all geared up

What kind of tachometer are you running? Is it the kind that has a 4-6-8 switch on it? If it does, check that. The lowest gear found in a S10/15 rear axle is 4.10:1. If you had that, for it to be in top gear, at 50 MPH, spinning 4K RPM you'd have to be running about 14-1/2" tall tires.

Ratio options for the 2001 Blazer were 3.42:1 (tag will say GU6) and 3.73:1 (tag will say GT4). There are two different axles that year, one with a 7-5/8" ring gear, and the other with an 8-1/2" ring gear.

What the tach should read (not corrected for transmissive losses), based on a 27" tire is as follows:

3.42:1 @ 50 MPH, in top gear: 2128 RPM
3.73:1 @ 50 MPH, in top gear: 2321 RPM

If you had a different year S10 axle, with the 4.10:1 gears:

4.10:1 @ 50 MPH, in top gear: 2551 RPM

If somebody changed the ring and pinion to a deeper one the lowest gears are as follows:

7-5/8" ring gear: 4.56:1
4.56:1 @ 50 MPH, in top gear: 2837 RPM

8-1/2" ring gear: 5.57:1
5.57:1 @ 50 MPH, in top gear: 3466 RPM

The deepest practical gear that you would use on this axle, in a 4x4 rig, with 35" tires, would be 4.88:1.

8-1/2" ring gear: 4.88:1
4.88:1 @ 50 MPH, in top gear: 3036 RPM

In any case, not 4K RPM.

Since the TH350 torque converter does not lock up, there is a small amount of loss associated with it. Your actual RPM numbers may be 50-150 RPM higher.

 

Is the vacuum modulator hooked up correctly? If not, the transmission will stay in second.

 

i think i got it ..THANKS TO ALL WHO ADDED SOME INFO .

 

All you have to do is jack up the rear, spin a rear tire 1 time, see how many revs the drive shaft goes around, do the math and you'll be real close. I'll also 2nd the mis-wired tach thing. Some tachs have a switch on the back of them.

Bob