Hi Guys,
When I bought my '86 TransAm a couple of years ago, the bloke I bought it off said he had a posi-drive diff installed into the original case. Now I think I'm missing something because if I jack one wheel off the ground and turn it, it spins freely and the drive-shaft turns. Is this a standard Diff' or what? Shouldn't it lock?
Also the ratio is a bit of a mystery. I'm getting 60MPH at aprox' 1500RPM!(5th gear-T5). I've checked this using a:General traffic flow and b:another late model vehicle. Also my 'speedo' seems to read about 5-10MPH slow.
Anyone know the correct method of telling what kind of Diff' it is and what ratio?
Some expertise in this area would be a great help... Thanks for your time... Regards...
Geoff

 

RPM / GEAR RATIO X TIRE DIA. / 336 =MPH

use 4th gear to check your rpm@60mph

4th gear is 1:1 ratio fifth is overdrive.

eg 2300 / 2.73 X 24in / 336 = 60 with a 2.73 and a 24 inch tire

you can measure your tire's effective diameter by measuring the
distance from the center of the wheel to the ground with a tape measure and multiply by 2.

 

Thanks F-bird... That's a handy little formula...
regards...
Geoff

 

If the speedo is off, finding the gear ratio will be a little difficult using MPH. Jack it up and spin both rear tires exactly one turn the same direction(it helps to have additional people) and then count how many times the driveshaft turns. The driveshaft turns will equal the gear ratio. Or pop the cover and the ring gear should be marked--or count the teeth. The fluid probably needs changed in the rear anyway. When you do the posi test, your 5 speed needs to be in gear. If you don't have posi, the rear wheels will spin freely with the car in gear(but spin opposite directions. Posi will give you a good amount of resistance.

 

FIND THE AXL CODE ON THE FRONT OF THE RIGHT AXL HOUSING, POST THE FIRST THREE NUMBERS AND OR LETTERS,AND I CAN TELL YOU EVERY THING YOU NEED TO KNOW.I HAVE A COMPLEAT LIST OF AXL CODES AND INFO ON EACH AXL FROM 82 TO 92,RATIO AND TYPE.