My question is gear change or converter reworked? I changed tire to a 275 drag radial and now I'm on the 6400 rev limiter chip in 1/4 with 410 gears. When I ran 29" slicks and the tire growth was perfect at trap speed. Will going to a 3.73 gear be better or send my 9 1/2" converter back for rebuild?

Did your 60 ft time go up or down when you went to the 275 vs the 29" slick? A 275 is only about 28" by calculations so I'm guessing that's why your RPM went up. Is going back to 29" slicks not an option?

If you have an accurate tachometer, figure out what the RPM at your trap speed should be with no converter slip. The formula is 336/tire height X rear gear X MPH = RPM. Then use your observed RPM and figure out what the converter slip is. That will tell you whether you need to send the converter back.

My 60 foots are about the same, maybe a little better 1.58 The problem is no tire growth on top end. Slicks are now not an option.

Not quite sure how this works, can you help me out here. 336/28= 12x4.10=49.2x121=5953.2 So if I divide that number from my RPM that is my slippage amount? (1.075)

So, you are wanting a tighter converter? That will probably hurt your 60ft time. I would think a good converter would already be locked up at well past stall speed if you are spinning it at 6400 across the line. What is your MPH at the end? With no slip, 27.5" tires would be 128MPH. 28" tires would be 130MPH and 29" tires would be 135MPH. I think 3.73 gears would be too low. If you could find 3.90 gears for that rear, that would be closer. With 3.90 gears, your 6400 RPM would net 27.5" tires at 134, about the same as your 4.10 gears at 29" tires. With 3.73 gears, your 6400 RPM would be 140 with 27.5" tires, a bigger gap than your tire size. It all depends on what you want to tune it to cross the line RPM wise.

At 121 MPH, your RPM with no slip should be 5953. You are seeing 6400, so the converter is slipping 500 RPM. 6400 -5953.2 = 446.8 (446.8/5953.2) X 100 = 7.5% Not sure what is acceptable, but that seems pretty good.

Ok Guys I can tell you that with the set up I have,by changing the 4.10 to the 3.73 gear and leaving my converter alone, my 60 ft got a little better and my RPM dropped 400 rpm. So that brought me back the 6400 rpm that I'm comfortable with @ 126.51mph. Thanks for all the input.