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Thread: Speedo gear choices....

  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...






    My other rides (that actually do run & drive)

    1969 Fairlane Cobra 428CJ 4-Speed

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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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    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

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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.

    88 Thunderbird/Cougar EVTM
    (Thanks to Trinom for hosting)

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    Speedo was accurate with 245/45R17, 3.27 rear and 18T gear before the swap (Should point out that my AOD has all 4R70W internals, including the output shaft and its 7 tooth drive gear), in order to keep same accuracy with my 3.73, I'd need a 20.5 tooth gear. Since that's stupid, seeing as you can't have a half gear tooth, You have to make up the difference with a tire change.
    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.

    88 Thunderbird/Cougar EVTM
    (Thanks to Trinom for hosting)

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    I'm going to get the GPS on the phone going and do another test, then, if it's the same, I'll try the new gear. Thanks Guys. I appreciate the advice Tom. As always.
    '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..

  10. #10
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    Quote Originally Posted by TheFoeYouKnow View Post
    Speedo was accurate with 245/45R17, 3.27 rear and 18T gear before the swap (Should point out that my AOD has all 4R70W internals, including the output shaft and its 7 tooth drive gear), in order to keep same accuracy with my 3.73, I'd need a 20.5 tooth gear. Since that's stupid, seeing as you can't have a half gear tooth,You have to make up the difference with a tire change .
    LOL yeah right... With the drive/driven gear system and mechanical speedo there was almost never a vehicle with speedo closer than 2-3 mph @ 60 mph... Just because one ordered the optional(larger) tires, the factory probably didn't use a different speedo gear... In some applications different rear ratios used same speedo gears...

    My Cobra Jet is prime example, same driven gear was used with 3.25 & 3.50 rear ratios... Both were slightly off calibration in opposite directions... Now it's definitely off calibration as I swapped the 3.50 to 3.00 and the factory 16T driven gear is as short as there is... The 6T output shaft drive gear needs to be changed to a 7T... The top loaders used behind lesser engines got the 7T, but version used behind 427/428/429 with the 1 3/8" input & 31 spline output shaft only used the 6T...






    My other rides (that actually do run & drive)

    1969 Fairlane Cobra 428CJ 4-Speed

    1972 Comet GT - 306 C4

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