Hi, I have an 86 Trans Am with the 305 TPI in it. It's an automatic with a slapshift/manual valve body, 9 bolt BW, and it has a cold air intake, cat delete, and a flowmaster muffler. It has the stock 2.73's in it right now and they are way to sluggish. I was wondering if I upgraded to 4.10 gears and 28/29 inch tire height, would I still be able to cruise alright around under 60 mph or would it just be too much for my car? I won't be taking it on the highway much and just want to be able to really launch it and have some better passing power. I plan on adding many more mods and a bigger engine in the future. I do drive the car quite often though, but I'm not too worried about decreased gas mileage. Thanks for any input.

 

I also have a 4:10 gear I need to put in but holding off til next spring when we change motors//rear ends.I have an open rear but the next one will be posi with the 4:10.
I don't know how to determine tire size//gears final out put but with overdrive it __seems__ like it would work out pretty good.
I'm sure we both will learn from your thread!Thanks for asking-I was going to wait til spring to ask.

 

im running 4:10s theyre not bad ,2200@ 60 mph. i had 2:73s made a hell of a difference in launch, im running 28" tall tires also.i do think though that 3:42s or 3:73s would be better because it changes into drive @ 30-35 mph.if i change im going with 3:73s.they also keep it from jumping out of overdrive every time you hit a slight hill,jimmy

 

#1.....If you have a 9-bolt, you have 2.77s, not 2.73s
#2....4.10 would be a 10-bolt gear ratio. Close to that for a 9-bolt would be 3.9 or 4.11. Yeah, not much difference, until you go to order. A 4.10 wouldn't be close to fitting in your carrier.

4.11s in a TPI engine....Lots of low end torque.

How much traction do you really have? I've had 3.73s in a 10-bolt before. Traction is already an issue with my 315s & 3.27 gears. I can't imagine going to 4.11s.

 

http://www.chevymania.com/ in the technical articles thier are calculater for speeds at what rpms you just need to know your transmission gear ratio and it will do all the math for yea

 

My car with 25.65" (stock height) tires and 3.23's shift into overdrive at 30-35mph.

 

well he's running larger than stock tires most likely thats why and he must be getting on it it should shift abit lower unless he done work to the trani

 

i dont think 4.11 would be so bad. im making the jump from 3.73 to 4.11 with the 9 inch swap(i needed for the cam). so yea i wouldnt be against it expecially if its gettin a motor down the road

 

This info has been a big help, thanks a lot guys. I wouldn't have to worry about it downshifting on me though with going up hills because it's a full manual valve body so I shift it, also the trans has been rebuilt recently. Which tire height do you think would be more suitable? The 28 inch or 29 inch.

 

Running 4.11's in my 9" as well. Around 2500@60mph. Still running stock converter, so that will be changed out this winter. They don't make speedometer gears when you get that high so I have to go 5 over to get relatively close to the speed limit. I suppose it keeps me out of as much trouble this way. Larger tires would help.

Traction goes out the window. I could break it loose easy with the 305 TBI with timing at 0* base. Throw 2-3* at it and feels like a different car.

Gas mileage went bye-bye. I would see how fast I could take off without breaking traction from every light. Too much fun.

Sounds like you've been through the tranny and probably have a shift-kit? If you do get the gears, watch out for 2nd. I've nearly spun a few times taking a corner to fast when 2nd hits.

 

Something that hasn't been mentioned is the drive shaft. When going from a 2.77 to a 4.11 the drive shaft speed is going to go way up. Even though the engine RPMs aren't that high with an overdrive transmission, the drive shaft speed is going to be high. It doesn't seem to be a problem with everyone, but many cars develope a vibration at certain speeds after a gear change. This is because the drive shaft will distort at the higher RPMs. It will flex in the center causing a vibration. Going to a larger diameter drive shaft usually fixes the problem.

 

im goin to a autometer 5 in speedo and a programablef sender unit to take care of that problem.

kinda hope it works i havnt had a speedo in 2 years lol

 

315s are going to be a traction problem anyway, because they're so wide, they increase the total area, in square inches, of tread touching the pavement, and the contact pressure, in pounds per square inch, is reduced. This makes them easier to spin, until you increase the vertical loading on them. They look great, but a 315-width street radial is actually worse than a 275-width street radial. with drag radials being so much stickier, the 315s seem to bite well enough to get the weight transferred back on launch, and that's where you can get an advantage from the extra width. Now, as to the issue of gear ratio, with a 700R-4, a 3.73:1 is the lowest you want to go, even with a rev-happy big-cammed LSx. If you must have 4.10s or 4.11s, plan on swapping to a 200-4R.

 

If you want to upgrade the 9-bolt, you've got a 2-series carrier so you'll need a 3-series carrier too.

The gears get significantly weaker above 3.73 too... especially with the 10-bolt. The 9-bolt is a bit stronger but still only a 7.75 inch gear and I decided going higher than 3.70 wasnt worth it. 9-bolt parts just cost too much to blow it up. I'd rather it survive until I can afford a 9 inch.