It;s funny how physicists and engineers often find it necessary to state things as absolutes and yet ignore the subtleties of their own fields. I have direct experience with this as my father is a particle physicist who insists that tires with tread have more grip on dry roads than slicks.


I actually agree with you on most points but your insistence on your own superior knowledge and your failure to take into account that you may possibly be fallible in some of your judgements is galling.

#1.
Right on, agreed, except that when you drive especially in the city a great deal of the fuel that you are using is in the overcoming of inertia. You yourself stated very correctly the by moving the weight of the tire farther away from the center of the rim you require more torque to overcome the inertia of the wheel and tire. Even if that tire is the same wieght as the stock, the torque required to overcome it's inertia would be higher. But we all know that with 19" wheels the tires will be significantly heavier, compounding this effect. Again you make a sweeping generalization about spoilers, yes a giant aluminum wing will most definitely hurt your mileage, as it a source of drag, but with a square back like that of the XB, a properly designed spoiler could actually decrease turbulence and therefore drag over the rear of the car and have the effect of increasing fuel mileage. Now choosing esthetics over function is also one of my pet peeves, but on the subject o larger wheels I often think that the compromise is worth it as the wheels often make the car.

#2.
Nice caps, and I agree, except that my inital response clearly stated that the rolling resistance of a wider tire has a huge effect, I am not aware of anyone selling a 185/35/19 tire, so ignoring this reality is silly.

#3
Alex Trebec, I think that's overstepping a little.

#4
The s2000 is fast because it has a low aerodynamic profile and it's gearing and horsepower. Blankly stating that horsepower makes you go fast and that torque makes you accelerate is being blind to the realities of gearing and aerodynamics. Why can a mercedes deisel with littel horsepower but a great deal of torque attain a high speed? Because it has good aerodynamics and gearing that places it's engine in the right places in the torque curve to attain those speeds.

#5
Why not just use the formula I gave? It tells you the same thing, of course if the peak power is @ 19,000 RPM they deliver low torque. And thanks for making my point about gearing that much more clear.


Again I agree on principle with your last point but again you ignore the subtleties.
No, bolt ons will not in most cases increase fuel mileage unless they lead to more complete combustion or the ability to use less throttle to get moving.
Efficiency does not necessarily increase with more throttle, there is a peak efficieny for each engine which is often around 3000-4000 RPM at a relatively small throttle opening. If you are flooring it at all times to accelerate you are going out of that efficiency range. Thi is why a buick with a pushrod 3.8 gets better gas mileage than a ford contour with an OHC 2.5, the 3.8 driver can ride around on lazy efficient torque while the contour driver has to give his engine more throttle (and therefore inject more gas) to get moving.

Your air throttle theory ignores the fact that when you give the engine more air you must at the same time give it more fuel to maintian the proper ratio and achieve combustion. Injecting more fuel over a certain amount of time will amazingly use more fuel, this is why flooring it all the time will net you terrible gas mileage. The reason that the cylinder deactivation schemes net better gas mileage is because they deactivate the cylinders under light load when the car is not floored and less acceleration is needed such as on the highway at a steady speed. They use less gas because they are not injecting gas into 1/2 of the cylinders and again we use less fuel.


I don't mean to flame either, but I will not simply knuckle under beacause someone claims to have superior credentials. I also think that your attempts to belittle me are clumsy and heavy handed.

 

what does starting off in 2nd gear have to do with gas mileage?? this has absolutely nothing to do with efficiency or mileage... YES it will be slower, but how can gearing cause worse mileage??

back up this statement with some sort of fact... PLEASE...

 

well, you and I can have a discussion and we can talk facts and figures but I used many a backspace to keep my information clear to the layman, not to be insulting to anyone but I am trying to expand people's comprehension... so the concept is my point, frustration is my anchor...

I understand your frustration with your father's tire theory... anyone who follows F1 racing knows the grooves are there to serve as a penalty... I will agree that the tread will help in anything other than dry/clean pavement, but for performance on a track - slicks win...

and to the points... to backup my explanations and to continue on your points....
#1... everyone seems to be going with the stock spoiler on the xB... so I'll stick to this one... it is a rear window deflector used to dry and clear the back window... it doesn't make downforce... it is there for asthetics... I agree the box needs help on the butt end... and I want larger rims, but ride quality and not blending with the ricers keeps me on 17's (i have to wait until the winter is over.. snow is due here in about 4 hours...)

#2... his original point was the MPG was killed by 19".. it's the 225 factor that is causing more of an issue... but really it comes down to essentially a gearing change and expecting to maintain the original performance/speed/accel... something I thought about today is that he is probably not accounting for the larger roll out of the wheel, so his calculated mileage will be less but the actual MPG will be closer than realized to the original figure...

#3... my point was that Alex serves as the interim to the knowlege... he isn't the smartest man on tv, but his teleprompter (read - source of information) makes him seem this way... not trying to come off as a jack ___, but the messenger always gets shot...

#4... the S2000 is not aerodynamic... sorry... it is "sleek" or "sexy" but the Cd is 0.32... the Insight is aerodynamic... Cd is 0.25... a bmw 3series is 0.31... it is quick because it is light and because it revs high... the combination of gearing and high rev limiter allows the engine to maintain a higher ratio to get more torque to the ground... 150ft-lbs ain't much, but a 13:1 1st gear makes gobbs of power... not to forget that this car can hang that gear until around 8500 rpm... this copares to the 11.8:1 1st gear in our xb's... so while the s2000 puts 1950 ft-lbs to the ground, the xb is putting down 1239... (drivetrain losses disregarded due to them canceling each other out in a comparison) I never balked at the gearing effect, but here's numbers for perusal...

#5... okay, here's the difference between torque and hp... standard kid's merry-go-round at the partk... 20' in diameter... put your hand on the outside and push tangential to the circle... you just applied torque, even before the unit moved... you applied a force AT a distance... now take the same force you just applied and apply it continuously as spin begins, measure the distance your hand moves, multiply... this is work... called horsepower... because you applied a force OVER a distance... no matter what you do to a stock xb, the car will hit a terminal velocity, probably around 125 or 130... no matter how you change the gearing the car will NEVER NEVER NEVER break this speed... because the aerodynamic drag is slowing the car equal (we'll call this negative work) to the amount of avialable horsepower (positive work)... gear ratios don't up horsepower - the ultimate setup will attempt to place the peak horsepower and the gear ratio to work out at the terminal velocity... so if you are spinning 6400rpm at 125mph... you could use a better gearing to get the engine down to 6000rpm at maybe 127mph... this isn't changing the available horsepower it is letting the engine run most efficiently when you need the most power...

on the air throttle and engine efficiency points... you can't compare two unlike engines... take a 4 cylinder and an 8 cylinder that use the exact same technologies/setup and you will then have an apples to apples comparison... your analogy ignores this point... you are taking two engines designed for different performance and attempting to get them to aim at efficiency... my point is that an engine that is sized for 70mph will get better mileage than an engine that is designed to pull a boat and hit 13 in the 1/4... same vehicle with my two engines and the 2nd car will ALWAYS be operating in a more efficient range, the throttle pedal reduces volumetric efficiency which is how we keep the car from speeding away... but for MPG purposes a 1.5l will always trump a 3.0l in the same vehicle... (as stated before - all other factors being equal)

hopefully this post clarifies some of my loose ends, but my point remains the same... MPG is next to impossible to bolt on...

 

crap, that post made me tired...

 

Wow I actually almost entirely agree with every point, you have made your statements much clearer. I still disagree that aerodynamics have nothing to do with top speed, but your discussion of gearing is very agreeable. Although I bet that the XB due to it;s upright profile would top out at less than 125 even if it was geared for max engine revs at max speed with the stock motor. I appreciate your dropping of the tone, and I thank you for the extended discussion. Oddly based on my own thinking I would perform much the same modifications I'm going to go with 17" wheels as light as possible and with an eye towards tire weight. It would be interesting to see someone produce a rear spoiller that would address teh rear turbulence on the XB as it might make quite a bit of difference.


Ok good talking with ya back to pullign my exhaust off the Subaru, anyone want a turboxs turboback for a WRX ?

 

I didn't say that aerodynamics isn't a factor... the only thing that even came close is that the Cd on the S2000 is so crappy... top speed is still limited to the point where current hp = drag... so a high coefficient of drag will always produce a slower top end...

but sports cars are not trying to be "slick" (read - aerodynamic)... sports cars are trying to reduce downforce... the S2000 is a great wedge... it probably produces somewhere between 200 and 400 lbs of downforce at top speed... I know my '01 BMW felt more secure at 125mph than it did at 75... my point on aerodynamics is that cars that have low Cd get better mileage for two reasons... #1... they are aerodynamic... and #2 that are designed to get better mileage so a better engine (read - more efficient) is chosen for the application...

 

I think sportscars are tryin to reduce lift but I could be wrong, I'm not aware that the s2000 has any significant downforce , and a .32 is not that crappy of a CD especially for a convertible, hell my subaru had a .g33.
But this horse is dead, I'm tired of beating it..