Question on calculating RPM at given speed x gear

[99SVTAddict]

Is there a calculation that would tell you what RPM you are at given this:

Viper T56
4.30 rear gear
27" tall tire
122.4 MPH.

I would like to know what RPM I was at when crossing the traps in 4th gear.
I know there's a calculator out there but can't remember where and I don't know if it takes into account the tires size (if that even matters).

Thanks.
Chuck

[99SVTAddict]

I just found this link http://www.izook.com/gearcalc.htm

Near the bottom it says "Find RPM"

Tire 27"
Trans ratio 1:1 (4th gear)
4.30 gear
122.4 mph

Says it's 6549 RPM.

If I had a 26" tire, I would cross the traps at 6800RPM's. I redline at 7K.

Which would be a better tire size for the next track event....27 or 26" ET Street.

[Crazzy_Al]

Originally Posted by 99SVTAddict

Is there a calculation that would tell you what RPM you are at given this:

Viper T56
4.30 rear gear
27" tall tire
122.4 MPH.

I would like to know what RPM I was at when crossing the traps in 4th gear.
I know there's a calculator out there but can't remember where and I don't know if it takes into account the tires size (if that even matters).

Thanks.
Chuck

You suck, I rule...

http://www.tchracing.com/tech/gearcalc.php

Your tire's diameter is 27.000 inches
At 122.4 MPH in 4th gear, your motor is turning at 6,552.4 RPM

[99SVTAddict]

Your's is a little more accurate.

A 26" tire would be good for 126MPH basically.
So, which is the better tire size for my application????

[1Quick88]

at 7k are you making your peak Horsepower?

[Strtracer]

Sidewall height will affect traction significantly. While switching to a shorter tire will increase effective gear ratio, if the tire spins, you will slow down. I would stay with the 27's or get a 28, you will be able to launch at a higher rpm, therefore lower you 60ft time, and your ET. the idea here is to allow the sidewall to "wrap" more when you launch, which allows you to hit the tire harder without it breaking loose. Do you have a dyno read out, you should be shifting 2-300 rpm past peak HP. Get yourself a shift lite and program it to come on at peak HP, by the time you react to the lite, you will be at the desired RPM, the motor will fall back to below peak, and you will runup to the next gear quicker. If you short shift, it will fall too far below peak power and take a longer time to recover. Also, taller tires will generally allow you to run a higher tire pressure, which will give you a more stable run at the top end. Idea here is to launch as close to peak torque, and run the motor to peak power.

[NTIMID8]

GEARGEAR RATIOFINAL DRIVE RATIOTIRE DIAMETER ENGINE RPMCALCULATED SPEED @ RPM(GR)(FDR)(OD) (IN)(RPM)(MPH)4TH1.004.3026.156500.00117.604TH1.004.3026.156600.00119.414TH1.004.3026.156700.00121.224TH1.004.3026.156800.00123.034TH1.004.3026.156900.00124.844TH1.004.3026.157000.00126.654TH1.004.3027.246500.00122.504TH1.004.3027.246600.00124.394TH1.004.3027.246700.00126.274TH1.004.3027.246800.00128.154TH1.004.3027.246900.00130.04

[Crazzy_Al]

Originally Posted by NTIMID8

GEARGEAR RATIOFINAL DRIVE RATIOTIRE DIAMETER ENGINE RPMCALCULATED SPEED @ RPM(GR)(FDR)(OD) (IN)(RPM)(MPH)4TH1.004.3026.156500.00117.604TH1.004.3026.156600.00119.414TH1.004.3026.156700.00121.224TH1.004.3026.156800.00123.034TH1.004.3026.156900.00124.844TH1.004.3026.157000.00126.654TH1.004.3027.246500.00122.504TH1.004.3027.246600.00124.394TH1.004.3027.246700.00126.274TH1.004.3027.246800.00128.154TH1.004.3027.246900.00130.04

What?!?!

[99SVTAddict]

Funny stuff Prick.

Peak power is at 7,000RPM so I was shifting at redline.
The tires I believe measure at 27x11.5x15 (a friends Hoosier Streets)

I had the tires at 16PSI. Launching around 4500 RPM's. I'm not sure if the tires were spinning or not as I had the helmet on and the car is loud. I did get a little tire shake on my 11.55 run in first gear around 5500RPM's but drove through it....got 2 hops.

Things were happening quick in my mind as I don't have much seat time going down the 1/4 mile.

Another thing I noticed...my wheel well lip was rubbing the tire by about 1/8 of an ich. I noticed it did this when I came down the driveway at an angle and all the weight was on one wheel. Does this sound like a BS problem from the wheel I was on and/or do the lips have to be rolled on the car.

[Strtracer]

Originally Posted by 99SVTAddict

Another thing I noticed...my wheel well lip was rubbing the tire by about 1/8 of an ich. I noticed it did this when I came down the driveway at an angle and all the weight was on one wheel. Does this sound like a BS problem from the wheel I was on and/or do the lips have to be rolled on the car.

Get under the car and see if you have room on the inside of the wheelwell, if so, you could change the back spacing to eliminate the problem, if not, then you need to roll the fender lip. My DR's do the same thing, but that will change when I replace them with the taller size. DOT tires are measure different than track only tires. A DOT 27x11.5 has a tread width of 9.5 inches, a 28x9 slick would have a 9" tread with, and a 26x10 slick would be 10" wide at the tread