72Vert442

I am trying to determine the rear gear ratio. Came from the factory with 2.73. At 60 it is turning 2500 rpm. 65 turns 2800rpm. Seems way to high for a 2.73, no?

Tried the method of lifting the rear off the ground and counting the driveshaft turns per one wheel revolution. That made no sense as the driveshaft only turned 1. 3/4 turns for 1 revolution of the wheel. What am I missing?

joe_padavano

What you are missing is that if only one wheel is turning, the differential adds a 2:1 multiplier. That would get you 2 x 1.75 = 3.5, which could be either 3.42 or 3.73 depending on how accurately you counted. This is why I count ten turns of the wheel, since it's much easier to determine 37.3 vs. 34.2 turns of the driveshaft. The RPMs at speed will depend on your tire diameter, which you didn't tell us.

ASSUMING about a 26.5" diameter tire (225/70-14), 3.42 gears would be turning about 2650 at 60 MPH and about 2800 at 65. If your tires are smaller (like a 245/60-14, which is only about 25.5" in diameter) then lower numerical gears would give you those numbers.

72Vert442

Thanks for the info. Not sure peoples logic of changing gears in a nice highway cruiser. Are you really drag racing a 180 hp car? Taking away the ability feature to turn low rpms at interstate speeds to gain low end punch doesnt make sense to me.

monzaz

Tire size like Joe said. Do you have the correct factory size wheels THAT will also affect the ratio the rear and rpm at certain speeds.
CHANGING gears: EVERYONE did it back in the day...It was the easiest way to get the big car out of the hole. No matter what size engine you had. lol. Most really did not take these cars out of town or on the express way in the 80's over drive was starting to come into being in the late 80's and the power on most cars was even worse with smaller cube motors. so the mid 70's still seems like plenty more power than a 80's car. Emissions was another hand cuff of doing anything to motors...so again gear ratio was a way to get the grunt. not till late 80's with the brink of electronic timing and fuel injection was it starting to look better.
Jim
JD Race

joe_padavano

You have different priorities. I just drove my 62 on Hot Rod Power Tour for over 2,000 miles round trip. I have 3.36 gears and small 205/70-14 tires. Engine was turning 3000-3200 RPM on the freeway legs. It sounds best there.

Fun71

Same with my car. I have had several gear ratios from 2.56 to 3.55, and the engine really seems to run best on the highway at around 2800 RPM.

A tidbit of information: when I swapped the original 2.56 rear for a 3.55 rear, the in-town mileage stayed the same. And it was sooooo much more fun to drive.