I keep thinking the gearing determines top speed...- perhaps I'm wrong? Anyone care to take this one?

Cris

 

both, gearing is the limit, but without power gearing is useless

 

All things aside...bottom line is that top speed of a car really depends on how tall the final gearing is.

Cris

 

yup

 

Limitations and examples from the 2/98 article in Porsche UK magazine. There are, of course, other mechanical limitations that affect top speed (e.g.: lack of downforce devices making high-speed handling unpredictable, lack of fuel container rated for high-speed impact, etc. but these are not major physical limitations and do not affect most cars properly outfitted for high-speed work... it could be argued, of course, that tires also fall into this category...)

1) Tire speed rating
*) Use of a tire beyond its speed rating is dangerous and not encouraged by the factory. While this may not limit top speed on each run, a tire failure is catastrophic enough to warrant its listing here.
2) Aerodynamically limited
*) The car's drag coefficient is such that tremendous amounts of power would be required to significantly increase the car's speed. An examples is the Chrysler Viper RT/10 with a drag coefficient of 0.56 where the amounts of horsepower needed to push the car through the air more quickly would be tremendous compared to present engine output.
3) Gear limited
*) The car bounces off redline at the limit. Examples are short-geared cars, including the Porsche 930 Slantnose Comp E and other competition cars (especially hillclimb cars) not geared for high speed runs. Most competition cars that run at Silverstone are nearly gear limited, most competition cars that run at Le Mans are far from it.

 

Which is now probably the most common top speed determiner. All German cars except Porsche are limited to 155mph (250kph). Yes, even M5s, yes, even AMGs.

 

Most cars are aerodynamically (power) limited. Most do not get to the redline in top gear before petering out. All Audis, if they had no chip but no speed limiter would top out at less than redline in top gear.

By saying that gearing is the limiting factor, you are implying that by changing the gearing, the car might go faster. This is true of some cars (like the Vette Z06), but any increase in top speed on any Audi would be minor or zero just by changing gearing.

No, most cars that we speak of are electronically limited. Next most common would be power (aero) limited. With the high overdrives installed on cars now, hitting the redline in top gear isn't a problem very often.

 

my apr ECMS in stock mode has hit redline in 5th

 

if all are redline limited.... they would reach the same top speed, even if one has more hp than another, right? Granted, one would prob. reach top speed quicker than the other.

Cris

 

Especially the 4.2, at least on the non-S models. Those are geared very low to get good mpg.

The 2.7T probably gets closest due to its lowish gearing and good power.