If you'd like to figure it out, use the following formula:

(Max RPM x Tire Diameter) / 866.88*

18" tire diameters:

295/30/18 = 24.96
295/35/18 = 26.13
305/30/18 = 25.20
315/30/18 = 25.44

19" tire diameters:

295/30/19 = 25.96
305/30/19 = 26.20
315/30/19 = 26.44

866.88 is the Absolute Final Drive Ratio (6th gear ratio of 0.75 x final drive ratio of 3.44 x 336). This will be the same for all TT's and GT2's that are running stock gears.

Now, remember...all of this is assuming that your car has enough power to reach it's theoretical top speed.

I figured out mine today.

With my current 295/35/18 rear tires on, my top speed should be 235 mph at 7,800 rpm in 6th gear. If I bump my redline to 8k, the top speed jumps to 241 mph.

With 315/30/18's, it should be 229 mph at 7,800 rpm, and 234 mph at 8k rpm.

Now, how much power does it require to hit your top speed?

This formula tells you how much HP is required to hit certain speeds.

(Frontal Area) x (Drag Coefficient) x .000007 x Speed you want to achieve (cubed) = Wheel HP required.

Insertig the correct numbers above, we get the following:

-To hit the stock top speed of 189 mph in a 996TT, it requires 344 HP to the wheels.

-To hit 235 mph in a 996TT, it would take 661 HP to the wheels.

-To hit 235 mph in a GT2 (higher drag coefficient; .34 vs .31 for the 996TT), it would take 725 HP to the wheels.

-And for a 996TT to hit 253 MPH like the Veyron did, it would require 825 HP to the wheels. A 996 GT2 would require 905 HP to the wheels.

 

Scott,

What if you were to change your gears to something that would allow more mph? Wouldn't that also effect how fast you could go?

 

Buddy,

Yes. If you simply swapped 6th gear for a taller gear (lower numerical ratio), it would allow you to have a higher top speed, assuming you had enough power to overcome the wind resistance.

 

And there is a lot of wind resistance at 200+mph!

 

I just added the HP required to achieve the theoretical top speed in the first post.

 

>>866.88 is the Absolute Final Drive Ratio (6th gear ratio of 0.75 x final drive ratio of 3.44).<<

I think 0.75 x 3.44 = 2.58?

Calculating my 285/30/18 (rolling footprint = 6.315') & final drive = 0.70 x 3.44 yields 205.87 mph @ 7000 RPM. I have observed that # 205 & know it to be true.


Cheers

R

 

I'm completly speculating, so don't shoot the noob with the question. Won't the tires also "grow" with the centrifigul force at that speed meaning that if the wheel is still spinning that fast, you'll actually be going even faster yet? I mean, it couldn't grow as much as a drag slick, but even if it went up 1/8", that's a quarter inch total for the tire!! Of course it could be the other way too, downforce squishing the tire smaller...... OK, too many variables, your way is easier.

 

Thanks. I accidentally left one number out of the formula in my first post; 336 (its there now).

You multiply .75 x 3.44 (= 2.58) x 336 = 866.88. 866.88 will always be the number you divide your max rpm x tire diameter by if you have a stock geared 996TT or GT2.

So, for your car: 24.7 (your tire diameter) x 7,000 rpm / .75 x 3.44 = 2.58 x 336 (866.88) = 199.4 mph. 205 mph would only be possible if your redline is 7,200 rpm.

Also, to reach 205 mph, it would require 440 HP to the wheels (535 BHP).

 

LOL..ya, figuring tire height at speed is way beyond my capabilities.

 

Yes. Tires will bulge out at high speeds. Also, different tires of the
same nominal size may have serious differences in real diameter.
The main point here is that the power needed increases with the
third power of the velocity, so the small apparent difference between
two cars, one that can get XXX top mph and another that gets XXX+4
top mph may actually be a huge difference in day-to-day power for
acceleration at street speeds. In other words, top speed is not a
very useful distinguisher of real-world car performance.

 

I knew how you got there with the numbers..... BTW,my P Zero footprint is exactly 6.315' as measured. Top gear ratio IS 0.70... I have both radar & GPS events @ 211 mph ...7350 is my redline compliments of Mr.Ruf.


Cheers

R

P.S.
tire growth does not seem to be a factor at high speed......

 

I completely agree, Joe. And neither is 0-60.

 

Ahh, 7,350 rpm works out perfectly. Thanks!

 

Scott,
Cool stuff, many of us would love to TRY our top speed,
and Mr. Roland Roth has (Ruf Racer). Anybody that wants to do the Silver State
Classic or other Open Road Racing events should talk to him. He has gone
over 200 mph many, many times (even more than that wild talkin Viper guy on 6speed (Vicious).)

 

I would definitely prefer to know 0-60 than top speed. However
what I'd like to know would be 20mph to redline, from a steady
20 mph in second gear. That would capture power without worrying
about a launch or shifting. The steady start would show whether
there was lag.
Some of our group's super cars might have traction problems,
but not too much I hope. This speed range (20-70) is where I
live, anyway.

Joe