gear ratio basics

[Chryco Psycho]

Gearing
I will try to clarify some of the questiojns relate to gearing ,
I will deal with rear end gearing first most common ratios are 2.76, 2.94, 3.23, 3.55 3.73,
3.91, 4.10 , 4.30
The 83/4 diff has more then 4  different carriers , all are interchangable
, the 1820657, & 741 carriers are both 1 3/8" pinion & are the weakest style carrier
the 742 carrier has a 1 3/4 diameter pinion as is the strongest of the carriers
the 489 carriers has the biggest pinion at 1 7/8" but has a crush sleeve instead of spacers
& therefore is not as strong unless the crush sleeve is replaced with a solid spacer.
The Dana 60 has only the 3.55 , 3.73 [not factory] & 4.10 ratios available.
Similar ratios are available for the 7 1/4 , 8 1/4 & 9 1/4 diffs as well
High numerical [lower] gearsmake the car feel lighter due to more leverage & the car will accelerate
faster but this increases highway RPM & limits top end speed , while lower munerical
[higher] gears do the opposite.
to illustrate the difference with different gearing 
At 70 MPH your RPM will be
using a 26" tire  & using 28" tire       
2.76 = 2496 rpm   &  = 2318 rpm  & at 6000 rpm speed is 168.2 MPH
2.94 = 2659 rpm   &  = 2469 rpm  & at 6000 rpm speed is 157.9 MPH
3.23 = 2922 rpm   &  = 2713 rpm  & at 6000 rpm speed is 143.7 MPH
3.55 = 3211 rpm   &  = 2982 rpm  & at 6000 rpm speed is 130.8 mph
3.73 = 3374 rpm   &  = 3133 rpm  & at 6000 rpm speed is 124.5 mph
3.91 = 3537 rpm   &  = 3284 rpm  & at 6000 rpm speed is 118.7 mph
4.10 = 3709 rpm   &  = 3444 rpm  & at 6000 rpm speed is 113.2 mph
4.30 = 3890 rpm   &  = 3612 rpm  & at 6000 rpm speed is 108.0 mph
4.56 = 4125 rpm   &  = 3830 rpm  & at 6000 rpm speed is 101.8 mph

the formula for rpm is- mph X gear X 336 divided by tire diameter

the formula for mph is - [rpm X tire diamter] divided by [gear ratio X 336]

The reason the car feels lighter with lower gears is power multiplication  for example
A 440 is rated near 500 ft lbs torque so we will use 500 so with
2.76 gears the torque is multiplied 2.76 X so 1380 ft lbs is applied to the rear tires
4.30 gears multiply the torque 4.30 X for 2150 ft lbs at the tires
both of these are assuming you are in the direct gear in the tranny 3rd in an auto & 4th with a 4 spd
if you add in the tranny gearing you get a 2.45 ratio in 1st gear with an auto tranny
so 500 X 2.45 X 2.79 = 3418 ft lbs  torque
or 500 X 2.45 X 4.30 = 5267 ft lbs torque
typical tranny ratios are
auto 2.45 1st gear , 1.45 2nd , 1:1 3rd & 2.20 reverse
4 spd 2.65 1st , 1.93 2nd , 1.39 3rd , 1:1 4th  & 2.57 reverse
for interest the I will list the 833 4 Spd OD which is a direct bolt in into a 4 spd car
  3.09 1st , ??? 2nd , 1:1 3rd , .73 4th  this allows a lot of flexability with gearing
for example you now get   500 X 3.09 X 2.79 = 4310 ft lbs at the tire
or 500 X 3.09 X 4.30 = 6644 ft lbs
the other advantage is using lower gearing you can easily cruise on the highway , for example
4.30 X .73 in 4th gear = 3.14 rear gears so your rpm would be less in Od then using a 3.23
rear gear 
as you change the diff gearing you will need to use a matching speedo gear to correct the speedometer accuracy .
Here is a Mopar dash that you can input info into http://vexer.com/automotive-tools/speed-rpm-calculator

[cataclysm80]

Great info there, thanks!

Soon I'm going to be trying to decide what gears to use with a 727 auto trans that has a Gear Vendors overdrive attached.

[Marc70challenger]

Nice!  Thanks mate!

[anlauto]

Wow, that's a great addition of information....this new site is going to rock !