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Thread: Speedo gear choices....

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  1. #1
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    Default Speedo gear choices....

    I recently installed a disc rear and went from 3.55 to 3.73 gears. I also installed the recommended 21 tooth reddish colored speedo gear. The car is reading about 7 to 8 mph too fast. The rear tires are Nitto NT555 G2's, 275-40-17. Any idea which gear for a more accurate speedo reading?
    '88 Sport--T-5,HURST shifter,TMoss GT-40 intake,Ed Curtis cam,Trick Flow heads,Scorpion rockers,70mm t-body,3G,mini starter,Taurus fan,BBK headers,O/R H-Pipe, Flowamaster 40's, deep and deeper Cobra R wheels,Mass Air, Mass Air and 24's,8.8 with 3.73's,140 mph speedo,Mach 1 chin spoiler,SN-95 springs,CHE control arms,aluminum drive shaft and a lot more..

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    Quote Originally Posted by vinnietbird View Post
    I recently installed a disc rear and went from 3.55 to 3.73 gears. I also installed the recommended 21 tooth reddish colored speedo gear. The car is reading about 7 to 8 mph too fast. The rear tires are Nitto NT555 G2's, 275-40-17. Any idea which gear for a more accurate speedo reading?
    Reading was no doubt too fast with 3.55, changing ratio to 3.73 won't cause a 7-8 mph error...

    Too fast means there aren't enough teeth on the driven gear, for genuine Ford, 21T is it... There is a aftermarket 23T available, due to thin teeth reported by some to have short longevity...

    Best solution would be change the drive gear on output shaft from a 8T(what it likely has) to 7T, of course you have to remove the transmission tail housing to change it(no fun at all on T-5)...

    https://performanceparts.ford.com/do...UsageChart.pdf

    BTW the 23T will drop reading approx 6-7 MPH...






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    This would work as well, since our cars have electric speedometers: https://lmr.com/item/M4209ADPTAC/94-...-M-4209ADPT-AC

    Pricey fix though.
    '88 Thunderbird LX: 306, Edelbrock Performer heads, Comp 266HR cam, Edelbrock Performer RPM intake, Edelbrock 70mm TB, 76mm C&L MAF, 30lb injectors, 2.5" exhaust, AOD with 2800 PI converter, 8.8 with 3.73 gears, 31 spline Traction-Lok, 31 spline Moser axles, 04 Cobra front arms, Maximum Motorsports extreme duty rear arms, subframes.
    '11 Focus, '12 Mustang 3.7, '17 Accord EX-L V6

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    Quote Originally Posted by thunderjet302 View Post
    This would work as well, since our cars have electric speedometers: https://lmr.com/item/M4209ADPTAC/94-...-M-4209ADPT-AC

    Pricey fix though.
    Or this will work. More cost less effort though
    '88 Thunderbird LX: 306, Edelbrock Performer heads, Comp 266HR cam, Edelbrock Performer RPM intake, Edelbrock 70mm TB, 76mm C&L MAF, 30lb injectors, 2.5" exhaust, AOD with 2800 PI converter, 8.8 with 3.73 gears, 31 spline Traction-Lok, 31 spline Moser axles, 04 Cobra front arms, Maximum Motorsports extreme duty rear arms, subframes.
    '11 Focus, '12 Mustang 3.7, '17 Accord EX-L V6

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    Quote Originally Posted by thunderjet302 View Post
    Or this will work. More cost less effort though
    I'm not sure that would work for our cars?
    Mike

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    Quote Originally Posted by mcb82gt View Post
    I'm not sure that would work for our cars?
    It will. We have a VSS speedometer, like 94 and up Mustangs.
    '88 Thunderbird LX: 306, Edelbrock Performer heads, Comp 266HR cam, Edelbrock Performer RPM intake, Edelbrock 70mm TB, 76mm C&L MAF, 30lb injectors, 2.5" exhaust, AOD with 2800 PI converter, 8.8 with 3.73 gears, 31 spline Traction-Lok, 31 spline Moser axles, 04 Cobra front arms, Maximum Motorsports extreme duty rear arms, subframes.
    '11 Focus, '12 Mustang 3.7, '17 Accord EX-L V6

  7. #7
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    Or get some taller tires .... I ended up putting 255/50/17 tires on (getting harder to find appropriate speed ratings unfortunately) after the old one wore out.....lived with Speedo being out till then.

    Only other choice would be to send the Speedo out for a recalibration.

    John

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    Quote Originally Posted by Skunk View Post
    Or get some taller tires .... I ended up putting 255/50/17 tires on (getting harder to find appropriate speed ratings unfortunately) after the old one wore out.....lived with Speedo being out till then.

    Only other choice would be to send the Speedo out for a recalibration.

    John
    That just does same thing as reducing gear ratio... Assuming Vin likes the 3.73, using tires to equal the gear combo of 3.27 is a giant step backwards... Could need something like a G78-15 to be tall enough...

    I'm 99% my fresh AOD has a 8T output shaft, I'll be using the 23T gear...






    My other rides (that actually do run & drive)

    1969 Fairlane Cobra 428CJ 4-Speed

    1972 Comet GT - 306 C4

  9. #9
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    It may be a step backwards but it is very close to the stock 16" TC tire height and keeps the Speedo a little more accurate for my car.

    I never bothered to calculate gear comparison to a Mustang but with a turbo....that 327 works well for me. Thanks for posting that info.

    John

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    I put on 255/40ZR17's rear and 235/45ZR17's front after my 21T 3.73 swap. GPS verified error of 1@60, 2.5@100
    Last edited by TheFoeYouKnow; 09-11-2017 at 10:24 PM.
    1988 Thunderbird 5.0
    GT40P, TFS-1, KB domes, 30#, MAF conversion, Explorer GT40 Upper/Lower, Smog delete, Wide Ratio AOD Mod, Unlocked Speedo, 3G Charging System Upgrade, Hi-Torque Mini Starter, 3.73 LSD, BBK 1 5/8 unequal headers, H-Pipe.

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