TBI 305 and gears
 

what is the best all around gears for a 305 tbi. i don;t have enough money to do an engine swap so im just going to leave that in there but i think a new set of gears and posi would really help me out. i think 3.23's or 3.42's would really be the two options id go with.

which ones?

I would go with the 3.42's for sure.

ive got 3.42s outta the bone yard(whole rear end, 35K on it for 100$), it really woke my car up, now if i can find the speedo gears to get my speedo back on track

Go to your local GM dealer and ask for 3.42 speedo gears out of a '90/'91/'92 305 TPI car.

http://memimage.cardomain.net/member...4_115_full.jpg

I would give you the part numbers but I forgot to write them down before I installed them. A quick search should give you the numbers.


Refer to this thread for more info.

Speedo gear Install

I'd do 3.73s just for the hell of it...

I have 3.73s with a eaton posi matched to my modifided 305 TBI and they work great.

I didnt think 3.73 would be good since all the power is low end in these things.

You got a T5 - you'll be just fine...

For the stock powerband and stock cam the 3.42's are your best choice. Until you have an engine combo that spins to 6k I would stick with the 3.42's. They are a great overall street gear. The close ratios in the T5 also make it a good fit.

FWIW, I'm considering 3.23 or 3.27 simply as a compromise between economy and speed gain. Anything has to be better than what I have now with the 2.77. I also want the posi and disk brakes too.

Can anybody talk about the potential difference between the 3.23's and 3.42's in terms of performance gain? I'm guessing a second or two if that in a quarter mile, right?

you mean a tenth???

I just recently swaped from 2.73s to 3.42s and love it. The car feels more like a V8 and the front end really lifts up when I go. It's just all around much nicer. I never been to the track with the 2.73s so I can't tell you what you'll get. But it's just more impressive and fun to drive. The speedo gears are available at the dealership for @ 5 bux each. I can tell you I lost about 2 MPG but I've been heavy footed having fun with the new rear:D

Your ET may not change much but you should see a mph or two in your trap. You won't drop a few seconds off your ET until you add 300 more hp :)

when I put in my 3.42's and powertrax locker my straight line performance was like night and day.now if I could get rid of my locker and get a posi I'd be a happy camper.

Why don't you like the locker? I enjoy mine over a posi...

Errr wait - what powertrax do you have?

The no-slip locker,I can't stand the banging in the rear if I brake into a turn.

Oh, I got the "streetable" locker. It's not as friendly as a limited-slip, but it's don't kick out peg leg under stressful situations either (which is why I bought it).

What is the RPMs at when you cruise at 70 mph for both the 3.42 and 3.73?

I am at 3.08 right now and feel that 3.42 is on the low side and 3.73 is on the high side of what I am looking for. Just want to be able to cruise down the highway without the Rs too high. Leaning towards the 3.42s so far.

Somewhere in the 2500-2700 range I believe with a 26" tire...

I forgot exactly, but I had a 700r4 at that time...
If ya got OD, don't worry.

With a manual gearbox in 5th gear at 70mph, you'd be just under 2k RPM with 3.42's and a 26" tire.

I would not go any lower than 3.42's. I recently swapped from 3.08 open to 3.73 with Detroit Truetrac posi (manual trans). Night and day differnece in how the car feels, the best mod I've made yet. The Truetrac was just over $300 IIRC and it works great, no noise like lockers. Much cheaper than conventional type posi's, but no clutches to wear out or special oil additives.

Some have said 3.73's are too much for a 305, but I'm glad I went with these rather than the 3.42. Yes I'm running about 350 rpm higher when crusing in 5th gear (mileage). At 70 mph I'm running at about ~2300 rpm. So mileage went down a tiny bit, but well worth the tradoff for what I gained down low. If you want to know for sure what you're cruise rpm will be, just do a simple ratio of the new gears to the old and multiply by your current cruise rpm. Example for 3.08 that cruises at 2000 rpm @ 70 mph. If you swap in 3.73's, then: 3.73/3.08 x 2000 = 2422 rpm. With 3.42's you would end up at 2220 rpm.

3.42 vs 3.73 . . . either way you'll have a big smile on your face the first time you pull out of the driveway.:)

running 3.23 on stock cam, fits perfectly with the non existent powerband...

For guys wanting to predict engine rpm changes with rear gear swaps, I've attached a simple spreadsheet to play with. The trans gear ratios are set up for a 1991 T5 manual trans, but you can change the tranny gears if you have a different trans. Use the table on the left for your baseline, the table on the right for the new.

Below the tables is a little calculator that will tell you what your rpm's will be after shifting at desired target point (redline). Set the desired shift rpm in cell B23. Cells E25-E28 will report the "new" rpm after each gear shift. This helps check where you'll be in the powerband after each shift.

It's in a zip file, so you'll need to unzip first.

91RockS, that sheet rocks! I am hoping I can get a set on 3.55 gears because that is really where i would like to be. I seem to recall the guy at the shop telling me he could get them. Barring that the 3.42 look good for me.

Kammi10, Do you think you have 2.73 or 3.08 in there now?

its a 2.73

When I was looking for gears, I don't remember seeing 3.55's for a GM 7.5" 10 bolt rear end. Maybe you'll get lucky and find them somewhere. If you're just switching gears, but not the carrier, you need to buy the correct gears for your carrier. There are two different gear offsets based on which carrier you have. The catalogs will say something like "for 3.08 carriers and below or "for carriers above 3.08". So it's based on what gears came in the axle from the factory (assuming the diff carrier has never been swapped).