This may be a stupid question, I don't know. I have an 86 Iroc, originally a 305 4-bbl car with an open rear. I just got my tranny rebuilt and have some real low *** gears in the back. I know this cause before I had the tranny rebuilt I was cruising at 65mph and the tach was reading about 2100, and I thought that was low. Now that the converter is actually locking up I'm cruising at 65 and the engine's only doing like 1600-1700 rpm! So, I'm looking at buying a Powertrax posi unit, and I wanna throw some 3.55's in there while I'm at it. I had no idea that there were 2 different size rears in these cars. If anyone could tell me what size my car has I'd appreciate it. Thanks.


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86 IROC w/T-tops
Engine: 355ci/apx. 300hp
Tranny: rebuilt 700-r4 with shift kit, 2400 stall converter w/lock up kit, trans cooler

 

It should be 7 1/2.

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1989 IROC-Z 5.7L
NOS 150HP kit
ProBuilt 700R4, PI Vigilante 2800 stall lockup
Baer Brakes 12" Sport System

 

Although the gear sizes are slightly different, they both bolt up to the same carrier. You could buy 7 5/8 gears (usually all they sell now anyway) and bolt them onto your 7 1/2 carrier or any aftermarket posi etc.

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Follow my racing progress on Stephen's racing page
and check out the race car

87 IROC-Z Pro ET Bracket Race Car
383 stroker (carbed) with double hump cast iron heads and pump gas
461 Big Block installed and ready for the 2001 racing season

Best results before the 383 blew up
Best ET on a time slip: 11.857 altitude corrected to 11.163
Best MPH on a time slip: 117.87 altitude corrected to 126.10
Altitude corrected rear wheel HP based on power to weight ratio: 476.5
Best 60 foot: 1.662

Racing at 3500 feet elevation but most race days it's over 5000 feet density altitude!
Member of the Calgary Drag Racing Association

87 IROC bracket car, 91 454SS daily driver, 95 Homebuilt Harley

 

I thought GM switched to the 7 5/8" gears in 85, but I could be wrong.


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West Coast GM Shootout 2001!
1991 Camaro Z28
5.7L 5-Speed (originally 305)
13.23 @ 107.62 MPH
Southern California
Member: SoCal 3rd Gen F-Bodies
Webmaster: SoCal F-Bodies
-=ICON Motorsports=-

 

I thought the switch to 7-5/8" came with 28 splines, but I could be wrong.