Tall gearing for economy

[MAP]

Greetings,

I have a 3.42 posi rear from an '87 GN that has served me well in the past, but at $3/gal for gas and for daily driving, I'd like to reduce this to the 2.7-2.9:1 range. Does anyone make a gearset for this, or does/did there exist an OEM application that would make for a direct swap?

Thanks,
MAP

 

[ITSNOTAGN]

any gm gearset for the 8.5 bolts right in, 2.73 and higher. It is my suggestion that you go to the 3.08, as the 2.73 is too high for the car IMHO. The 3.08 will drop cruising rpm from 2-2100@60mph to around 1800-1850. Its a big difference, but the 2.73 goes too far, down to about 1500 and it doesnt pull it that well.
Gary

 

[MAP]

Hi Gary,

Thanks for that input. Do I understand correctly that I could buy a 2.73:1 (or what-have-you) gearset for this rear directly from GM? If so, how - GM parts direct possibly? I have to admit that I never bought gears directly from GM before, so I'm ignorant in this area.

Something close to 2.8:1 would be ideal for me. It would take a while to explain - but the basic reasons are very strong bottom-end torque from the motor right from idle; a tight converter with lockup; and 70-75mph speed limits on local highways. Another thing too: I have some internal contacts with the petroleum industry who think that the likeliest scenario is $6/gal gas within five years, and that's assuming no Gulf-area wars.

Then again, a 3.42:1 rear with a fifth automatic OD gear would be nice, but at the present time, I don't want to spring for a GV overdrive unit to make this happen.

Thanks again!

Best,
MAP

 

[Madcat455]

Does it have to be POSI???

If not, then just grab any complete 7.5 from a V6 cutlass out of a bone yard... it'll probably cost $100 (not in demand), and that way you can just swap in the stock rear when gas drops or gas funds grow.

My 7.5 in an old cutlass lasted pretty good behind the torque of a 12:1 Buick 455(6 months)... so it should handle the turbo 6 as long as you don't try launching off of a T-brake:wink:

 

[MAP]

Hi 455,

Yes, it has to be posi, and it has to be the 8.5" ring gear. I've killed too many of those 7.5" rears over the years to ever go back to them. YMMV.

Thanks,
MAP

 

[novaderrik]

after you spend a couple hundred dollars buying new gears, and another several hundred getting them installed, you might gain a couple of mpg- and you mgiht even lose some mileage, due to the motor beign below it's torque band at highway speeds.
and even if you do get a slight gain in mileage, how many miles will you have to drive it before it pays for itself? even at today's prices- and the prices we might see in the future- the $500-$600 you will have into this buys a LOT of gas that goes a LOT of miles.
if you want efficiency, you'd be better off spending money where it can actually help- hell, just doing some tuning and regular maintainence can get you some pretty amazing efficiency increases for very little cash outlay. another "free" area to find some mpg is in the aero package- experiment with blocking off the grill and see what the results are. on my 74 Monte- which is, admittedly, a brick compared to the relatively sleek G bodies of the 80's- i found the 3 mpg that i LOST when i put an overdrive trans in it by blocking off the grille.

 

[MAP]

Hi Nova,

Those are all good points. The gear change is actually being done in consultation with David Vizard, and we're re-designing the entire drivetrain. I'm also making extensive use of modeling software to look at the performance/economy tradeoffs. About value - I use a four-year maximum amortization rule, and the projected cost is favorable in this view, assuming that the average price of gas will be about $4.25/gal during this period.

But back to the present topic: I found, quite to my surprise, that a few manufacturers make 2.73:1, one (US Gear) makes 2.93:1, and a whole slew makes 3.08:1. GM, as I learned, still supplies 2.73:1 and 3.08:1. Of these three, I'd say 2.93:1 seems best on paper, but US Gear is an unknown entity to me. Does anyone have any knowledge about their ring-pinion sets? If US Gear is doubtful, I might consider using 3.08:1 from GM, and increasing the rear wheel diameter to reduce the effective ratio.

Thanks,
MAP

 

[bison]

I doubt putting in taller gears will do anything for mileage. A light foot is the best bet for fuel savings. Cruising at 55-60 will do wonders for mileage vs. 70-75 cruising speeds. And like what was already mentioned you would spend $500 to change gears and not be guaranteed a gain in mpg. Even if you gained 2 mpg (which is very doubtful) you would only get about 35 miles more per tankful on the highway. Maybe $4-5 savings per tank. Looks like about 125 tank fulls and nothing but highway cruising to just break even . Not worth imo.

 

[mikestertwo]

I doubt putting in taller gears will do anything for mileage. A light foot is the best bet for fuel savings. Not worth imo.

I put a 7.5" 3.08 in my 86' GN a few years back and it didn't change the mileage at all.

 

[CTX-SLPR]

2.93's I think were in some B-bodies so GM made them at one point. Check ImpalaSSForum.com to see if anyone has a car they are parting out with them. I think its the towing package wagon ratio.

 

[MAP]

Hi Folks,

Again, thanks to everyone for all inputs.

My purpose here isn't to debate about whether the change is warranted or not; that decision has already been made. The motor isn't a TR.

My request is about the availability of ratios - you can start another thread if you wish to discuss the pros and cons.

Thank you,
MAP

 

[Brer Rabbit]

Ive got a gear set for U.

I have 3.08s in my car right now, I love the way the car cruises on the highway. The car is a pleasure to drive and it does not hurt the performance. I have another gear set that is 2.93 sitting hear that U can have for $50.00 shipped. I can't imagine any difference in the 2.93 compared to my 3.08. This is an 8.5" ring and pinion. Let me know if ur interested.

 

[ITSNOTAGN]

I would jump on that. I didnt realise it wasnt for a TR. I also run a v8 in my car and going to a 3.08 gear, it is a huge improvement with the TQ of the v8. If you think you can pull it, the 2.93 would be ideal.
Gary

 

[malibuick231]

You can get 3.08 gears from Precision Gear. I ran them in my car and I loved them. The car I have now has 3.73 gears and I hate them and will be swapping to 3.08 soon.