The Rant - Grrrrrr!!

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Famine
Remember, wheels are part of your gearing... You'd only need an 18% bigger wheel/tyre combo - that's the equivalent of an 18" rim with the same profile on a car originally shipped with a 15" rim - to see that effect, and Habit is wearing colossal Hoosier drag slicks... Compare them to the stock 15x5.5 60-series the Challenger usually wore (they're the ones on the front, if that helps)...
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Thats a valid point. I still dont think your going to gain 20mph+. Im pretty sure that would only produce a gain of about 5-10mph.
 
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It's percentages, rather than absolutes.

A 15x5.5, 60-series wheel/tyre has a rolling radius of about 71.6 inches, so for every time the axle is told to turn, the car moves 71.6 inches. At 131mph that'd be 1,931 revolutions per minute - so we're assuming that the powertrain results in a maximum turn rate of 1,931rpm on the hub.

Now in order for 1,931rpm on the hub to result in 155mph you'd need a tyre/wheel combination that is just 18% bigger - 155mph is 18% more than 131mph. So instead of being 23" tall (15" inner diameter plus two lots of 60% of 6.5 inches) and 71.6" round it needs to be 27" tall and 84.5" round. Take your pick. And then, you need to make sure you have enough power to punch through the air to reach the new geared maximum* - well, she has a 9.2 litre, 850hp V8.

Job done, as they say.


*Power required [hp, at hub] to reach given speed = given speed * [(weight [lb] * 0.0135) + (given speed * given speed * Coefficient of Drag * Frontal Area * local air density)]/375
 
Famine
And then, you need to make sure you have enough power to punch through the air to reach the new geared maximum* - well, she has a 9.2 litre, 850hp V8.

Job done, as they say.
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Quick question about this bit, oh wise purple one: drag is a factor in top speed as well as gearing, right? If the car has more power (why did I think of Tim Taylor when I typed that?) does this lessen the effect of drag?
 
No, it just allows you to overcome it.

Famine
*Power required [hp, at hub] to reach given speed = given speed * [(weight [lb] * 0.0135) + (given speed * given speed * Coefficient of Drag * Frontal Area * local air density)]/375
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Quick'n'dirty example. You have a car with a drag coefficient of 0.31, a frontal area of 20.5 square feet, a weight of 2,200lb and 170hp crank giving 145hp wheel. Can it do, say, 145mph?

Power required [hp, at hub] to reach given speed = given speed * [(weight [lb] * 0.0135) + (given speed * given speed * Coefficient of Drag * Frontal Area * local air density)]/375

Power required [hp, at hub] to reach 145mph = 145 * [(2200lb * 0.0135) + (145 * 145 * 0.31 * 20.5 * 0.00256)]/375

Power required [hp, at hub] to reach 145mph = 145 * [(29.7lb) + (342.1lb)]/375

Power required [hp, at hub] to reach 145mph = 145 * 371.8/375

Power required [hp, at hub] to reach 145mph = 143.7hp

Or "yes, just". Where gearing comes into play here is that the car needs to be producing the 145hp peak it produces at the crank at exactly the same point in the rev range that the engine is turning fast enough to turn the input shaft of the gearbox fast enough to turn the output shaft of the gearbox fast enough to turn the wheels fast enough to do that 145mph - you need that power to punch through the air at that speed, so producing it later or earlier will limit the actual speed you can do (known as the aero limit - the limit caused by the aerodynamics of the car).

So let's say it produces the 170hp crank peak at 6,500rpm. It would need to be geared, in top, to 22.3mph per 1,000rpm crank on stock wheels (as discussed above, changing wheel size will change gearing). Assuming the wheels are 16"x7 with 225 section, 50 profile tyres - which would be a circumference of 78.1" - they would need to turn at a rate of 1,961rpm while the engine is turning at 6,500rpm, which is a gearing step-down of 3.315:1. Taking a typical top gear ratio to be 0.85:1, it would need a Final drive of 3.900:1 to achieve the 145mph at the appropriate point on the engine's rev range.


Looks massively complex, but it's really just a case of punching numbers in. I should add that there are two assumptions in the power calculation which vary slightly depending on local conditions - air density, for instance - but while denser air is harder to punch through, it also makes more power for the engine. So those calculations are plus or minus a couple of percent.
 
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Ah right. So, other than when we're talking about cars trying to beat the landspeed record, do cars ever reach the point where their aerodynamics are the only limiting factor to the top speed? Like they hit their terminal velocity, sort of thing?

What about adding a NOS sticker and doing a single wiper conversion? How much faster will a car go then?
 
Hit the nail on the head! Skylines, Supras e.t.c should be able to have the tuning options they have in real life/old GT's. I would love to see 1200bhp Skylines leaving Veyrons for dead on SSR7 and 700bhp Time Attack spec Evo's and Scoobies competing for quickest times on any track!
 
ScouserInExile
Ah right. So, other than when we're talking about cars trying to beat the landspeed record, do cars ever reach the point where their aerodynamics are the only limiting factor to the top speed? Like they hit their terminal velocity, sort of thing?
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Sure. Most road cars will reach an aero limit before they reach a geared limit - an aero limit won't result in bits of mechanicals falling off at great speed, whereas running at the top of your rev range and gearing for any significant length of time will be quite terminal for many components.

ScouserInExile
What about adding a NOS sticker and doing a single wiper conversion? How much faster will a car go then?
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5hp per sticker and the wiper reduces the Cd. Just need to fold the back seats down for a lower centre of gravity and you're sorted.

Incidentally, if you know your aero limit and know that you've got gearing left, you can work out how much faster your car will go with more power in just a couple of steps:

New speed = Cube root of (New power/old power) * old speed

So if we give the above 145hp wheel car another 35hp at the wheels:

New speed = Cube root of (180hp/145hp) * 145mph

New speed = Cube root of 1.241 * 145mph

New speed = 1.075 * 145mph

New speed = 155.8mph

Air resistance increase by a cube of speed, you see (look in the above calculation - you'll see the speed occurs three times as a multiplier), so to go 10% (1.1 times) faster you need 33.1% (1.1 * 1.1 * 1.1) more power.
 

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