I have attached a zip'd XLS which will give you the numbers you want.
Plug in tyre width, section and wheel diameter and you get speed in each gear - I have arbitrarily set rpm to 7000 in all gears apart from 6th which is 6500, but you can fiddle with the values to your hearts content. Diff ratios included are 3.7 and 4.08 to 1. Just change the pinion/ring gear tooth counts for other ratios.
Vmax in my car has never exceeded 250kph (+ or - a few kph) in either 5th or 6th (at Philip Island GP Circuit which has a slightly down-hill main straight which you come onto at 180-190kph). Without a tail-wind, 5th is higher Vmax.
This thread has confused torque (which is the available force which can be applied to move/accelerate the car - i.e.: increase speed) and horsepower which the rate at which work can be done (i.e. affects max speed).
It is torque which sets the rate at which a vehicle is accelerated and hp (torque in lbs/ft x engine rpms/5252) which is doing the work of pushing the air out of the path of the accelerating vehicle and ultimately sets maximum speed - when hp and air resistance reach equilibrium. NOTE: Air-resistance varies as the square of vehicle speed - double the speed requires 4 times the power.
Gearing comes in to play as it affects the engine speed (and it is the amount of torque available at a given engine speed which determines the amount of power available to push the air out of the way of the moving vehicle bodywork).
The bottom line (depending on additional variables such as wheel drag - wider front tyres will always go a tiny bit slower that narrower tyres), is that horsepower available (given the gearing) in a 6MT and stock engine runs out somewhere around 255kph, give or take 10kph depending on relative wind-speed (tail or head-wind and how much). Ultra lo-rolling resistance tyres will improve Vmax by a few %.
In my own case, the car will run faster into T1 at PI in 5th (will pull red line give or take 100rpms) and 6th is a tad slower because of taller gearing.
If gearing is too short (higher numerical number for diff ratio) it is possible to "run out of revs" in that available power is greater than that required to push the air out of the way f the body when the engine reaches its max rpms.
Last edited by BGTV8; 01-22-2015 at 07:42 PM.
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