I am looking to buy a 1987 firebird formula, 305 TPI, 5 speed, with a few minor bolt ons. There MIGHT have been some work done to the engine, but I have not confirmed it as of yet. My question is, the car is reving around 2000 rpm at around 100 km/h (sorry, i live in canada), or about 62 mph. What drive ratio do I have? Is it the stock 3.08 or is it the borg warner rear? Thanx for all your help, excellent site

 

https://www.thirdgen.org/newdesign/vin/

 

Thanks...problem is....i stupidly didn't take down the VIN number when I looked at it...I know, stupid error...i usually do check it out...but at the same time, I think the rear end might have been changed so that is the reason for my question, since it wouldn't show up on the VIN number obviously if it had..., also, what kind of gears are the "sweet spot" for the 305 TPI? and what kind of mileage can I expect for each. Thanks again to anyone replies!

 

www.f-body.org/gears.url

 

ignore the "/url" part

 

You have a 5-speed right? Well then put it in 4th gear and go down the highway at 70mph, dont know what that is in kph. Then, what ever RPM your at, divide by 1000. For example, mine reads 2730.
2730/1000 = 2.73.

 

that will vary in accuracy depending on tire height.
The only way to see which rear you have is to look at it. 9 bolt has 9 bolts on the cover and a 10 bolt has 10. Assuming it has 245/50R16's, 2000 rpm at 62mph is about 3.73 gears in 5th gear--assuming the tach and speedo is accurate.

 

The VIN won't show the gear ratio.

More than likely you have the B-W rear. It's easy enough to tell: count the bolts that hold the cover on. 10 bolts = {drum roll please} 10-bolt; 9 = B-W.

That is very low RPMs at cruise speed, assuming your tach is accurate (not necessarily a good assumption). With what should have come in it if it had the standard ratio, which would have been a 3.23 (or 3.27 if it's a 9-bolt), it should be at around 2400 RPM at that speed.

First, get your tach checked, against a tune-up analyzer at a shop or something. You can easily check your speedo by diriving a known measured distance (like between distance markers on the highway) at some speed as indicated on the speedo, and timing how long it takes to cover it, and calculating the actual speed. If it's accurate, I'd bet you have 2.73 if it's a 10-bolt or 2.77 if it's a 9-bolt, which would not be the original rear end.

 

I run 70 mph with 275/60 r15's on the rear and only turn 2100 rpm. Thats w/ a 700R4.

3.08 rear gears too.


Hope this helps!!!