Hello Everybody,

I know most people will tell me to go all the way with 3.42 gears, but I would like to hear a variety of opinions as to why or why not. This will be for a slightly modified 305 tpi with a 700R4 and shift kit. I'm looking to keep the car streetable.

Thanks for your help,
Jim

1991 Pontiac Trans Am
305 H.O.- total rebuild w/ mild cam
700R4- total rebuild w/ shift kit
SLP headers and 3" cat back
2.73 posi rear diff.
functional cold/ ram air
ported plenum
upgraded ingnition
AFPR
75 hp shot of nitrous

TO COME:
new rear diff. w/ higher gears
burned chip
heads
? new cam
? higher flow intake

 

Oh yeah, I have upgraded the suspension, including torque arm, lower control arms, and poly. bushings.

I have done a search and found that most people go from 2.73 to 3.42 and are happy. But I would like to know all opinions i.e. gas milage, highway driving, top speed concerns, traction problems, etc.

Thanks again,
Jim

 

I swapped from a 2.73 to a 3.42 and i dont have the first complaint. I love it, off the line was an awesome difference, top end diference too. The car is still every bit as streetable as it used to be, i can still sruise the interstate at 70, 75 or 80 just fine. You wont loose but maybe 2 or 3 MPG. As far as traction goes, well it becomes to be a little more of an issue, but it sure is fun. I dont think youll be dissapointed with a 3.42.

 

How fast do you want to go?
I put 3.73's in my Formula and with the Auto I still could hit 140mph.
With the T5 I just put in I beleive my top speed dropped to about 130mph.

I get upto 25mpg on the highway if I just cruise, in the city it drops to about 12 since I usually get on it at every stop light.

If your car can pull past 5K rpms, then go with the 3.73's

 

Thanks for all the advice guys, I appreciate it. I'm pretty sure 3.42 is the biggest I'd go with a 700R4 and a motor that pulls to about 5500 rpm. If it helps, my peak power and torque are made between 3500 and 4500 rpm.

Thanks again

 

if your peak power and torque is between 3500-4500, then definitely no higher than 3.42's...anything higher and you're gonna wind that motor out too quickly.

 

Exactly--my car came with 3.73's and with the stock 305 they're just a little too low.

 

I'm thinking of using gm's 3.42 gear ratio

 

thats a good start, factory gears are usually MUCH quieter than aftermarket gears.

 

i have 3.27's and my gas mileage has basically stayed the same from when i had the stock peg leg 2.73's.. i love the rear tho... dropped some off my quarter and still get decent gas mileage...oh and my car has a 150k mile LO3 with slight mods and 700R4 ...

 

Zepher--why did your top speed go down when you put in the T5? Is it ratios or something else?

 

I'm trying to decide if I should get the gm 3.42 or richmond 3.73? decisions desicions

 

I believe the overdrive ratio is higher in the WC T5, 0.72 I think. 700R4 is .62 or something like that.

 

Other way around. 700r4's overdrive is .70 while T5 is .63

 

I went from 2.73 to 3.42 and I'd recommend it heartily. Very nice difference in off the line performance and the mileage around town actually increased from a best of 18 mpg to 19.5!

At highway speeds you are just coming into the engine's powerband, so when you go to pass someone you can tell you're right in the sweet spot for power

But I think 3.23s would be about 95% as good as the 3.42s. Whichever is the better deal is the one to grab.

 

If that is the case, I should be going faster with the T5, but I am going slower.

 

What would you all recommend over all for a 350tpi's powerband 3.73's or 3.42s?

 

Not enough power to make it go faster with that much of an overdrive. Think of the C4 vettes, 5th gear had a higher top speed then 6th by like 20mph.

 

I think that most people have found that 3.73s are borderline with TPI if you are just going for 1/4 mile performance. Many find that they run out of rpms in 3rd gear before reaching the finish line.

But if you are just into stop light to stop light action it doesn't matter.

Still, 3.42s and 3.23s will be a much better trade-off for a daily driver that sees some highway travel, too.

 

No, it's got plenty of pull, but at 130mph in 5th I am at 5K rpms.

On the highway I am taching 3000 at about 80mph in 5th.
With the Auto I was taching about 25-2700 at 80.

 

The T5 had two 5th gear ratios, the lg4/lo3 cars got the .63 while the l69/lb9 cars got the .7X ( I think it was .76 ).

 

You using a stock tach??? Was a post a while ago about guys finding out at the dyno that the stock tach was way off. People thinking they are near red line but 1000-1500+ rpms off.

 

I'm looking for 1/4 mile performance. I could care less for lite 2 lite races.

So where can I get the gm 3.42 gears from? dealer or sumthin?


__________________

 

V6 3rd gen's come with 3:42 stock and a V6 and V8 rear end are the same.

some 4cyls had 4:11s stock, same thing on the rear end.

 

why would a v6 or 4 come with a steeper gear?

 

Makes it easier for the motor to get the car moving.

I went from 3.23 to 3.73 and the car gets going from a stop much quicker since the engine turns up quicker. Just like riding a 10 speed, in the lowest gear where you pedal a lot and the bike starts moving with no effort.

 

Yep, that sounds about right. I could cruise at 35 mph in 5th gear in town with my L69 T-5 and 3.73's in the rear.

Never tried 3.43's, but I think it would have been a better compromise...

 

got it...i understand perfectly

 

so I can go to the local parts retailer and ask for a set of 3.42 gears out of a 88 v6 firebird? And they will work fine in my gta?

 

Probably much cheaper to just buy a whole rear end with 3:42s

I can't see a problem with used gears. Just take off the cover and inspect them. Maybe just swap the whole rear. That way you can rebuild the rear brakes and paint everything nice.

 

Smith:rockon:Faneca:rockon:Hartings :rockon:Vincent :rockon:ross :rockon:

 

My '84 Z came with 3.73s. They are great, but the RPMs are a bit high on the freeway. I probably would have gone with 3.42s if I had the option.

 

ok

 

I just saw this.

Um yea isnt that neat.

Its help level the playing field.
Let you still get good MPG, while being able to take off better.

If my car had a 2:73 or a 3:08 it would be boaring as heck.
But probably a 35mpg car.

 

I dunno if I want a 6 banger rear but I wouldn't mind a 3.42

 

i got a few questions here..

I have an 88GTA with a 305 lightly modded. I'm not a pro on rear ends at all and my friend recommended changing my gears because I want to get my acceleration off the line better. I want to eventually get a 350 dropped in maybe next year or the year after. Would it be best to go with the 3.42 like JADIII35 right now, and then buy 3.73 later when i do the engine swap, or should I go with the 3.73 right now. I think if I go with the 3.73 right now it will just be too much for my lil 305. What do you guys think?

Thanks

also does anyone know what gears came stock in my car. Because I have no clue. As I said, clueless when it comes to rear ends. Thanks.

 

For optimal 1/4 mile choice of gear needs to be determined by the engines power band. If your new engine has a cam large that 220 degrees then you should use at least a 3.42. The higher up the rpm band your engine makes power the larger numerical gear you need to spin the engine up into its band.

If you install 4.56 gears on a tpi it will very quickly pass through the cars sweet spot (1500-3500) and spend all its time gasping for air at the higher rpms.

I had a modified crossfire corvette with 3.08 gears that were well suited to the low rpm power of the car, I shifted at 5000rpm.

My car now makes higher rpm power from 3500-5500 with me shifting at 6000rpm. In this case the 3.73's I have now are a perfect match for the cam im using and im very happy with the results.

 

Would it be correct to say that higher gears, while in no way increasing engine HP, do increase rear-wheel HP?

 

gears add 0 hp to the engine or rear wheels...all it does is multiply the torque to get you moving harder and faster.

 

thats the perfect example why 3.73s suck with TPI, doesnt have the top end to need a gear like that.

3.23s or 3.42s are as good as it gets


gas mileage has to do with VE, basically the rpm(s) where an engine is most efficient, numerically lower ratios dont automatically mean better gas mileage, if the gearing is to low numerically IE: 2.73s then u could be hurting gas mileage by making the engine work harder to acclerate the car and maintain speed especially on the highway.

 

If I dynoed my car with the current 2.73, and then changed to a 3.42 and dynoed the car again, would any of the numbers change?
If gears multiply the torque, isn't that an increase in power at the rear wheels? I thinking along the lines of the power lost as it goes from the flywheel-through the trans-through the driveshaft-through the gears-out the axle to the wheels.
Also, what impacts acceleration more...passing your peak torque or your peak horsepower?
and wanting to learn:hail:

 

all rear gears do is "manipulate" your engines power band if that makes sense. Say for example you got 2.73's in your car now you're gonna have a higher top speed but its gonna take you "longer" to get there which also applies to max hp and torque too mainly because of the weak acceleration of the 2.73's. Don't just go wild and assume lower will automatically make you faster, if the gears are not matched to you engines power band, it can actually make you slower...i've seen it happen and right there you'd have spent a few hundred bux on a gear swap just to be SLOWER after.

 

ok I'll stick to a gm 3.42.

 

Good stuff--big subjects--the basics to building the right car--if you want hard acceleration you need to spin your motor faster so it'll pump more air/fuel mixture and make more power, so you put in lower gears--but the faster it spins the more expensive it is to build, the more likely it is to break and the more gas it uses--stuff to think of if it's for daily use.
My target now is to build a 350 for 2000 to 5500 rpm and run 3.23's behind my reinforced WC T5.
The 350 will be about 9/1 so I can burn regular gas cruising at 70 mph at 2000 rpm; power starts right there so I get a mildly lumpy idle; shift at 5500 so I run almost stock valve train, hydraulic cam; factory light flywheel and everything else stock--no expensive exotics anywhere. I'll run forged pistons and screw in studs with guide plates in case I get a wild hair and put in a dryer some day.
I've run a 348 with 3/2's and a power glide in a '58 BelAir to an 11/1 427 with 4.88's in a '69 Camaro--this 350/ 5 speed Camaro will be just right.

 

I called gm about the gear set and they are telling me $700! I called summit and they told me that they don't sell anything for the aussie borg warner 7.7.5 9 bolt! where can I get a set of gears?

 

Ohh its a 9 bolt much more expensive to build then a 10 bolt rear. Some TA's and Z28's came with a 3.23 ratio 9 bolt but they can cost some cash unless you find one at a pick n pull. #GM 7.75-345, http://www.ring-pinion.com/content/p...sp?vid=4&pid=4

 

does anyone know the part # on the 9 bolt rear end gears?(3.42)

 

how tall are the tires on camaros from the factory.

 

personally i believe 3.73 is really pushing it, especially with as short of tires as there ar eon these cars, its not like a truck in which have 31" or taller tires in 90% of cases,

id go with 3.42, it would be best of both worlds, and 3.27 BG too

good overall acceleration and gas milage, altogether,

tradeoffs, there is no perfect ratio out there really, but these two ratios are damn near close enough

im going to 3.42 from an 87 maro under my 82 t/a with peg leg POS 2.73

holy crap it will wake it up real quick, i cant wait, the 3.42 axle is sitting all pretty in back entrance way in this building


i cant wait to kill the 2.73 one, good riddance

why gm ever wanted to use 2.73 in a F body is beyond me, especially in a factory v8 t/a

just crazy,,

cant call it a trans am, its more a like a freaking turtle am

ive got an 84 c-10 pickup originally 250ci and is still original sm326 three on the tree and its 2.73 8.5" under it, just horrible it can barely pull itself from a dead stop lug lug lug hard on the engine, no oil pressure hardly at low RPM takeoff, just horrible ratio

crazy

not like in the 60s when 3.73 was base model ratio and under about all the 1/2 ton pickups, they were pretty darn quick and could haul a ton of weight and no lugging of engine at all

all about gas mileage now, not hauling/performance capabilities




good luck

 

well it depends on what size LOL

there are tire ratio calculators online, search google real quick, type in numbers, get actual height then

235 75 15 are 29.5" tall i know, used on old 1/2 ton pickups commonly, most all of them, G10/G20 vans too, etc


good luck