Rear Gear

[VetteManiac]

My 1980 Vette has a 1980 camaro z-28 motor in it. Going down the road I've noticed that at 65 mph I'm at 4000 rpms. This isn't good. My speedo is off, it is 5 mph below the real speed, would my tach be off also? Can anyone help me?

 

[AKRAY4PLAY]

sounds like the tach may be off (some are way out of spec), or you have 4.56 rear gears. try using ths calculators in the tools section. they will allow you to enter tire size, speed and rpm to calculate rear gear. the best thing you can do is count the drive shaft revolutions per one rev of the tire. jack up the rear of the car and put on jack stands. put the tranny in neutral and climb under. marking the pinion with a paint pen helps you count drive shaft revolutions. roll the tire exactly one turn, this will give you the rear ratio. if drive shaft turns four and one half times, then you have a 4.56:1 rear end, three and three quarters is 3.70:1, you get the picture. JUST REMEMBER TO PUT THE TRANNY BACK IN PARK BEFORE YOU LOWER THE CAR!! it would suck to lower the car and have it roll into something or worse you.

 

[VetteManiac]

Yeah it would suck to have it rolling about. I believe my tach is off, and my dad and I are gonna try to do something about that. Thanks for your help AKRAY.

 

[AKRAY4PLAY]

once you get the rear ratio figured out, run the numbers through the calculator and and it will give you the rpm for a given speed. that will tell you how far the tach is off. i am not sure, but the tach may have some way to adjust it a little, i'll find out.

 

[VetteManiac]

Thanks for all of your help.