the Calculators that I posted were just for reference, I cant remember the exact numbers but once you reach 175 to go faster you have to increase your HP exponentially. unless you can decrease your drag. We have cars that run at Bonneville and we have learned first hand how important every little thing is. We even calculate how much drag the tires have since this is a open wheel car.

There are tons of things that matter, one more is that it will require more HP to drive anything once you get passed a 1:1 ratio.

James, since I do not come from a top speed enviroment, I always wondered if a overdrive and rear gear set up that equals a the final drive of a 1:1 drive ratio would pull the same. For example the final drive with a .064 overdrive on a 3.90 ratio would be equal to a 2.50 ratio 1:1 trans high gear. Would the overdrive 3.90 run the same as the 2.50 ratio 1:1? Would it take more horsepower to pull the overdrive 3.90?

"A car having a final drive of 2.98 and a direct 4th gear has the same overall ratio as a car with a .80 overdive 5th and 3.73 final drive. The car with the overdrive will use more horspower and generate more heat thru the transmission, then the direct drive box. However maybe the 3.73 rear may offer more low speed punch on turns."

took this from near the bottom of the page ----> http://www.5speeds.com/ratios.html

this is kinda a different answer than your looking for i think... but it sheds light in that area...

I guess I can understand the heat part pretty well, but it seems insignificant for a top speed blast. Actually, while it might be long term hard on parts, heat to some degree is good. For example you want heat in your gear boxes before you run you car be it at the dragstrip, roundy round, road coarse, etc. There is always a warm up period.

I really wonder about the extra horsepower to pull the overdrive. I am usually pretty good at figuring this stuff out, but this one has me stumped.

It would seem that if the final drive ratio is the same, then it is the same. I struggle to figure out where the horsepower absorption comes from in the overdrive situation, if the final ratio is the same. My assumption that is even if a rocket scientist does the equation, the net result is .07635 HP loss or something, LOL!

just a guess
 

I am thinking the overdrive would take more HP. Based on roatational energy and friction. Just for discussion. The drive shaft turns 2 times to get the wheels to turn one. 2 to 1 ratio. Lets say that it takes 4 rotation to turn the tires once, 4 to 1. Put an over drive on the 4 to 1 to make it a 2 to 1 and the drive shaft will need to spin 2 times faster or 8 times to equal the ratio along with the gears. More heat, friction, mechanical loss from the multiplication of the overdrive.

Just my guess.

NASCAR guys are running a 1:1 final drive.

You got it right and just for reference wind resistance cubes with each 1 mph increase. to put this in perspective it takes roughly 500 whp to get a 3rd or 4th gen camaro to hit 200 with out tkeing into account rolling resistance which squares with speed.

Close - wind resistance is a square function of velocity as in v^2. Rolling resistance is only a function of the normal force (with no exponents), which is just the force exerted vertically on the tires due to the mass of the vehicle. The only thing that might increase rolling resistance would be any sort of downforce a car might produce at speed. Otherwise, once a car is rolling and neglecting things like a tire's size change, the rolling resistance is the same at any speed.

As for a 'wall' at 150 mph - I wouldn't think of it as a wall necessarily, but I'd imagine that the forces get to be big enough that a car can take off and act like an airplane without the right aero work. There's really no need for any production vehicle to worry about these kinds of speeds unless you're talking Corvette, Camaro, Mustang, and few others. Joe Schmoe probably doesn't care, but lift forces big enough to counteract gravity = big lawsuit.

Ryan

At the same engine rpm,the OD trans would be spinning the ring & pinion(also driveshaft) alot faster = more friction & drag.

It takes approximately 450rwhp and the right gearing to push a C4 Corvette to 200mph. The aero characteristics become a little scary above 175,though.
My 90 ZR-1 with 420rwhp and a 3.45 rear cog would run 192mph in 5th gear (ZF6 trans). Doug Rippie was driving and I was in the passenger seat (with Simpson harnesses as tight as possible :lol: ).