Expanding on my previous post, Porsche lists the transmission ration for 6th gear as 0.75:1.

They list the Final Drive as 3.44:1

I am unclear on the applicability of "Final Drive".
Anyone?

 

If you divide RPM's by final gear and divide that by 10, you come up with..

(7000/3.44)/10= 203.48

Not sure that is how it's done...sort of a reverse engineering stab in the dark...

 

Ruf-Racer , you are almost correct .. first of all its 6400RPM ,
speed = rpm/gear/tire , what about gravity ? weight/Air ...
go and sue porsche for the speedo ? if this was in a ferrari then the speedo would of read 230MPH or somewhere in there .. porsche speedos are very accurate .. im going for another run this week and il let someone stand with a speed gun and get the accurate speed too .. also i will have a GPS mounted in car to make sure .. if the speedo is wrong , then im sure iit will only be off by a couple of miles no more .. will find out this week

 

Wear a seat belt and mount the video cam so you can keep both hands on the wheel my friend.

 

White tt ... You crazy bastard. Ok now do it again with a gps system in the car. The speeds from those are good to within .1 of mph.

I'm glad I was patient and waited for you. You crazy dude, Buddy your brother?

Seriously don’t do it again, I don't want that on my conscience.

 

The tyres might make a difference as you mentioned at the start of this post. If they are smaller or larger then the mph on the speedo wont be totally acurate as the wheels will no longer be rotating at the same rate.

 

OK, here is my calculation:
Given:

• 640mm diameter tires (Yoko 315's)
• 6700rpm redline looking in the back of the owner's manual at the horsepower curve cutoff on page 208
• 6th gear ratio of 0.75:1
• Final Drive ratio of 3.44:1

We have:

Wheel circumference = Pi*d=2010.62mm

And,

(Engine speed)*(Gear Ratio)*(Min/Hr conversion)*(Length traveled/wheel revolution)*(Mile/MM conversion)/(Final Drive Ratio)=Maximum possible mph based on gearing

Or, the real numbers,

(6700rev/min)*(1/0.75)*(60min/hr)*(2010.62mm/rev)*(1mile/1609344mm)/3.44=194.66mph

Porsche states a 189mph top speed which would exactly equate to a redline engine speed of 6500rpm so I think my calculation is accurate given my calculation uses 6700rpm as a redline.

So, one of several things is odd.

1. The Porsche speedo is wrong by a large margin (10 mph or more at the upper end)
2. White changed his gears
3. White went beyond redline (but I thought you can't do that with the rev limiter)
4. Other

Disclaimer:
I was just curious so I ran the numbers myself. I do not mean any disrespect towards anyone and I am not making any implications.

 

Okie, thats still pretty dam fast either way.

Another thing to consider is excessive tyre wear will change the diameter. Hopefully the tyres are in good condition for that speed though

The law over here in Uk, I think states that a speedometer may be out by 5% either way, when supplied.

 

5% of 200 mph = 10 mph

 

My top speed at 7000 rpm's wih 315 tires is 189-190 mph. That is very close R.

 

Chad,
By my calculation, at 7000rpm's and assuming tire diameter of 640mm, your top speed should be 203.4mph.

 

>>>Ruf,
You picqued some interest here since I hae always been interested in top speed runs. I would like to go through the process of calculating the theoretical top speed of a stock geared 996TT at redline. What is the ratio of engine rpm to wheel rpm in 6th gear?? Anybody??<<<

Ken,
Rear end ratio = 3.44 & top gear=0.75 therefore final drive ratio = 3.44 x 0.75= 2.58 final drive ratio in top gear! My Ruf top gear =0.70 = 2.408 final drive! OK?
an 18" P Zero footprint is 6.315'. An easy way to grind out numbers is to go to tirerack and look up the "revs per mile" listed for the tire of interest. The tirerack data are off by a few per cent and I prefer the Pi D method.

FWIW I know after at least 150 runs above 200MPH my speedo (both digital & analog) is totally out in the trees! I also know my Garmin GP3+,with a high gain external antenna is quite accurate. It's a toss up between a radar gun and the GPS as to which it more accurate if the radar gun is properly used. Much error may be introduced with the radar gun due to "cosine angle." I also know that after many runs through a carefully calibrated timing trap the results match very closely my dual Garmin GPs set up! Hope that helps.

Peace

Roland

PS
Ken,feel free to mail me. I will gladly share the little bit I know about running fast! OK?

 

Exactly, i've never heard of anyone use a stock speedo to determine speeds over 180-190. Everyone uses GPS, look at all the crotch rockets using them, go to any bike meet and nearly everyone has them.





btw, Roland, been lookin for you at donut derlicts lately, haven't seen you around. Got any new cars I should be looking for? I think I may of seen you drive by in the RUF at west bay cafe in sunset the other week. But for some reason i thought you might of sold it.

 

I knew better that my pathetic attempt at the speed calculation was ridiculous(coincidental that it was relatively in the ball park). I was going to delete my post for embarassment reasons, particularly since I scored an A in Calc 3 in college...Figured I would just take my medicine...

 

Hey Soon,

Yea that was me! Jay the west bay owner is a golf partner. Nothing new,same old boring machines!Look me up in the Ruf Corral @ Dunkels' on 2/1/04.

R