Anyone know any more about this equation

[chi town brat]

A quote from the ecomodder website:

Blue Bomber Man said - To get an idea of the impact of lighter wheels, take your vehicles weight and divide it by your HP. This will tell you how much weight each HP drives. Say you have a 100 hp car that weighs 2000 lbs. Works out to 20 lbs/hp. So cutting 20 pounds from your car would be the equivalent to improving your engine by 1 hp.

Now this isn't really a comparison of apples to apples because the wheels are rotational mass. I think the conversion is roughly 4:1 so if you cut 20 pounds of wheel mass it is the same as cutting 80 pounds off the car.

A debate has been going on about wheel size, weight, and rotational mass. I have a set of 15" rota slips weighing in at 11.9 lbs. I am being told that I need to switch to a 14" Honda Civic HX wheel which weighs in at 11.75 lbs. The weight is negligible so then it goes to wheel mass that a 15" tire weighs more than the 14" tire.

I can see that argument but with the mentioned equation would it really make a difference?

[Loren]

I agree with the formula, as I've experienced it, and I've even seen the results of reduced inertial weight (flywheel and lightweight wheels) on a dyno.

The difference between 14 and 15" wheels that have the same weight, the same width tire, and the same overall diameter isn't going to be enough to worry about, though. The 15" tire may be a touch heavier, it all depends on what size and what model of tire you choose. The difference can be a pound... or it can be 3 pounds, just depends on the tire.

Odds are that you can ultimately find a narrower and lighter 14" tire, however. But, from what I've read, going with a low rolling resistance tire will pay off more than downsizing the tire.

Tires : Test Results : The Tire Rack

Even then, you're only looking at maybe 1-2 mpg difference for a typical car.

[omgwtfbbq!]

Would you notice the difference? Almost certainly not outside of a lab. There would be an added advantage from the decreased rotational inertia since the 14" wheels are smaller (obviously) than the 15" wheels.

[Loren]

You'd notice a 3 pound difference on a lightweight Honda, but I'm sure she already knows that.

1 pound difference? Nah. You'd probably not even be able to identify it statistically, it's just not going to make that much difference.

[SixCylinders]

The thing about reducing weight is you have to keep in mind what you are compromising. I'm planning to move up in wheel diameter, width, tire width and as a consequence rotational weight but I'm doing this for other reasons. My car is missing about 400-500lbs of dead weight, including cruise control which is one of those things that might help gas mileage....if it doesn't make me fall asleep and crash into a tree during a 50 mile plus drive.

The lower you go in weight the more it matters of course, going from 6000 to 5800 is laughable in a full sized truck but even 100lbs from an old VW (that weighs under 1800lbs) can be HUGE. F1 teams at some point complained to Champion I believe that the spark plug was THE densest part in their cars, as a result the spark plug design seen to the left was put into production, on the far right is a Ford Taurus plug for comparison's sake.

http://www.caranddriver.com/var/ezwe...line_image.jpg

You just have to ask yourself "how serious am I about this?".

[Loren]

Cruise control only helps fuel economy if you're a driver with a lead foot. Using technology to keep your speed in check will return better economy than driving 10-15-20 mph faster.

But, if you're capable of employing "hypermiling" techniques on your own, you'll return at least 10% better than cruise control will.

[chi town brat]

You'd notice a 3 pound difference on a lightweight Honda, but I'm sure she already knows that.

1 pound difference? Nah. You'd probably not even be able to identify it statistically, it's just not going to make that much difference.

The difference between 14 and 15" wheels that have the same weight, the same width tire, and the same overall diameter isn't going to be enough to worry about, though. The 15" tire may be a touch heavier, it all depends on what size and what model of tire you choose. The difference can be a pound... or it can be 3 pounds, just depends on the tire.

Would I notice a 3lb difference is an interesting question to me. My friend wants to put his HX wheels on my CRX once we find some spacers as I have an Si model and the wheels won't clear my brake calipers. I guess the road test will tell all.

The Model of tire has been difficult to choose. I need something for the street that is good to go on the drag strip as well. I am looking at the Falken Azeni 65-K. I wonder what the weight difference is from the 14" to the 15"?

[chi town brat]

The thing about reducing weight is you have to keep in mind what you are compromising. I'm planning to move up in wheel diameter, width, tire width and as a consequence rotational weight but I'm doing this for other reasons. My car is missing about 400-500lbs of dead weight, including cruise control which is one of those things that might help gas mileage....if it doesn't make me fall asleep and crash into a tree during a 50 mile plus drive.

The lower you go in weight the more it matters of course, going from 6000 to 5800 is laughable in a full sized truck but even 100lbs from an old VW (that weighs under 1800lbs) can be HUGE. F1 teams at some point complained to Champion I believe that the spark plug was THE densest part in their cars, as a result the spark plug design seen to the left was put into production, on the far right is a Ford Taurus plug for comparison's sake.

http://www.caranddriver.com/var/ezwe...line_image.jpg

You just have to ask yourself "how serious am I about this?".

Cruise control scares the hell out of me. The point of driving is to be in control of the car at all times. Well that is for me anyway.

I've never seen a spark plug design like that before.

[omgwtfbbq!]

Cruise control yields better MPG for my car. But its kind of an odd case. Cruising on the interstate at constant speed with/without cruise yields a 2-3 mpg difference on the instantaneous MPG readout. I think it has something to do with the ECU controlling the throttle and having a good idea about whats coming next as far as injector timing, etc.

[Loren]

Angela, I'm pretty sure you'll notice 3 pounds difference in several ways. Let us know.

On the tangent topic of cruise control, CC will probably appear to be giving you better MPG as long as the road is straight and flat. Where it fails is when you introduce hills. CC wants to maintain speed above all else, that's its mission in life. So, when you start up a hill and speed starts to decline, CC punches the throttle as hard as necessary to maintain speed. Now, you, as a mpg-concious hypermiler, will see the hill before you get there and maybe breathe on the throttle just a little more to gain 5 mph before you get to the hill... and then allow the car to lose 5-10 mph on the way up the hill, and then on the downhill side, you'll gently regain your speed and then some... all while using less fuel than your CC would have.

[omgwtfbbq!]

I didn't try the comparison in the mountains, but I did notice my mountainous terrain MPG was significantly higher than the flat terrain MPG, both with cruise control. I believe that was the record run for my entry on the fuel mileage list.

[redinjuns]

Maybe the cc issue needs its own thread,... but what the hell.
It helps me in the work truck, usually by a couple of mpgs but I don't let do all it likes. As I start up the numerous hills along my drive I'll hit the decel button as I feel the truck losing momentum. If i get it right the truck will slow somewhat, and not downshift it stays in the OD gear and lugs abit. After cresting I can use the accel button to regain my speed in the OD gear. The key is to make the computer not downshift, and you have to hit the button slow to do that.

[kegaration]

awsome good question
i think we got apples and oranges and peanuts all mixed up
and the example just was close enough to look right
im shure some smart bastard can anser this correct and we wont know what the hell happend
i just look at the 2 kinds of weight that make sense to me spinning mass (tires and shit)
and payload (people seat spare tire)
hp is and old school measurement and is open to a whole new bitch fest
i use mechanical and i think its the ability to move 550 pounds one foot in one second or so

i would just weight the bastards and pick the lightest package
where the weight placed on the spinning mass is all the same if the weight is the same and you need x amount to move it
if its 5 feet long or 6 inches the leverage will work if the weight is the same

will you notice 3 pounds i guess it depends on you
there are other things that will impact it more