The problem is on a 1991 RS Camaro that orginally came with a 305, but now has a 350. It still has TBI setup and came with the 700R4 Trans that had been rebuilt. The car came with 2.73 gears from the factory and do not believe that it has been changed.
While driving on the highway at 70mph the Tach is showing 4500 RPM. It appears trans is shifting ok, but something is off. How would you check trans shift points, or that the OEM tach is right?

 

The OEM tachs arent very accurate but their not usually out by 3000RPM lol

My guess is that:

1. Not going into 4th (OD)
2. TCC is not engaging
3. gears have been swapped
4. combination of all of those

Edit: Ok, @ 4500 RPM in OD with 2.73 gears you should be doing something in the neighborhood of 180MPH...

4500RPM in 3rd and 4.11 gears will get you around 80MPH.

If everything is fine with 2.73 and its shifting into 4th you should be just under 2000RPM

 

What he said.

Does it feel like 4500rpm, should be easy to tell by seat of the pants lol. Kinda winding it up there.

Any of what he listed could cause your problem, plus compounding issues things could get real fun.

Factory tachs are known to be quite horrnedous on some cars too, thats why I asked if it feels like 4500rpm.

Get a data logger of some sort on their and see what it says, or feel by your rear if its 4500rpm or not.

Than proceed to check everything manually. Put the rear up and count how many times the driveshaft/pinion moves to the tire to get the real rear end ratio etc.

later
Jeremy

 

Boy do I wish I was going 180!!!! I will check the to see if the TCC is engaging and might hook up another tack. It doesn't necessarily feel like 4500rpm, b/c if I gun it, it will hit 7000rpm and I would think at 7000RPM the engine would explode.
I will keep you posted.

 

My 1992 has 3.73 gears and at about 90mph I am running about 4000rpm.

Right now the car is in the shop.

I am interested in the resolutions here.

I would suspect the gears are no longer 2.73

 

tire diameter also plays a big role in your speed too. Grab a measurement from the outside of the tire to the center of the rim.

You would still have to have a tire with a 5" radius to only get 70mph in 4th @ 4500RPM

 

When I get the car back I will get exact numbers for all and runt them by you.

 

off the top of your head do you remember what size tires are on there. (215/45r15)??

 

Wait a second, wouldn't you be able to hear if your car is turning 4500 rpm? If your car is hitting 7k rpm, and the normal shift point is about 4500, that would put your tach 2500 rpm's off. Thus meaning you are really pulling 2000 rpm at 70 mph. Your tach is off my friend.

 

But then you get into the whole 3rd gear only while floored. which could take him to 8k, you have to let off before it will shift into 4th unless he has a modified trans.

Either way a guy should be able to tell if his motors spinning 4 or 7k RPM without a tach...

 

I hooked up a scanner to my car and my tach is way off!
When my tach said 4500, the scanner was showing 1950. When my tach showed 7000, the scanner showed a little over 3000.
Looks like under full throttle the car shifts at 3000rpm, I wish it was closer to 4000, but I guess I can not change easily.

I have 245/50R16 tires. The RS wheels were changed out for IROC wheels.

If I just add an aftermarket tach, will it work or do I need to also check the connection point?

Thanks for everyone's help!

 

Check your TV cable, it should be shifting higher than that @ WOT

If your going to add a tach i would run a new wire directly out to the dizzy, just incase the original wiring is funky.

 

I have the both of the problems - tach an TV cable.I drove my 83 for the first time for an extended period of time today.I noticed the tach problem before and figured either a bad wire or tach,although as simple as tachs are not much to else to go wrong.I found the tach and the dist wires had most probably been pinched at some point by the air cleaner hsg(it's an LU5 CFI).I'll adjust my TV cable tomorrow.It's not like the old days when you had just a vac modulator and detent cable.TV cable combines all into one.And I've seen more than one trans burned up becuase it wan't adjusted right.

 

Update on mine with exact numbers.
69-70 MPH turns at 2500RPM

17" Wheels and the rear tires are 275/40ZR17

 

I have a 91 rs with the lo3, 700r4, and 2.73 gears. On the stock tach, it reads 2200-2300rpms at 70mph. Stock rims and tire size.