Frambs546225

Hello, I had just installed a 9 inch rear end with a 3.50 gear and I have some questions. I originally had the stock rear end in which I'm guessing was using a 2.73 gear, prior to switching it would run about 3700-4000 rpm at 60-70 mph. Now it at 5000-5300. I was wondering if that's normally even though to me it seems really high. It has a TH350 transmission with an unknown torque converter. Original 350 engine also. I was wondering if the torque converter could maybe cause it to be that high or maybe the way the transmission is geared. Any help would be great to get a better understanding. Thank you

oldcutlass

I think your tach is wrong

Rallye469

Seeing as you didn’t give any data I filled in some blanks…

With a 26” tire, 2.73 gears @ 3800rpms and 5% converter slippage(high end of normal) you should be going about 102mph, not 70mph.
So I think your converter is REALLY loose.
You should be at 2600 to drive at 70mph

With a 26” tire, 3.50 gears @ 5200rpms and 5% slippage, you should be at 109mph (again, points to a really loose converter if you’re only doing 65)
You should be at about 3300 at 70.

get rid of that converter.

matt69olds

Are you sure it’s going into 3rd gear?

I’m betting you find the tach is way off, or the speedometer is inaccurate.

66_Jetstar

Aftermarket tach thats in the 6 cylinder switch position and not 8?

gs72

I was going to say sort of the same thing. My car has a TH400 stock convertor and 3.08 rear gears and it runs about 2700 rpm at 70. I would check your tach accuracy first.

BangScreech4-4-2

Here's one way to tell if you're getting an accurate tach reading: at 65 MPH is the engine making a hell of a racket?

monzaz

lol. These are all correct and great answers to figure this out.
At that RPM your brain should be aching...LOL. EARs ringing...lol.
1. check the trans for correct High gear engagement 3rd gear... (count your shifts...1...2....3)
2. Check tach for correct cylinder count on the back of the tach (IF AFTERMARKET) there is a switch for 4, 6 and 8 cylinder.
3. Check RPM with a timing gun to see if tach is reading correctly if a factory gauge or an aftermarket gauge.
4. Torque converter REALLY shot...??? slipping very badly...???
5. Tire height (could be some of it but you would need to be at like 17" tall tires (THE RUBBER PART!!!!) to run 2.73 gears at 70 mph.
So just kind of squeezing it all into one big mess... lol.
Jim
JD

no1oldsfan

Definitely look at the tach. Does the motor sound like it is winding that high? You can tell if your motor is revving that high just by the sound. 3.50's revving that high does not sound even close to correct. Something is off. Like I said, to your ear does it sound like your engine is revving that high? Is that a FuManChew tach?

Chugger

Check your trans fluid level. If it's really low that could cause the symptoms

REO442

This sounds like the same problem in the 60's song=== BEEP BEEP, check it out!

cfair

With 3.23’s and 26.4” tires, my Starfire runs just about 2900-3,000 rpm around 60-65, if that comparison helps.