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Discussion Starter · #1 ·
I know I have read a few things on here about this, but now I can't find anything on this subject to save my butt. I have a feeling my RPM's are higher than they should be after going from 3.21 to a 3.73 ring and pinion. Seems like I remember they should be around 2300 at 70. Im hitting 2500 at 70 and about 2650 at 75. I know your RPM's increase about 300 after a change like that but that seems higher than it should be. Right or wrong? Keep in mind, other than the ring and pinion, new axles, and new bearings through out the rear end, everything is stock that I know of. No obvious changes to the cars original setup that I can tell by looking at it. I still haven't had my speedo re-calibrated yet after the change, so I am basing my speed off the GPS. I have tested the GPS in other vehicles and it is right on. So there you go. Thats all I know, so let me know if I am wrong or not please.
 

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You should have a T-45 transmission. With 3.73s, second gear 20 mph = 2000 rpms, 40 mph = 4,000 rpm. In fourth gear, 40 mph = 2000 rpm, 60 mph = 3000 rpm. In fifth gear, 60 mph = 2000 rpms, 75 mph = 2500 rpms.

Check out Cliffyk's chart:
PaladinMicro

Did someone put a gauge overlay on your car? You might want to put the odometer in test mode and check to see if the needles on the gauges are correct.
 

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i know i have 3:73 and a t45. 30.7mph in 4th is 1500rpms with 275/40/17s
 

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Discussion Starter · #4 ·
No overlay, all stock other than what I mentioned. According to the door panel I have a t45. Having said that, my RPM's don't match up to any chart or calculator that I can find. at idle I am at about 600 RPM's. I believe that is where it should be. So I am running more RPM's than I should be. Any suggestions as to what could be the culprit?
 

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Then you probobly have 4.10. Your seller probobly made a mistake and sold you 4.10 instead of 3.73
 

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This is a common problem when a previous owner has changed the instrument cluster overlay. There is an easy test to make sure the needles on your gauges are in the right place. On our cars the odometer has a test mode that will test all of the gauges. Hold the odometer button down and turn the key on without starting the motor. The odometer will go through some light test functions. Release the button. When it is done push the button a few time until the display read tac0000. The odometer is now a digital tachometer. The digital display in the odometer should match exactly the needle on the gauge. Setting the odometer to Eng0000 turns it into a digital speedometer. It should match exactly the needle on the speedometer. When you turn off the car the odometer test function ends.
 

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Then you probobly have 4.10. Your seller probobly made a mistake and sold you 4.10 instead of 3.73
That is a possibility. It is easy to check as well. Chock the front tires, jack the rear wheels off the ground, put the car in neutral. Place a mark on the driveshaft and a mark on the wheel. Turn the wheel one complete turn and count the number of times the driveshaft goes around. 3 3/4 times is 3.73 gears. A little over 4 times is 4.10 gears.
 

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Discussion Starter · #10 ·
no I have 3.73's without a doubt. Prior to having them put on I was running RPM's that were in line with 3.55's when I had the stock 3.27's still in. I bought this car in Dec. so I don't know what the hero did to it prior to me buying it, but clearly something is not right. Is there something that could have been modified that would cause it to run 200 to 300 RPM's higher than it should. Something I cant see right off, like a tune or something? Right now it is running about 500 RPM's higher than stock. Figure 300 for the 3.73's which leaves me with the mystery 200. I know just about nothing when it comes to all these modifications other than what I learned during the overhaul of the rear end. Seems like something has been done and my hope is that someone can point me in a likely direction to look. Gas is too high to be pissing it away on those extra RPM's. I drive 86 miles round trip to work so it adds up quickly!
 

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Discussion Starter · #11 ·
I'll go check the gages out right now. I'm assuming that while it is in the digital mode I am to drive it around and compare the two right
 

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Discussion Starter · #12 ·
Ok. tested the gauges everything matches what the needles are indicating. Also it is idling around 730. At 60 the RPMs are around 2145.
 

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Discussion Starter · #13 ·
Well that matches right up with 3.73's on PaladinMicro I guess I'm an idiot and got all worked up for nothing. Thanks and sorry to those who responded. Thanks alot eagle2000gt for the tip on the digital tac. Made it alot easer to get an accurate RPM rate for the speed.
 
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