I'm thinking on installing 3.90s, but I was curious what rpm that i would see going through the traps. Anyone???

Thanks,
Mike

 

Tire height and proposed trap speed, please?

 

Hey Colonel is the loaded tire height 25.6" in diameter with 245 50 16? Because assuming 6200rpms rev limiter my car would only do 103mph in third. From my calculations the tire diameter would be closer to 25.1" tall.

 

I get 25.645" for a 245/50/16. That's 245/25.4 (converting mm to in)x.5 (aspect ratio)x2(two sidewalls)+16 (rim height)=25.645

What gears and tranny are you running, dug?

 

Oops, forgot the details. Its a stock m6 with 3.42 gears. The first time I raced it I didnt think the car would trap fast enough to use 4th gear and hit the rev limiter in third well before the end of the track. The time slip showed 103mph. Using 25.6" tall tire in the calculations will get me 106mph in third. So it looks like the sumitonos are little bit shorter. No big deal. Every tire is different. I was just curious if loaded tires were typically slightly shorter than the advertised size.

I was gonna measure the roll out of the tires but never got around to it.

 

Yeah, sounds like they may be a little shorter than advertised. This one is close.

I would assume an actual speed of 106 if the time slip read 103 since the trap speed is calculated from an average of the last 66 feet of the track. 106 MPH gets me 6185 RPM in 3rd with 25.6" tires.

The timing equipment at the track might play into this as well. I've seen 'em read different traps from one lane to the next...quite a bit different in fact. NOT that this is common.

Tire circumference shouldn't change much with tire pressure. Tire height may but it'll still be the same distance around, which is what matters when calculating gearing.

 

http://www.f-body.org/gears/

This site will help you. I just ordered a 12 bolt with 3.90 instead of 4.11. I hope I did the right thing. Look at my post and it will tell you the trap speed in your 1to 1 gear. LAter