Hey guys,

I've been toying with the rear gear calculator to try to find out what rear I have in my car. And if I am reading correctly, it looks like I have 4.11 gears in my Formula. In 5th gear at about 60 MPH, I am at a little over 2k on the tach. Now, everytime I put the proper info (as I know it to be) I get strange numbers. I put in the tire diameter (overall 25.7) and I get that I should be going 150 mph in 5th gear at 2000 rpm. Any ideas?


Thanks

 

Jack it up, spin a rear tire (does the other tire spin), if yes, you have a POSI. Then, make a mark on your driveshaft, make a mark on your tire. Spin you tire 1 turn (while counting your driveshaft turns). If the driveshaft turns come out to be around 4 or slightly higher, than you probaly have 4.11's. If it's lower than 4, say 3.75 turns, you more than likly have 3.73's.

If you don't have a POSI, you will need to multiply the driveshaft turns by 2 in order to determine your gear Ratio.

Hope this helps!

 

I have 3 ideas.

1. Assuming you're doing the "test" right, your tach reads WWWAAAAAYYYYYYY high, unless somebody has swapped the gears (the tach being off is far more likely). Your car would have come with 3.08 gears. I'm betting that's what's in there.

2. You're not using the calculator correctly.

3. The only way to know for sure what gears you have, or whether it has a posi, is to take off the cover and look. All of these "tests" rely on various other things to be working perfectly, and for your observations do be made acurately. They are very easily misinterpreted, and errors are introduced by all sorts of things. Besides, if you haven't changed your rear end fluid lately, it's overdue; and the best way to do that is to pop the cover off. So while you're catching up on all your overdue routine maintenance, make a point of looking at your gears, count the teeth, and see what you've really got. I think you'll find the "tests" aren't worth the bandwidth you used to download the instructions.

 

That was part of the plan. I just picked the car up on Friday night, so there are a tonne of things that need to be done to it. I already have my tune up stuff coming in, as well as new sway bar bushings and end links.

As for the rear gear ratio, I knew it comes with 3.08s but I am not sure if they were ever changed, since the previous owner is like the 4th owner. And it does whine a great deal too. Not something I am used to.


Thanks

 

The gear ratio is usually stamped on the edge of the ring gear; it would say something like "41:11" for example if it was 7.73s (41 divided by 11 = 3.7272727272....) or "40:13" if it's a 3.08 (40/13 = 3.076923076923076923....) If you can find that marking, you don't even have to count the teeth yourself.

 

Sweet!! Thanks for all the info. I am so used to driving my auto firebird, and that would lope around 1400 rpms at 65 mph.

 

uh...the calculator on this site says radius not diamiter...radius is from the center out, and diameter is the distance accross, the radius of our tires is allitle less than 13" (I just use 13" and round off my results)

 

Yeah, i'm a retard for not looking at the site. I found out that the rear was swapped and the speedo is off by 10 miles an hour. So when I am doing 40, it reads 30. I used the calculator again, and it appears I have a 3.55 gear (or there abouts) in the back of the car. Now I thought that was a Ford ratio. I know it's not a 9 inch, and it looks like a regular 10 bolt. Weird.

 

if it's off it's going to be off by a presentage, not by a set MPH, the faster you go the more it will be off, also GM did make a 3.43, rear, which is close enough to your 3.55 calc to be in it's margin of error.