American-Muscle1989

Hi all,

My 1998 Z28 drives at 2300 rpm at 70mph and 2600 rpm at 80mph in overdrive on the flat.
I have the car in D with a box around it (overdrive) when I am doing this and it is on a flat road.

My question is that this seems high and I wondered if it was. Is my overdrive not working properly?

I am new to the car so I will check the gearbox and diff servicing records to see if it needs a service.

I heard that there is a possibility of different gears being fitted for the car before I got it which would affect the rpms at speed.

Any and all help would be appreciated.

Thank you

97camaro383

iTrader: (31)

My car has 3.73s and i think it was around 2500rpm at 70mph in overdrive. Yours is proably close to what it should be.

jimmyblue

iTrader: (11)

This is what I turn with 3.42s and slightly oversized
tires. So on 16" rims and 3.23s it's roughly right I
guess.

NC98Z

iTrader: (1)

http://www.pszweb.com/car/gears.htm

RPM WS6

iTrader: (3)

I remember my 3.23 cars turing about 2100rpm at 70mph with stock tires.

The gears may have been swapped, or the tire size may not be correct, or the tach may be off, or the speedometer might be off. Can you tell if the torque converter is locking? Excessive random misfires can keep it unlocked, and keep rpms higher than they should be.

American-Muscle1989

Hi guys,

That is really interesting and the data is great, thank you very much.
Mine has 18 inch rims on the back from a corvette but I'm not sure about the tire profile.

Mine is a 1998 so will it certainly have 3.23 gears?
Is there a way to tell, will it be on some paperwork or do I need some electronic plug ins to tell me?


Thanks

How do I tell if your torque converter is working?

Thanks

RPM WS6

iTrader: (3)

A '98 Z28 with auto trans could have come originally with either 3.23 or 2.73 gears. There should be a sticker on the driver's door jamb with a bunch of 3-digit codes (and your VIN number at the top). This sticker is called an SPID (service parts identification) sticker, and the codes found on the sticker are RPOs (regular production order/option).

If you see "GU2", then it originally came with 2.73s. If you see "GU5", then it originally came with 3.23s. Of course, this is assuming that the car still has it's original gears.

Based on your rpm vs speed data, assuming that your overall tire diameter is stock and that your speedometer and tach are correct, then you might have a 3.42 gear currently in place. This was also a stock gear ratio for a '98 Z28, but would have only come with the manual trans. The RPO for this ratio was "GU6", but if you see that RPO then it means that your car was originally equipped with a manual trans (and so you would also see RPOs "MM6" and "MN6"). An original auto car would have RPOs "MX0" and "M30".

If your car still has the original '98 axle, then you should be able to jack up the rear of the car and rotate a rear tire one full turn, counting how many times the driveshaft turns. 2.73s would be 2-3/4 turns, 3.23s would be 3-1/4 turns, and 3.42s would be just a bit less than 3-1/2 turns.

This will not work the same on a '99+ torsen rear, since those act like an open rear when not loaded.

When you're cruising along at a steady speed above 45mph (like any expressway speed), if you push the accelerator down just a little bit, does the engine seem to rev/respond easily or does it seem to lug/feel a bit heavy and unwilling to rev as easily as it would from ~35mph? If the TC is locking properly, you should notice little if any change in rpm by opening the throttle just a small amount from a steady cruise at those speeds (a lugging sort of feeling). Once you give it enough throttle, it should suddenly start to rev much easier until you return to a steady cruising state.

American-Muscle1989

Hiya,

That's excellent info thank you, I will look up the numbers on the label tomorrow and report back

99Bluz28

iTrader: (35)

Yeah, those rpms look like it has 3.42 gears. 3.23 gear would be like 2100 at 70mph, and 2400 at 80mph.

American-Muscle1989

Hi guys,

OK I have taken pictures of the labels and tires for you.

Here is the picture of the label and I can see GU5 as listed.

So that means that I have 3.23 gears yes?

Currently the car has 245/40/17 tires on the fronts and 275/40/18 tires on the back.

Would this explain my stats of 2300rpm at 70mph and 2600 at 80mph?


Also, slightly different question, would my speedo be off slightly due to the different sized wheels. The seller said it could well be.

Thanks as always

RPM WS6

iTrader: (3)

Correct, it means that this car came off the assembly line with 3.23s.

A 275/40/18 tire is 1" taller than your original 245/50/16 tires would have been, so unless someone has recalibrated for the new tire height, then your speedometer would in fact be incorrect.

When your speedometer reads 70mph, you are actually going more like 73mph. 2300 is still just a tad high for 3.23s at that speed with the TC locked, but the factory tach might not be 100% accurate.

99Bluz28

iTrader: (35)

With a 275/40/18 I'd have to say your car has 3.42 or 3.73 gears, if it had 3.23 gears it should be about 2000rpm at 70 and 2300rpm at 80.

IMO, I don't think you rpm gauge is off by 15%, but unless a tuner changer you tire size your car is more likely doing 83mph when it reads 80mph, and about 73 mph when is reads 70mph.

American-Muscle1989

Ok interesting, I will go out on a flat section and make sure of my figures but it seems I am pretty happy that everything is working out.

RPM WS6

iTrader: (3)

Pay close attention to pedal feel (based on what I mentioned above) at those speeds as well, to check for proper TCC locking.

Fry_

Not sure if this helps or may just add more confusion but my 98 T/A's tach reads almost exactly 200rpm high from idle all of the way to red line. If you have a smart phone you could download a free app to check you cars speed with it's gps then you'd know if your speedometer is accurate.

American-Muscle1989

I will check for proper TCC operation thanks

2002 blue z28

iTrader: (1)

Hey there, don't mean to jack the thread or anything. Is there a way to check what gears are in the car without pulling the rear end cover off?

RPM WS6

iTrader: (3)

Do you still have the stock Torsen differential in your '02? If someone has swapped it to an Auburn/Eaton style posi, then this method will work:

As stated, if it's the stock Torsen posi, then the method is different. It would need to be checked like an open differential, which if I remember correctly you have to rotate the tire two full turns rather than one.

2002 blue z28

iTrader: (1)

I believe it's the stock rear end. I was just curious because at 70 mph my rpm's are around 2500. I'll have to try out that method you mentioned.

NC98Z

iTrader: (1)

This is one of the best calculators that I've seen. If you can't figure out what gear ratio is in your car using it I'm sorry.