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#1
04-14-2022, 07:03 PM
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Th400 speedo gear
Anyone know what gear set I would need for my th400 speedometer setup? This is for my original 69 gto th400 I just rebuilt. I have 3.73 gears in the rear with a 30" rear tire. Tried a few of the online calculators but just got frustrated with those.
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1969 GTO street strip project 11.1 forged 461, highport heads 1995 Trans-am 420 ci sb 14:1 compression 9"ford 9.89@132 1.34 60ft SOLD! |
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#2
04-14-2022, 09:41 PM
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Are you talking these gears? https://transmissioncenter.net/shop/...r-driven-gear/ I'm running the blue with 3.73's and a 28 inch tall tire. Cna't remember how close we are to correct speed, but I believe it's close.
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68 Firebird. IA2 block, 505 cu in, SD Performance E-head, Solid roller 3600 weight. Reid TH400 3:90 gear. 28" slick. Best so far 9.95@135 mph. 1.39 60 ft. 76 Trans am, TKX .81 o/d, 3.73 Moser rearend, 468 with KRE D-ports, Doug headers, 3" Exhaust 950 Quickfuel. |
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#3
04-14-2022, 09:48 PM
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need more info. Are you sure 30"?? that's pretty large. what was your original tire and gear ratio unless you know the drive gear tooth count in your Th400.
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1979 Firebird Trans Am 301/4spd (Now 428) 1977 Firebird Formula 400/Auto 2007 Grand Prix GXP 5.3L |
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#4
04-15-2022, 06:38 AM
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Need to know what your drive gear tooth count (and/or color) is to use the calculators.
That's the one on your output shaft that drives the driven speedometer gear.
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 1965 LeMans Post Coupe..............I'm not waitin for the undertaker....I'm lookin forward to the uppertaker!..... |
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#5
04-15-2022, 06:50 AM
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It's definitely 30 inch tire. Actually 30.01 according to M/T website. It's a 295 65 15. I think the Tailshaft gear is orange but it's loose right now so I will get a count. There was no speedo gear in the case though someone removed it, leaked like crazy out the speedometer cable.
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1969 GTO street strip project 11.1 forged 461, highport heads 1995 Trans-am 420 ci sb 14:1 compression 9"ford 9.89@132 1.34 60ft SOLD! |
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#6
04-15-2022, 07:43 AM
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I have the 16 tooth tailshaft gear my housing for the gear adjusts from 36-39
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1969 GTO street strip project 11.1 forged 461, highport heads 1995 Trans-am 420 ci sb 14:1 compression 9"ford 9.89@132 1.34 60ft SOLD! |
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#7
04-15-2022, 09:00 PM
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Steel 16 or tan plastic 18 drive teeth are the most popular Pontiac tail drive gears. Calc away at https://www.tciauto.com/speedometer-gear-calculator
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#8
04-15-2022, 09:47 PM
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Thanks for the handy link to the calculator. I also posted this question on a race th400 and was told 15 on the tail shaft and 38 on the driven gear tooth. The calculator verified this to be accurate. I ordered a grey tail gear and a blue driven gear.
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1969 GTO street strip project 11.1 forged 461, highport heads 1995 Trans-am 420 ci sb 14:1 compression 9"ford 9.89@132 1.34 60ft SOLD! |
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