Gear Ratio???
 

I'm trying to figure out what rear differential is in my '72 without counting the teeth. I run 65 mph @ 3100rpm's on the highway. Anyone want to give me an estimate??? The trans is a TH400.

Thanks in advance,
Craig

RE: Gear Ratio???
 

Depends on tire size. I'm guessing you have a 3.55 rearend.

Here's the formula if you don't want to jack up the rearend and count the driveshaft revs per wheel revs.

Rear ratio = (2.98 times tire outer diameter in inches) divided by miles/hour per 1,000 rpm

This should get you close enuf that you can tell what the actual ratio is.

RE: Gear Ratio???
 

Thanks, I'll look at what size they are tonight. I know that they are bigger than stock tires (Goodyear Eagle GT II's) as my speedo says 10mph faster than I'm actually going. I took a handheld gps with me and 76 on the speedo was 75 on the gps.

RE: Gear Ratio???
 

I've got 255-60X15 Eagle GTs and a 3:55. I get 22 mph per 1,000 rpm in 4th.

RE: Gear Ratio???
 

Ooops...Typo in my previous post. It's was actually 65 on the gps and 76 on the speedo. Thanks for your input!

RE: Gear Ratio???
 

Thanks, my tires are 255/60/15's as well. I wonder if the ratio of a turbo400 in third is the same as a four-speed car in 4th?? If they are the same....then my rear is 3:55. [sm=gears.gif]

RE: Gear Ratio???
 

If you come off the line pretty good you probabbly have a 3:55.
If you bog a bit then figure on a 3:36
If you engine is screaming at 55MPH figure on 3:73 or so on.

RE: Gear Ratio???
 

Both transmissions are 1.00 to 1, so yes, you do have 3.55 gears.

RE: Gear Ratio???
 

I would say your close to 3.50

RE: Gear Ratio???
 

Thanks alot!! Does anyone know if the 3.55 gears were standard or an option?