Tom W

Andy: You are assuming that your speedometer is accurate at that speed (which is highly unlikely). A better way to get your speed is to use a GPS based system. On my track car, if I use the wheel speed sensors and the published tire diameter to calculate ground speed (what your speedometer is doing) I get values that can be off by almost 10% at about 100 mph (when compared to what the GPS tells me).

wellcraft290

I can't believe we are actually talking about only going 152 mph hehe

I think we will have problems with tope end speed depending on the car gears, horsepower and good old physics of wind resitance and running out of horsepower to push thru the wind

RaveDisc

Mine is a C4, so it has a G64 ? What kind of gear ratio is that ?

@Tom W : Yes, you are right. But still, my problem is that the car can't go more than 6'200rpms in 5th. I have a gps so I will try to test it anyway.

But this is incredible, 150mph would mean 240kph real speed. Like a honda s2000 (I give a lot of credit to the s2k but hey... we're talking about 3.6 flat-6 engine masterpiece! ) ... Even my beretta GTZ isn't that far...

@Wellcraft290 : I really hope wind may have been the problem last time I checked it on the freeway because there was some wind. I guess I'll try again !

Let me think, maybe I should get myself on a diet and weight 120pounds, take the seats out (of course the driver seat also !), A/C, fill the tank with about 1 gallon of fuel, and then maybe I can hope to attain 6'700rpms in 5th (in a tunnel) ! Hahaha... Just joking..

Have a nice day,
Andy

Bill Verburg

The g64/00 used on 964 C4
has an 8:31 cwp, and slightly shorter gearing in 2 and 3 than the g50/03 used in 964 C2.

Internally the transmissions have very different designs and in general do not share parts or layout.

Bull

"Or is it possible that my 16" wheels are limiting the potential of the car at high speed and that 17" wheels would improve it ?"

16" vs. 17" wheels should not make a noticeable difference, assuming they are both using factory spec size tires, as the larger wheel/tire will have a very similar circumference due to a lower profile tire than the 16s. If anything, the wider 17s will lower the top speed due to slightly increased wind resistance.

Most speedometers are at least 2% - 5% optimistic. So, at 161mph they would read 164 to 169mph.

Many variables impact the top speed on any given day including options (weight), wind speed, road angle, tire pressures, condition of the drive train, etc. Then there are the variations in the accuracy of the fairly inaccurate devices you are using to measure speed, rpms, etc. Also, not every car of the same year, model, options, etc. came out of the Factory door with the exact same horsepower, drive train losses and top speed. I don't believe that + or - 5% of the Factory numbers would be out of spec.

RaveDisc

About the wheel size, I do think that 17" or 18" wheels would add top speed. The wheel is bigger, and the tire has a lower profile, so the circumference stays the same. Ok, but the bigger wheel creates more inertia/momentum, thus increasing the top speed.
On the other hand, a smaller wheel would permit better accelerations (vs. the bigger wheel that would restrict acceleration a little) IMO.

Okay, now the truth, the facts... I used the gps, with accuracy from 4-7meters...
At 100kph (62mph) it showed 99kph on GPS... so pretty accurate, huh ?
At 258kph (161mph), it showed 240kph (150mph) on GPS... That's a 7% difference.

I putted less fuel than last time, and some Octane booster, it didn't change the top speed for a single kph. I'm puzzled.

By the way, I wanted to try how the car brakes at such speed, it's brilliant. But since I did it, the car mileage (total mileage and trip mileage) is increasing faster than before and the needle for speedometer is behaving differently.
Did it happen to you ?

Andy