Hi, i havent really ever taken my car out on the highway until recently and ive noticed my tach says the engine is turning over roughly 4k rpms at 70mph. Its an 88 z28 with the 350tpi and 700r4 trans. No shift kit or anything, yet. Basically stock engine, just bolt ons.

The tach is right, i wired in an aftermarket one myself.

The trans is fine, it shifts great. It goes into 4th at about 40mph. I can also feel the trans lock up engage.

I cant think of any other reason for it to be revving that high other than if someone before me changed out the rear gears. Anyone have any ideas?? Thanks!

 

When on the highway try shifting into d not overdrive and see if overdrive is engaging

 

This very same question seems to come up regularly , and the solution always comes back to Tires/wheels .... If you got any kinds of low profile rims , and the total Tire/rim diameter is smaller than the factory supplied parts , well then you have "geared down" the final drive by the amount of the reduction in tire size .... It don't take much of a change to make a noticeable difference in final gearing , just a couple of inches smaller is enough to have ya wondering what's up with the RPMs ......

So , , Got Rims ????

 

Sounds like the tranny is not shifting into Overdrive. Or your running 13" wheels with low-pro tires.

Mine, with stock height tires, is running about 2000-2100 at 63 mph.

 

Even based on a 3.73 rear gear in 3rd gear he would have to have 22 in tires and no way is that possible, talk about ground scraper. Id say transmission is not working properly. Clutches goin bad cant hold in gear maybe.

 

Aftermarket tach set on 4 cyl. scale?

 

Does the engine sound like its turning high revs at 70 MPH. I would think you could really hear it if it was.

 

The factory tachs are NOTORIOUSLY inaccurate at this point. Mine reads about 1500 RPMs higher than it should at cruise. Many are far worse. The factory tach is just a dash ornament. Unless you can HEAR teh engine turning 4000 RPMs (and you'd know), dont worry about it.

 

i got the same thing with my car except mines a 5-speed

 

you don't have the tach set on "6 cylinder" do you? that will throw it off by 20%..

 

no i dont i set it to 8 but the rpm's dont even pass 3000

 

If it won't go past 3000 at all & stops there? Then it is pretty apparent that you have a faulty tach.

 

or I may have wired it wrong.....would u happen to know were eaxctly i tap into?its a carbed engine.

 

Thanks for the input everyone. My car is the z28 and came with the 16' rims originaly, but someone took them off and put the 15' RS rims on it for whatever reason. So I dont think thats the problem. Good idea though. I have also checked tire pressure (just for the hell of it) and they're all at 32, im gonna put them at 35 tho (not going to make a big difference, if any, i know)


OD is definely engaging and I can feel the torque convertor lock-up as well.

I do have an aftermarket tach and I wired it myself. It is done corretly, and it is set for 8cyl. The motor is definetly turning the rpms the tach is telling me, i can feel the car "wanting" to shift and if i let off the gas it slows way down, like it would do if you had it in first and didnt shift into 2nd just let off the gas. But It is in 4th im 100% positive of this.

The trans shifts fine, I dont have any problems with it. Is it possible that the detent cable being adjusted improperly could cause this?? It shifts great and at the right speeds but im just wondering if thats possible.

 

Ok how fast (mph) is first gear at say 3000 rpm?

 

If everything else seems ok, then I'd look in the rear end and count teeth if necessary. Maybe the previous owner put some real low gears in it. When you say it goes into OD at 40MPH, do you mean floored? I doubt it's your detent cable. If it was slipping that bad, your tranny would have been toast a long time ago.

 

I will probably do this soon, I have to replace my radiator this weekend but that should be the next thing I do.


if I'm driving it normal it usually shifts out of first at 10-12MPH. When it goes into fourth ihave to be going 40 and when it shifts into 4th it will be at around 2000rpm but if I slow down to about 37ish it will turn roughly 1500. When I bought the car the detention cable was wayyy out of adjustmentioned but it had sat for a few years,being driven very scarcely in that period of time. I readjusted the detent and I think it's good now,but that's why I asked.

As far as speed in first at 3k, I'm not really sure to be honest. I think probably 15 but I'll check and get back about that.

 

U dont have to count teeth for a close approximate guess at gear size raise the car mark the driveshaft and tire, put car in neutral spin tire slowly twice dont stop and keep spinning tire and count driveshaft revolutions per one tire revolution.

 

Thats a good idea, ill give that a try. Is there anything else it could be though? And if someone put different gears in the rear, what would have to be in there to make it do almost 4k at 70mph? 4.11's??

 

My 700r4 shifted into OD around 40 and locked the torque converter before 45 mph. Thats right on for a 2.73-3.23 range gear.

Your tach is wrong.

 

jack up the rear of the car.. put the trans in neutral... see how many times the driveshaft turns for each revolution of the tires.. if it's a posi, this will be straightforward.. if it's not a posi, then you need to prevent one wheel from turning..

 

There's only a few possibilities here.

First, the tach is wrong. You haven't told us whether the engine actually sounds like it's going 4000 RPM; it should be very obvious - my car does 2700 RPM at highway speeds, and it absolutely screams; 4k will be very loud. Either it's defective, connected incorrectly, or the wrong style - seems to me it's reading double what it should be; around 2000 RPM@70mph is pretty typical for the less optioned cars.

Second, you have some really steep gears in the rear end - well over 4.11, around to 5:1 by my quick maths if you have the original tire diameter. This seems highly unlikely to me.

Third, you aren't actually in overdrive, or the torque converter isn't locked. Even though you said you're positive it's in fourth and the torque converter is locked, it's possible that one of those isn't actually happening - either it's locking up in third (RPMs seem about right) or it isn't locking up.

Finally, transmission could be shot. I'd consider this only slightly more likely than some dolt installing a 5:1 rear gearset.