Corvette Forum : DigitalCorvettes.com Corvette Forums banner

1 - 20 of 23 Posts

·
Registered
Joined
·
741 Posts
Discussion Starter #1
I have a 79 L-82 with THM 350 and an 3:55 rear. In third gear at redline, which is 6500 RPM, I am at about 110 mph. This is crap! Is there something wrong with my car? According to my top speed spread sheet calculator, with the tires I am using, 255 60R 15, I should get a theoretical top speed of 147 at 6500 RPMs. So WTF, could wind resistance be causing that much of a difference???

Thanks,
Dude
 

·
Registered
Joined
·
167 Posts
I don't know what to tell you about how to increase your top speed - but with 3.55 gears, I would have expected you'd be able to go faster than 110.

I've never maxed out any car's top speed other than with a speed limiter (or, more often, my own fear causing me to lift the throttle!), but I'd have to think that if you're redlining, then wind resistance isn't the problem. I could definitely be wrong, though.
 

·
Registered
Joined
·
741 Posts
Discussion Starter #4
OK a bit of a correction, the redline is 6000 rpm and the theoretical top speed should be 136.

I am sure the speedo is accurate a 70 MPH. And at 70 Im turning about 4000 RPM. But I am NOT sure after that.

Mikebotz, I ran the top speed calculator with a 4.11 final drive and the numbers were right on the money. However my diff is stamped as a 3.55??? In fact, its the stock diff which had never been out of the car. What the heck?
 

·
Registered
Joined
·
167 Posts
OK a bit of a correction, the redline is 6000 rpm and the theoretical top speed should be 136.

I am sure the speedo is accurate a 70 MPH. And at 70 Im turning about 4000 RPM. But I am NOT sure after that.

Mikebotz, I ran the top speed calculator with a 4.11 final drive and the numbers were right on the money. However my diff is stamped as a 3.55??? In fact, its the stock diff which had never been out of the car. What the heck?
Well, for 50% more RPMs (6000 vs 4000) wouldn't you just get 50% more speed (105 mph vs 70 mph)?

4000 RPMs at 70 mph sounds like a lot of revolutions for a 3.55 rear end IMO.
 

·
Registered
Joined
·
167 Posts
That's 66.66%, not 50%. If you want to double 4000, it would be 8 grand.
Nope, you're wrong. Trust me, I calculate % increases all day long for my job, with salaries.

I said "50% more" - which means 50% on top of what's already there. 50% of 4,000 is 2,000, so on top of 4,000 is 6,000.

To check my math, take 4000 x 1.5 (or 4000 x 150%) and you get 6,000.:thumbsup:
 

·
Premium Member
Joined
·
5,157 Posts
It sure sounds like you've got something besides a 3.55:1 rear. Put the car on jack stands, leave it in neutral, and count the number of turns of the driveshaft to get the wheels to turn once. That will give you a real estimate to confirm what your spreadsheet is telling you (although it sounds about right to me).

Since you're using a spreadsheet, just reverse the formula and see what it pops out for a rear end ratio.

Steven

PS - Rooster, you're wrong about the percentage increase. You were looking at it backwards - you were taking 4000/6000, which is where you got the 66%.
 

·
Registered
Joined
·
741 Posts
Discussion Starter #9
OldVetteFan;

Thats a good idea! If I had a 3.55, then the drive shaft should turn over 3 and a half times for ever tire turn right??? If its a little over 4, then I have a 4.11????

Good idea, thanks,

Dude
 

·
Registered
Joined
·
741 Posts
Discussion Starter #10
I forgot to mention, I reversed the formula, and a 4.11 final drive ratio works perfectly for all my speed vs. rpm observations.

So, if it is a 4.11, then its NOT stock, but the diff housing IS stock because I decoded the stamping on it.

So, would it be better, easier to swap the rear end or put one of those aftermarket overdrive units that bolt to rear of the trans????


Or should I just not give a s---???

Dude
 

·
Registered
Joined
·
741 Posts
Discussion Starter #11
OK, the diff is Definitly a 3.55! So, I guess the trans is not original and was originally in a truck or something...
 

·
Registered
Joined
·
2,366 Posts
I forgot to mention, I reversed the formula, and a 4.11 final drive ratio works perfectly for all my speed vs. rpm observations.

So, if it is a 4.11, then its NOT stock, but the diff housing IS stock because I decoded the stamping on it.

So, would it be better, easier to swap the rear end or put one of those aftermarket overdrive units that bolt to rear of the trans????


Or should I just not give a s---???

Dude
The gears are on the in side. 4.11 gears could very easily have the stock housing.
 

·
Registered
Joined
·
3,100 Posts
I've seen 140MPH with 3.73's at 7000 RPM. Don't know what he tire diameter was, it's been too long ago (I was MUCH younger). 3.73's were showing 2X the tach, 60 was 3000, 70 was 3500 80 was 4000 etc., I'd have to suspect it's a gear thing until proof of something else.
 

·
Premium Member
Joined
·
10,260 Posts
Or should I just not give a s---???

Dude
how do you drive the car? Do you like the off-the-line acceleration? Do you take it to the strip? Do you mostly cruise and roadtrip?

If you don't put that many miles on it I'd say screw it and enjoy the seat of the pants feel of the taller gears.
 

·
Registered
Joined
·
53 Posts
You checked the rear and the the drive shaft turns 3 and a half turns for 1 turn of the wheels?

What trani has around a 1.2 final drive ratio?

If you are sure your speedo is correct and you checked the rear ratio then your trani has about a 1.2 final drive ratio.

Weird.
 

·
Registered
Joined
·
741 Posts
Discussion Starter #17
Update

The rear is a 3.55 confirmed!!!. At 3000 rpm I am at 55. 4000=75; 5000=95
and 6000=110 maybe a little over.

The only thing it could be is the transmission.

Yellow73, If I were at 100 mph in second gear, Id be at 7000 prm.

SO, according to the numbers, my THM 350 has a third gear of 1.2 to 1 instead of a 1 to 1.

Anybody ever hear of this???????????????????????????????????????
 

·
Premium Member
Joined
·
5,157 Posts
The rear is a 3.55 confirmed!!!. At 3000 rpm I am at 55. 4000=75; 5000=95
and 6000=110 maybe a little over.

The only thing it could be is the transmission.

Yellow73, If I were at 100 mph in second gear, Id be at 7000 prm.

SO, according to the numbers, my THM 350 has a third gear of 1.2 to 1 instead of a 1 to 1.

Anybody ever hear of this???????????????????????????????????????
No.

Try this ...

Get on a highway with mile markers. Get your car to an indicated 60 mph. At a constant rate of 60 mph, you should cover that distance in 60 seconds. Your odometer should also register a mile between the markers. This should show how far off your speedometer is.

Also, have you independently verified that your tachometer is indicating the correct RPM?

Good luck! You've got something screwy going on here!

Steven
 

·
Banned
Joined
·
176 Posts
Well, for 50% more RPMs (6000 vs 4000) wouldn't you just get 50% more speed (105 mph vs 70 mph)?

4000 RPMs at 70 mph sounds like a lot of revolutions for a 3.55 rear end IMO.
I WAS RESPONDING TO ELDUDERINO THINKING HIS TOP SPEED WAS 136! He was virtually doubling the top speed and I was saying fine, if you can rev to 8 grand.
You might calculate salaries that way, but nobody calculates gear multiplication that way. Part / total = %/100
The total is 6000, the original number is 4000. 4000/6000 = 66.66%. It 's a ratio and that is proper math formula for that type of calculation.

OldVettefan, I was trying to do it "forwards" not "backwards" like this guy did.
 
1 - 20 of 23 Posts
Top