Hey guys, i have a 91 Trans Am with the 305 TPI engine (LB9), 700r4 transmission, and 2.73 gears in the rear end. Would i notice a significant increase with different gears, like 3.42? And what kind would you recommend? Something that's still comfortable to drive at high speeds on the highway but something that also gives the car more low end grunt that takes good advantage of the engine's power band.

 

Street/Strip I use 4.10s. Mainly a street car with some throttle enjoyment I would use 3.73s. And the difference is quiet noticeable.

 

How would the 3.73s effect highway driving, just wondering? I was looking at 3.27s possibly myself, not trying to high jack the thread, sorry if it seems that way.

 

3.73 is a good all around gear. good highway and excellent acceleration. You would notice a HUGE difference from your stock gears. Don't forget to do the speedo adjust either.

even a 3.42 or 3.55 gear would work well too. I would chose 3.55 or 3.73 personally, you'll enjoy it.

here is some interstate numbers to contemplate. you're 2.73 gears (with a 26inch tall tire) should be turning 1750 rpm at 70 mph. a 3.55 would turn 2271rpm at 70, and 3.73 would turn 2380 rpm at 70mph.

 

Nah don't worry about it! I'm glad you chimed in since I meant to ask the same question.



That sounds about right, once in overdrive the with the stock gears on the highway all you hear is wind noise with no engine haha. The 3.73 doesn't seem bad on the highway at all, little 4 cyl cars probably turn at higher revs at that speed. So you guys think that 3.73s are the best all around gear? Both for fun driving and highway cruising?

 

im doing swaps from 2.73 peg leg to a 3.42 posi I've been told with the gearing of a 700r4 that the 3.42s are the best choice because of how quick first gear is with anything higher you realy wont have a first gear so I've been told and read through my searches.but i have no actual experience as of yet.

 

What if you have a low power band?

 

more rpms=more fuel spent

 

Yeah, you have to consider that a 700r4 has a 3.06 1st gear, so 3.73 and especially 4.10's will give you an extremely short 1st gear. With 4.10's it'll probably shift into 2nd after the car has moved 10 feet! With the torque of a TPI motor and the deep 1st gear of a 700r4, 3.42's is really about the lowest you want to go for a typical street driven car. And trust me, 3.42's will be a HUGE difference from those pathetic 2.73's. I upgraded to 3.42's from 2.73's and the change was amazing. A few years ago I dropped back down to 3.23's and they're still night and day better than 2.73's. The other problem with running extremely low gears in a TPI motor is that they run outta breath around 4500-5000 rpms. Go 3.23 or 3.42 and you'll be a happy camper. Much better takeoff power compared to 2.73's and you'll still do just fine on the freeway.

 

does changing rear gears actualy shave time of off a 1/4 mile time or is it just more a seat of your pants change?

 

it just all depends...a taller tire will "slow" you down a little bit. but if you go too low with your gearing you will have helashious acceleration(if you get traction) but at the big end of the track you can rev out!! so you have to find a balance, im running a 3.73 with a 28" tire and a t56, i want to do 4.10. but for a stock 26 inch tire i wouldnt go over 3.73...id prolly do a 3.42 or 3.55. hipe this helps

 

you want to be just under your rev limiter at the 1/4 mile mark

 

In my old 84 camaro I went from 3.23 to 3.73's I picked up about .3 Went from 13.9 to 13.6 all motor, carbed 350. I was dead hooking out of the hole though, went from 1.9 to 1.8 60's.

 

I think 3.42 is the best choice for an all around gear. Good for track use, but still a decent highway gear.

 

thanks for all the tips guys! but what would be the 0-60 or 1/4 mile difference of a 2.73 car vs a 3.42 car?

 

I can't give you an exact difference in your 1/4 mile or 0-60 times, but I guarantee both of those will improve significantly going from 2.73 to 3.42. 2.73's are such a highway/top-end oriented gear that there's no way you'll hit your peak rpms quick enough in a 1/4 mile, not to mention how sluggish the car is getting out of the hole and taking off from a dead start. If you gear it too low, then you have the exact opposite effect of maxing out your rpms to quickly in each gear, not to mention that you'll exceed your rpm redline before going through the traps at the end of the 1/4. The other reason we're trying to shy you away from 3.73 or 4.10 is the fact that a TPI motor has gobs of low end torque, or grunt if you will, so it doesn't need that low of a gear to get it moving quickly from a dead stop. If you pay attention to the way the factory geared our cars you'll notice that the smaller the engine, the lower gears the car got. The 350 powered cars never got anything lower than 3.27, but if you look at the 305's and the V6's, they got 3.42 or 3.73. The lower gears make up for the lack of grunt, or pulling power that the smaller motors have. That's why if you're looking for a 3.42 or 3.73 rear, the best cars to find them under are the 6 or 4 cylinder 3rd gens. In fact, 2.73's behind a powerful 350 or larger motor don't hinder them terribly bad, but when I had 2.73's behind my L03 305 TBI motor, it was slow as hell taking off. My buddy ran 2.73's behind his LT4 hot-cammed, AFR headed, 383 LT1 motor in his 95 Formula and he could roast the tires until they were bald! So the smaller/less torquey the motor, the more gear you need.

 

Not always true.....if you have a really tall gear, it can sometimes take more fuel to turn that gear, kinda the same as lugging the motor down.

 

Good point. My in town mileage actually improved when I went from 2.73 to 3.42, and the difference on the freeway was so negligible it was hardly noticeable. If you have to floor it all the time just to get the car moving, you're gonna suck up more gas. I noticed the same thing when it comes to a 2 barrel carb versus a 4 barrel. My mileage improved running a 4 barrel carb with the small primaries, as long as I didn't dump the huge Rochester secondaries all the time! A lot of that also has to do with the cam you have in your motor. If you can match your gears to the optimum cruise rpm of your particular cam, you can knock down some great MPG numbers.

 

Wow thanks for the tips guys! And no need to shy me away from 4.10, no way in hell i'm getting even near those. LOL. So it looks like 3.42s seem to be the best choice here?