Running 3500 at 70 want to run 2500 at 70

Im running a 3.42 posi with a th400 and 455.got a good start but once on the highway,when i get to 70 its around 3500 rpm. I would like to keep the th400 and get smoother highway drivin, like around, at most 2500rpm at 60-70mph.Wich one should I aim for :
2.56
2.73
2.93
3.08
My axle is a 71 and up 10 bolts, 8.5. (i dont know if they make all the gears i mentioned for the 8.5....)
Anyway, I have never run the car with another gears other than the 3.42, used to be not that bad with the 350, but since i got the 455 when i get on the highway, the girls dont like it, and i look like a fool!
I used to own a 80 trans am, and this car had no end.it had no start but the only thing holding it down was the underpowered 301, it would go on and on and on, 1mph at a time hehe

 

Well... Mathmatically speaking if 3.42 = 3500 @ 70mph then X should be = to 2500 @ 70mph which ciphers out (as Jethro would say) to around 2.47. So since that is not a viable option 2.56 should get you in the ball park. Torquey or not you'll definitely feel a droop in performance. Are you sure on the size you have now?

 

Here's where you'll be (approx) at 70 mph with the gears you listed:

2.56 2640 rpm
2.73 2816 rpm
2.93 3020 rpm
3.08 3178 rpm

I'm guessing you're running somewhere around a 26" tire, no? A taller tire will help to drop the cruise rpms too.

Devon

 

i guess 2.73 would be around where I want to be. But when you talk about a drop in performance, it sounds BAD! I mean, will I be able to do burnout? Got a rebuilt mostly stock 455, (headers+ oversize...stock sleezy cam!)

 

goodyear 255 60 15

 

rear end ( i like girls too, but this REAR eND turns me on too!!!) LOL

 

I doubt it will keep you from burnouts but that seat of the pants feel will suffer a little. Others on the board who race 1/4 mile could probably give a rough idea of ET loss from the ratio change.

 

My 70GS455 came with 2:93

The factory gear in my 70GS455 was 2:93 and I personally think that was the perfect street gear, not for drag racing but great for street fights and for real world driving.

I now have a 8.5 rear with 3:08, it works good for the street also. It seems like it wants to cruise right around 62mph...

Good Luck
Bert

 

Here is a link to a website with all the different calculators:

http://speedworldmotorplex.com/calc.htm

there are others out there, I like this one because they are all together, so you can calculate your tire height then go right to what gear is best, etc.

Keep the 3.42's and drive slower, plus sounds like your tach is off by 500 rpm. 60 mph will occur at 2550 rpm with 3.42 gears, 70 occurs at 2980

I've fallen in love with the 6 spd manuals (double overdrive), can't beat 75 mph at 2000 rpm and that's with a 3.46 rear gear and 26" tall tire.

 

Do what I and others have done here, swap in a 200-4R transmssion. My rpm's went from almost 3300 to 2200 at 70 mph.

 

goig 4-5 or even 6 speed would be a dream of mine, but if I would do so, I would want it the RIGHT way wich is original interior console and consolette, and all the hardware, pedals ....theres no end to it and its way too $$$ for me.Im 24 still livin with the parents..... o No: uzzled: I mean looks at those prices on ebay for a console!! Someone should reproduce all those interior 4-speed parts for the 70-72 GM A-bodies , that guys would go on and be rich!!!!As for 200r4 and 700r4 thats been in mind, but I would like to enjoy the car this summer, for once, and not work on it!..........And with all the time and money invest , your still stuck with n automatic......

 

I tend to agree with Larry. My car is a 3:42 also and shows right at 69-70 @ 3000. That's why I asked if you were sure on the gear ratio you have in my first reply. Sounds more like a 3:64 or an issue with the speedo or tach. You might want to check and see what color the speedo gear at the trans end of the cable is and check it against the correct color for your gear (I can't recall which one is correct but I'd bet the farm Larry or someone else here does).

 

rpms

I agree...Something is not right with the numbers. That tire combo is pretty tall and should work good with 3.42 even at 70 mph. Check The tach definetely and then tranny torque convertor could be slipping big time...and soon will eat the tranny. Or that gear ratio you think you have is not really 3.42. could be 3.73 if it is a 8.5. even 3.90 . 3.90 comes out pretty close to 3500 rpms at 70. Best to open that rear and check the ratio. then get someone elses tach and see if it is close or if you have a good digital timing light match the tach to that and see the difference. Lets just hope it is not the stall converter splipping. or do you have a 4 speed? Jim

 

You should look into the Gear Vendors Under/Overdrive attachments. I think the overdrive ends up being .78 which would get you in the 2750 range with your current gearset. With your 3.42's, it would be like having 2.67's in the rear.

Here's some more info: http://www.gearvendors.com/hrgmratios.html

 

I dont think the torque is slipping...what other problems would it create if it does?I think its the speedo gears thats wrong.With a 3.42 how much teeth is it suppose to have, or what color should it be? I might be going a lot faster than I think im going....I have race with friends, and it looks like that.But I never cleared this issue.Now with the winter clearly in place here I can check it.....Maybe its a 3.64...but I doubt it...

 

I have a set of 8.5 3.08's for 75.00 + shipping

 

thanks for the offer, i appreciate it, but IF i changr I would go for something like 2.73 or 2.93....