REAR END GEAR SELECTION

Hey guys I have a 1974 Plymouth Valiant 4 Door. alot of things were changed on this car. It has a 1972 360 HP engine in it with a small cam and some mild porting of the stock heads. Also it has a a-904 trans with a 8.25 rear end. The car goes good for what it is. Im not sure what rear end gear to get for running around the street and possibly the highway once in a while. I was thinking 3.90's but i think it might be too much for the street.

thanks alot

Vin  

I think your probably right 3.90's are probably too tall. It all depends on how tall your rear tire is.
If you are running a 26" tire 3.55 gears would be a good set up.
If you have a 28" tire, [not likely] then a 3.90 would be a good choice.  

I have a 360 in a challenger with a 904 and have an 8-3/4 rear with a 391 posi.I can drive it anywhere but it pegs 3000 rpm at 60 mph.  

I have learned that rear end gear ratio and " rolling radius " of rear wheels are connected for determining  our ultimate desire ( strip, street or highway .)
For example: I started with a 2.71 rearend( trans spins 2.71 times for each rotation of tire) and changed to a 3.23( a 19 % higher spin rate) so with all other things equal  when the speedometer reads a transmission spin of 3000rpm at 70mph , the car is really only traveling 57mph( 19% ) .

One way of compensating for the 19% difference ( in this case ) is to help  lower the rotation spin of the tires . This can be done by increasing the " rolling radius " such as a larger rim (15", 16" rims )and taller tires. ( ie 275/65 R 15)

I am wondering if there is a  transmission speedometer gear adjustment that can be done as well that evens things out .