So, in today's world, if your not doing 70-75 on the highway, you get run over....my 69, 383, 4 speed car pretty much screams at those speeds.....does that mean I have 3.55 gears? (Or higher)? Is there simple way to find present gear ratio? If wanting highway cruiser, what should I be running? PS....running 295/45/18 on rear Thanks

Jack up the rear and place jack stands under the axle tubes. Put a mark on the driveshaft, put a corresponding mark on the pig. Put a mark on the ground/floor by the tire and a corresponding mark on the tire. If it's an open rear turn the tire one complete turn while counting the number of turns on the driveshaft. If it's an open rear with one tire on the ground turn the tire two complete turn while counting the number of turns on the driveshaft. If it's a suregrip do one complete rotation while counting how many times the shaft turns. For instance, if it turns 3 1/2 times most likely 3:55's, 3 1/4 turns most likely 3:23's..........if it turns just shy of 4 turns most likely 3:91's. I run 3:55's and at 60 I'm turning around 2600-2800 rpm's, with a 26" tall tire. Running a taller tire will effectively slightly lower the gear ratio number numerically.

Thanks! I'll have to check rpms at 60, but highway's gonna be at least 70mph How do I know if open or sure grip?

Typically an open diff when rotating the passenger tire forward the driver side will rotate opposite, sure grip both in the same direction.

There are many online calculators to do that for you. Here is one. http://www.advanced-ev.com/Calculators/TireSize/

Your tire diameter should be about 28.45". At 75 MPH, 5.57:1 gear = 4,933 RPM At 75 MPH, 5.17:1 gear = 4,579 RPM At 75 MPH, 4.89:1 gear = 4,331 RPM At 75 MPH, 4.56:1 gear = 4,039 RPM At 75 MPH, 4.30:1 gear = 3,808 RPM At 75 MPH, 4.10:1 gear = 3,631 RPM At 75 MPH, 3.91:1 gear = 3,463 RPM At 75 MPH, 3.73:1 gear = 3,304 RPM At 75 MPH, 3.55:1 gear = 3,144 RPM At 75 MPH, 3.31:1 gear = 2,932 RPM At 75 MPH, 3.23:1 gear = 2,861 RPM At 75 MPH, 2.93:1 gear = 2,595 RPM At 75 MPH, 2.76:1 gear = 2,444 RPM

Glad you asked....I conducted this very experiment on my buddy's driveway a while ago... Suregrip marks..... and a Pegger...... Hopefully that concludes today's lesson.

One of the few down sides of a vintage Mopar. The short list of options for overdrive. I have a used GearVendors that I plan on putting in. However with 3.91, it will still be turning 2700 @ 70 mph. Right now it's 3500 and I have to stay out of the left hand lane or an SUV will run me over. LOL

Fun with Gear Vendors... Install thread: https://www.forbbodiesonly.com/moparforum/threads/80-grit.36174/page-6

I forget my exact tire diameter, but they are on 15' Cragars. '69 4 speed with a 3.23 rear end. My RPM is as follows. 50 MPH = 2000 RPM 55 MPH = 2200 RPM 60 MPH = 2400 RPM 65 MPH = 2600 RPM 70 MPH = 2800 RPM.

Bad sport is mostly right, but you don't multiply by two with a suregrip. Any limited slip diff will act like a spool if it's any good at all, if the tires are in the air, so simple observation of the number of turns is correct. If an open rear end, with one tire on the ground you must change the number of turns because of side gears and spider gears in the diff. It's easier just to jack up both, then they will both rotate (in opposite directions) and again count turns of the driveshaft for one turn of either tire.