Rear gear suggestions

[348tripower]

I fell into a smoking deal on a 3.91 sure grip for the wagon. It has a 3.23 open in it right now.  The trans is a five speed A580 final drive of .83.  Tires are 27 inches in diameter. I can change ring and pinion but I am looking for your suggestions as far as road worthiness. All my other rides had open rears. I like the idea of a posi. Seems not adding a tire equation I get 3.23 final and 2.68. Your thoughts?
Thanks,
Don

[wayne petty]

calculate engine speed at your normal cruising speed with both gear sets..

you might want to do an actual tire roll out measurement  as a 1/2 inch of difference from 27 to 27.5  goes from 628 to 611 turns per mile.


you may also want to even temp install an air fuel ratio display.. so you can fine tune the idle feed restriction if you are running a other than a quadrajet or if running a quadrajet you may want to play with the APT setting to get the best drivability and the best fuel economy at the lower end of the long distance cruise RPM.

what.. at idle.. you are feeding fuel only thru the idle feed ports that are controlled by the idle mixture screws...

off idle you get into the idle transition ports... these slots allow a variable amount of fuel to be pulled in to feed the engine..  before the mains start flowing.. but the idle transition also flows during the lower end of the main circuits operation...   if your idle feed restriction is too large..  you will get crappy fuel economy when you are operating from just off idle thru the low main circuit operation .. if its too lean.. you will have flat spots off idle or stumbling when coming back down thru the rpms.

you can tell in a few minutes with a fully warmed up engine..

slowly open the throttle.. take it up to about 2500 RPM..  drop it slowly to 1500 . then slowly increase back up to 2500 .. then drop it to 1200 and then slowly back to 2500..   drop it to 900 and back up to 2500..

you should if your idle transition circuit is properly calibrated have absolutely no issues doing this..

very few people play with the idle transition circuits.

the ldle feed restriction limits the maximum fuel flow to the idle transition slot..   since the idle transition circuit can be feeding fuel up  between 1500 and 2200 rpms.  this can make or break your pocket book..  

this tuning can make the difference between a 10 or 12 MPG car and a 20 to 23 MPG car  its really impossible to get more than about 23 MPG without going really lean and perhaps destroying the motor if you have a 350 engine size..   why do i say that.. because i worked on feedback carbs.  the full function quadrajets on a 350 could get 21 to 23MPG when tuned perfectly.. 305s could knock down 24/25MPG and 4.3 V6 motors could knock down 26 mpg..   obd1 fuel injected motors could do slightly better.. corvettes with L98 fuel injection could get 28 mpg pushing that wedge shape down the road with the 5 speed if you held your food steady on the pedal and were not constantly lifting and depressing like the cruise control does sometimes..  after i did a bunch of stuff on my older sisters 87 town car with the EFI 5.0 she could get 29MPG on the highway..   why don't OBD2 cars get that kind of fuel economy..  because they have to keep the air fuel ration slightly rich to keep the cats at operating temp to get the cleanest exhaust.  they monitor the air fuel ratio coming into the cat. but also monitor the exhaust gasses coming out to fine tune the air fuel mixture to keep them as clean as possible..

sorry to take you way off thread and jam idle transition down your carb throats..  if this is the car with the corvette fuel injection system..

you will want to monitor the block learn memory and the integrator.. both if the system is operating properly should be within a few points of 128.. but not fixed at 128..

[kb426]

Don, it appears you're looking at a 500 rpm increase with the ratio change. Choose mileage or performance. My stuff has the .69 overdrive ratio so I'm happy with my choices. My Dakota RT has a 3.90 gear but with the .69 overdrive, it's around 2200 rpm at 65 mph. However, it never makes gas mileage.

[348tripower]

I am going with a 3.55. It needs new gears so I will start there. Should be fine for what I am going to do with it.