hi guys i have a 1988 camaro iroc z 305 tpi 5 speed in 5th gear top speed is 70 at 5000 rpms i mark the drive shaft and the tire i count 3 1/2 i think it has 373 in it 9 bolt posi disk brake so i thinking it the rear end or someone put a 6 cylinder t5 in it i can not find any number on it to find out if some one know where the numbers are please let me know thanks

 

Your tachometer is off. Pretty common with these cars...

 

Normally a 1988 model with a disc-brake rear differential; would have originally had metal tags attached to the differential inspection cover (via the cover bolts).
Notice the metal tags on both the Left and Right sides of the inspection cover... shown below:




Most of these cars would have had a 2.77:1 or 3.27:1 ring and pinion gear-set (Borg Warner).

The Highest optioned 5.0L and 5-Speed combination IROC-Z cars (few 350s had this option) had a 3.45:1 gear-set (Borg Warner).
But 3.70:1 gears "stock" are so rare, that I do not think another will be found again.

There were "Dealer Package" installations with 3.70:1 gear-sets (Borg Warner) as well as a very rare "Dealer Package" 3.92:1 gear-set (Dana 44).

Also there was a GM Warranty-Repair Program...
Where if a vehicle with one of the Borg Warner differentials had repeat failures... a 3.31:1 or 3.92:1 (Dana 44) would be installed.

 

Most common place to find the 3.70s from the factory is 85 L69 5-spd Trans Am. To call that "common" is a bit of a stretch though. AFAIK they never used that in any Camaro, or any EFI car. AFAIK Camaros all came with 10-bolts up until 87, and no carbed one (the only ones that year were LG4) or TBI from 87 - 89 came with a 9-bolt, only TPI.

As said, dealers would put different gears in for people; but even that was not common at all. And the Dana 44 was even less so. I personally have never laid eyes on one.

3.45 was the common G92 gear in the 9-bolt for 305 TPI 5-spd cars in about 88 & 89. AFAIK it was never used behind an auto trans or behind a 350.

There are few if any aftermarket sources for the OE 3.70 ratio. 3.73 however is commonly available, so it's not impossible that you might have that now. Tags and whatnot obviously won't tell you if somebody has been doing any swappage.

Change your fluid. It obviously needs it anyway. Look at the parts while you're at it. Whatever gear you find in there, is what your car has, regardless of what it might have come with. EFFFF abuncha numbers, look at the parts.

And T.L. is right, your tach is wack, like everybody else's. With the .73 5th that you have and 3.45 gears, your engine should be at about 2225 - 2250 RPM at 70 mph.

 

thank guys going to put a new tech in it see what happens and going to take the cover off and look for the numbers there where no tags on the rear end