489 R&P, good choice?
You have to know, you can not guess at what your lowest corner rpm is and what your highest rpm is before going into the corner.
I haven’t tuned anything that turns corners for 15 years but only one guy in 25 knows what those rpm numbers are.
If your lowest rpm is 3500 and your highest is 6500 you can figure the gear ratio.
Really, the lowest rpm is for you to tune from. It doesn’t affect rear axle ratio but that number is critical
Any thing else is just a guess.
I’d say ask around where you race what gear ratio they are using but I doubt they know what the rpm is either.
It's not really as simple as the lowest rpm. Much of the time is spent pedaling the car through the middle of the corner and exit depending on how slick the track gets. Your looking more for the rpm when you can get the pedal all the way down. You need to be in the power range of the motor when you get to full throttle.
First you need to figure out what the safe mechanical rpm limit is for your motor. How hard can you safely turn it which out breaking it? Is this number lower than the rule mandated 7000 rpm? If so then set the rev limiter accordingly.
Next find a cam that optimizes that rpm limit.
Last choose a gear that will optimize the power that the motor makes where you need the power.
You may hit the rev limiter before you lift for the corner. Through experimentation you will have to decide if the power off the corner offsets what you lose hitting the chip. We see this scenario with the crate motors all the time. They almost always are faster trading hitting the chip for more power off the corner.
What track is it? Eagle? What is your safe rpm limit? Does your cam match your rpm limit? are you on 70's or 75's? Does your driver have experience? Is he or she a top five or top 10 finisher or rookie? How heavy is the car with driver?
I have customers at the tracks near you. If you answer these questions I think we can get you a good starting place gear ratio.
The top 4 finishers each night are likely going to be within one ratio of each other when comparing crate to crate and open to open.
You should be on a floater. :)