Weight Does matter?

[wwhalenc]

I underestimated the effect of weight on fuel economy.
I recently changed swapped my TDI Drivetrain from a '91 Jetta into an '84 Scirocco. Here's the deal:
While it doesn't look so, the Jetta in fact has much better aerodynamics than the slippery *looking* scirocco. However, the car was pretty heavy...with ME and a lot of crap in the car it was pushing 2800 at the track. I figure minus the crap it probably was weighing in at about 2500 at least. I think I was running 185/60/14. I was getting about 45-47 mpg with regular mixed driving
On the other hand, the rocco couldn't weigh more than 2200 as it stands. It's pretty lean right now...no power stuff...still no bumpers. 3000 miles post swap I'm getting a solid 49-51 mph mixed city/highway/mountain driving.
Conclusion: per the math you one would NOT expect that dramatic a difference from the weight difference...but there it is ... about a 10% increase. One would thing the inferior aero of the rocco would've trumped it's weight savings with any signifcant amount of highway driving. I figured the scirocco should have the edge for local driving (less fuel consumed to accellerate the car), with the jetta winning out on the highway where aero quickly becomes paramount to everything as speeds get high.

Possible confounding factors: No EGR, no cat on the rocco (this in an '84 car); Scirocco is much lower than it even should be which might closing the aero gap between the two cars;
I think rolling resistance is probably a little better on the rocco since it's lighter and it's running 195/60/14 (not sure what brand I had before, but on size alone these should have lower rolling resistance)

Alternate explanation: No EGR, No Cat in combination with the weight savings and Decreased rolling resistance (wider tires + lighter car) may be able to explain this.

 

[ThatRickGuy]

I would think that wider tires would increase your rolling resistance if anything.

Weight is important. At the track the common line is 100lbs = .1 seconds. There was even a great faked 'pimp my ride' article in some hot rod magazine a few years back. They took a rather blah import, threw spare tires on the back wheels (and tiny race wheels they traded rims for up front), used a sawzall to cut off the roof and door frames, tossed the back seat, lost the headlights, stereo, hood, trunk, upholstery, and I think the fenders too. With out sinking a dime into the engine they managed to cut a few full seconds off of the car's 1/4 mile times with just weight reduction and smaller wheels.

I know 1/4 times != mpg, but I would expect the energy savings to effect mpg to some extent (specificly in town driving.)

-Rick

 

[wwhalenc]

I think I saw that! Hilarious.
For mileage, the aero at highway speeds is hugely important (it increases per the Square of the speed, so after a while it starts ballooning the faster you go). Weight matters only in that it takes more energy to accelerate a heavier car, but once at speed the things that matter are
1) Wind resistance (gotten by Drag Cooefficient, Frontal surface area and Speed of car)
2) Friction losses (rolling resitance of tires, drivetrain friction)

I guess some other things are at work here too...the car's frontal surface area is probably smaller than the Jetta, so even though it has a larger Cd number, the overall wind resistance might be a wash (or even smaller?)

 

[Nitrowolf]

I believe the rule of thumb is 100lbs = 1mpg.

 

[Lug_Nut]

I reached a different conclusion

"Slippery" Cd is only one factor. This number must be multiplied by the frontal area to gain a valid number. A 747 has a much lower Cd than either 'dub, but one engine inlet alone has more frontal area than both cars combined.
The Ski-rock may have a few more points on the Cd, but the smaller frontal area mitigates most of that disadvantage. Put the bumper back on and restore the front ride height to limit air flow underneath.
My B4 have had more area than either, and more weight too, but still produced 48~52 mpg over the 360,000+ miles I drove.