My First Post!!! First, I want to say Hi to all my fellow C3 Vette drivers. Now, to my question. I have a '79 Vette with the 3 speed automatic tranny. I just drove my car on the Turnpike and I noticed that at 75-80 mph my tach is reading 4300-4450 RPM. Is that normal for 79 Vettes to turn that many RPMs at only 75-80 mph? My redline is at 5500 RPM so I am a little concerned to cruise at those RPMs for extended periods of time plus it really isn't helping my already poor gas mileage. Any input is appreciated. :chevy

 

sounds about right, 350 turbo tranny does not have the best highway final drive ratio, it is 1-1, your rpms at speed depend on rear end ratio and tire size,

this is what my car does with a 3.55 rear end gear,th350 tranny, and a 27 in tire or you could say 255 60 r15,
1000 rpm mph 22.67
2000 rpm mph 45.35
3000 rpm mph 68.03
4000 rpm mph 90.71
5000 rpm mph 113.36
6000 rpm mph 136.06
if your car has a stock rear in it their should be a stamp on the bottom of the differential housing. codes are as follows
OM 3.36
OH 3.55
OJ 3.70

your numbers are pretty high though. i ran them through with the tire ratio stated above, and it looks like you have a 4.56 rear gear

 

Someone may have installed a looser torque converter. If the car is new to you, you might want to double check the trans fluid level.
Good luck & welcome to the forum
:cheers:


[Modified by 71coupe, 2:15 AM 3/29/2004]

 

That is very high. Are you sure that your speedometer is accurate? Sometimes people change the rear gears without recalibrating the speedo. Were you passing everyone?

Does the tach move significantly if you pulse the throttle at that speed? Your tranny or torque converter could be slipping.

Give us some rpm and speed values in other gears to see if they're reasonable. Keep the revs up when you do that so the torque converter doesn't interfere.

 

I've had the Vette since I was 16 (I'm 19 now) so the car certainly isn't new to me, but the person I purchased it from definately messed around with the car while he had it. I wouldn't rule out that idea that the previous owner put a different rear end gear in the car and I will check out the number (if there is one) that is imprinted on the differential cover when I have the chance to get it on a lift. With that said, Here's all the info I have for you guys to help me out. My tires are P255/60R15 on the stock aluminum wheels (not the rally wheels). The speedo is accurate because I followed my dad down the Turnpike and I called him to make sure it was reading the same mph as his car (which is stock including the wheels) and they both read the same speed. As far as the tach being accurate, I'm pretty sure it is because it definately sounded like the engine was turning 4300-4400 RPM when I was cruising at 75-80. The only thing about the tach is that the needle sits at about 1500 RPM when I turn the car off but when I start it up it drops back down to 900-1000 RPM at idle and the needle will drop and raise accordingly when the idle speed screw on the carb is turned. The speedo/tach is also not the original that came with the car. The current speedo goes to 140mph and I think I read that one that came with the car only went to 80 or 90mph. The torque converter was replaced fairly recently (about 2 yrs ago). The tranny fluid was low on the Turnpike drive (when it read 4300-4400rpm) because the Chevy dealer I took it to for service decided not to fill it up even when I specifically told them to. :mad Unfortunately it was 1 1/2 quarts low. Thats everything I can think of for now.......Let me know if anything I posted (or anything else you can think of) might explain the high rpms. Thanks. :D


[Modified by 7Vette9, 11:28 PM 3/29/2004]

 

With a non-locking torque converter, and 255/60R15 (27" tall tires), here are the approx speeds (in MPH) you'd turn @ 4350 RPM with various gear ratios:
3.08:1: 103 MPH
3.36:1: 94 MPH
3.55:1: 89 MPH
3.70:1: 86 MPH
3.90:1: 81 MPH
4.11:1: 77 MPH
Hope this helps...
:thumbs:

 

This sounds very familiar. I have a '79 too. P255/60R15 tires, and 3.55 gears. On the freeway/highway my tach reads about the same, perhaps a little lower. My speedometer is out of calibration. It reads exactly 20MPH higher than I'm moving. I followed a friend once to measure and confirm. It's the rear end gearing in mine that's causing the high RPMs on the highway. Without an overdrive, this is what I'm stuck with.

Well, as you have seen, it's bad for fuel economy. I'm currently getting 7 MPG city, and 9 MPG highway. In the long run, it will wear on your engine faster. My car was set up for racing by the previous driver, so this is what happens on the highway as a result. If you're not racing your car, perhaps a 700R4 w/ overdrive would be better for you. BUT, if you race it, you will break it, even if built well. Your TH350 is more durable. It's all a trade off.

The later produced '79s had the 1980 85 MPH Speedo and two rear storage bins, not three.

 

You may not actually be turning the rpm you think you are. My Tach is not reading accuratly. What rpm are you turning in park at an idle. To verify that your tach is correct turn the idle adjustment screw in until your tach says 2500, turn the engine off, hook up an engine analyser and then turn the engine back on and see if the engine analyser reads the same as the tach. Hope this helps.