I am in the process of changing my gears in my 81 automatic, does anyone know what the standard ratio is.  I am considering 3:50 or 3:73, anyone do same?, would like some input before I change.  Thanks

Fred 

'81 automatic had 2.87.1 rear.....4spd had 2.72.1
These were the only available ratios for the specific trans.
A 3.73 will fit with no problems, but you'll give up quite a bit of fuel milage, compared to what you have now. A 3.55 would be a better choice, depending on what you use your car for.

If you want to go even further.   The 3.55 is a great choice, but then combine it with a 4 speed automatic.  The first gear ratio is a bit lower giving even a bit more launch, and with the overdrive, the highway mileage improves and the engine wear drops.  Good combo.

Just keep in mind the computer.  Correcting the speedo reading by changing the speed gears is all you need to do. 

kstyer38816.8169444444

Hey Guys,

Is there a "tutorial" on here somewhere that talks about the relationship between manual transmission ratios and rear end ratios. Can't figure out how they relate for the life of me.

Actually, I have seen a fairly long-winded debate/tech article on that subject, but I can't remember who posted it. It was a while ago, tho..."Search" and ye shall find!!!

Thanks Joel:

Thanks alot for the advise, i have one more question, could u tell what speedo gears to use for both the 3:55 and the 3:70, I have and 81 with a automatic turbo 350

 

thanks again

Fred

Try THIS CHART. The gear teeth count will be the same for the 350 as the 400 in this chart, but the part #s will not be the same. I can get any gears you or anyone else needs for this type of conversion, if you have trouble locating them locally.
If anyone here has an Assy. man. for the '81, it will have a chart for the 350 in it.

Thanks again Joel,  I will tell the guy doing the rear for me this,  If i have any further problems i will let you know,  also i think i am going to use the 3:70.  I dont do much highway driving, i only put about 1,000 mile a year on, mostly local,  I am looking for more punch off the line. ( like to old days).  Do you have any idea what my rpm would be arround 60 mph.  If you do i would appreciate your comments.

Thanks

Fred

  

Not sure, Fred. My '74, @ 60mph, is around 2500rpms, with the 3.36 diff. Without doing the math, which I hate anyway, I would guess around 3000/3200rpms, with the 3.70s. Tire size will also have a roll in overall rpms@ speed.

Fred,

Assuming a 28" diameter rear tire size, your rpm with 3.70 gears @ 60 mph will be 2664. If you have a 27" rear tire, the rpm will be 2763.

If you use 3.55 gears instead, at 60 mph using 28" tires your rpm will be 2556. If you change to 27" tires, rpms will increase to 2651.

All the data above assumes NO overdrive. If your trans has an overdrive gear, I need to know the ratio, then I can recalculate.

And of course, if your tires are a different diameter than 27" or 28", the numbers will change.