Does the 70 mph test really work? I was on the NJ Parkway driving down to LBI the other day, and at 70mph i dropped it into drive and my tach read 2560. If this is right then that would explain alot. I already knew the rear wasn't stock because it had posi and my rpo didn't. If it does work, how accurate is it? And does anyone have a 3.42 posi rear?

~Matt

 

as far as i know the 70 mph test works. I tried it on my car and it was taching at 3200-3250 which is good because i got a 3.23 posi. if your looking for a 3.42 posi and know something about rearends (or know a good mechanic) you can buy new gears for most rearends(10 bolts) in jegs or summit for less than 200.00. If yours is a 9 bolt, try 9bolt.com or slpeng.com

 

What is the 70MPH test?

 

When you are cruising at 70 mph, drop your car into drive and the rpms it reads should match your rear gear ratio. I remember someone saying something about it a while ago and i finally tried it. Thanks WS6t/a, it sounds like it is accurate. I think i will just get new gears and have my posi fixed. 2.56 is WAY too small. My 1/4 mile time makes a little more sense. I still don't understand my bad gas mileage though...

~Matt

 

If i remember correctly there are a set of numbers
stamped onto the right hand axle if you are looking at the cover
of the rear end.
I believe they were on the top, but I'm not sure.
I found the numbers in a manual once, (I'll check my chiltons) but don't ever recall seeing them in a manual again.
If you match these numbers up to the maual they will tell you what you have for rear end gears, if the internals haven't been changed.

 

I'm gonna have to try that, I will let ya know....

 

2.56 gears aren't in your car. The accuracy of the 70mph test depends on you tire size, tach accuracy, and speedometer accuracy.

At 70 mph with 3.73 gears and 245/45R16's on my 82, it calculates to 3550rpm at 70mph(getting rid of speedo and tach accuracy). Oops, the tire size does matter. If I had 245/40R16(23.7 inch tall tire) with 3.73's, then it comes out about right at 3700rpm.


At 70mph with 2.73's and 245/50R16 tires(I assume you have) it comes out to around 2550rpm. Standard gears in an 86 t/a with an automatic transmission are 2.73.

Gas mileage doesn't have that much to do with gears, you can actually get better mileage from deeper gears, depending on the engine specs.


https://www.thirdgen.org/newdesign/tech/techdb.shtml[/url]

www.f-body.org/gears