCM speed and GPS speed are correct, it sure does sound like there's a 3.90 and a 21 tooth gear in there. Let us know what you find.
I'm not so sure about that actually.

I was playing with your calculator, as opposed to the one I usually use which is http://www.andysautosport.com/learni...r/calculators/
and when I plug in the published numbers of what my rev/mile is (799) vs letting them calculate it (by plugging in the 255/40/18) it comes up very close to observed numbers (which I understand can be off by 100 or so RPM due to incline/decline/throttle input.

When I use your calculator, plug in 799rev/mile it tells me I need a 21 tooth gear (which I probably have) and it shows my calculated RPM at 71.5mph to be 2486.04 when my observed RPM from the scan tool was 2489. 3rpm is pretty damn close. playing with it more, my observed at 70.8mph was 2482, and the calculated was 2461, only off by 20rpm, and I was probably going up a slight incline.

I plugged the tire revolutions into the andysautosport calc and got very similar yet rounded off numbers. So it looks like my method of calculating was wrong, using the 255/40/18 instead of the actual revs per mile as listed by the manufacturer.

So, thanks very much for that calculator. Adding it to my calculators list!

97 Thunderbird LX (The GT Bird)
2003 Explorer WAP block with Modular Head Shop "street ported" heads and Stage 2 PI NA cams, 75mm Accufab throttle body, C&L upper intake plenum, Kooks 1.75" primary/3" collector headers, 2.5" full exhaust with mid mount Magnaflow dual in/out muffler, 24lb/hr injectors, 80mm MAF, Tuning from Don @ www.lasotaracing.com, CAI that feeds from fenderwell. Jmod, 3.73:1 TL in Mark VIII carrier, Mark VIII aluminum LCAs, 93 Mark VIII driveshaft, PBR brakes (soon to be Cobras), 18x9 wheels with 35mm offset, 255/40/ZR18 Tires, Front and Rear strut/shock bracing, GR-2 shocks, Eibach 1.5" springs, 1989 SC front and rear sway bars.
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post #8 of 12 (permalink) Old 05-04-2016, 01:36 PM
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Quote:
Originally Posted by Woodman View Post
the calculated was 2461, only off by 20rpm, and I was probably going up a slight incline.
Why would going up an incline make a difference? If the TC clutch is locked, it's locked.

Obviously, if it unlocks, it's a noticeable rise.

Al

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post #9 of 12 (permalink) Old 05-04-2016, 01:48 PM
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The TC lock isn't always 100% efficient. Slippage of a few RPM isn't unusual.
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post #10 of 12 (permalink) Old 05-04-2016, 02:24 PM Thread Starter
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Originally Posted by T6Rocket View Post
Why would going up an incline make a difference? If the TC clutch is locked, it's locked.

Obviously, if it unlocks, it's a noticeable rise.

Al
My assumption was since i had cruise on, I was getting just a little gas to keep the desired speed, and I figured a 20 year old tc may have a little slip.

97 Thunderbird LX (The GT Bird)
2003 Explorer WAP block with Modular Head Shop "street ported" heads and Stage 2 PI NA cams, 75mm Accufab throttle body, C&L upper intake plenum, Kooks 1.75" primary/3" collector headers, 2.5" full exhaust with mid mount Magnaflow dual in/out muffler, 24lb/hr injectors, 80mm MAF, Tuning from Don @ www.lasotaracing.com, CAI that feeds from fenderwell. Jmod, 3.73:1 TL in Mark VIII carrier, Mark VIII aluminum LCAs, 93 Mark VIII driveshaft, PBR brakes (soon to be Cobras), 18x9 wheels with 35mm offset, 255/40/ZR18 Tires, Front and Rear strut/shock bracing, GR-2 shocks, Eibach 1.5" springs, 1989 SC front and rear sway bars.
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post #11 of 12 (permalink) Old 05-05-2016, 09:35 AM
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Yes, I get a rise of 200 RPM or so when climbing a very steep hill with the cruise on going 75+ (very rare), but the TC is temporarily unlocked in that case.

My Toyota, with a 3400 stall, and T-Bird with 20 yr old stock stall do not change RPM one bit when it's truly locked. If you had 200 RPM of slippage with it locked, the clutch would burn up.

Al

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Previous Fords:
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post #12 of 12 (permalink) Old 05-05-2016, 10:32 AM Thread Starter
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20 rpm, not 200, close enough for government work.

97 Thunderbird LX (The GT Bird)
2003 Explorer WAP block with Modular Head Shop "street ported" heads and Stage 2 PI NA cams, 75mm Accufab throttle body, C&L upper intake plenum, Kooks 1.75" primary/3" collector headers, 2.5" full exhaust with mid mount Magnaflow dual in/out muffler, 24lb/hr injectors, 80mm MAF, Tuning from Don @ www.lasotaracing.com, CAI that feeds from fenderwell. Jmod, 3.73:1 TL in Mark VIII carrier, Mark VIII aluminum LCAs, 93 Mark VIII driveshaft, PBR brakes (soon to be Cobras), 18x9 wheels with 35mm offset, 255/40/ZR18 Tires, Front and Rear strut/shock bracing, GR-2 shocks, Eibach 1.5" springs, 1989 SC front and rear sway bars.
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