effects of rake on MPG??

[s1120]

What, if any effect would you all think a little rear up rake make on a 94 Saturn SL2?? Not a lot, but now it sets pretty level, and Im tossing around the idea of putting a tire on the rear thats a little thinner, and taller then stock. Input?? What do you think?

[GasSavers_Erik]

HolyCow made a pretty convincing argument that wider tires have less rolling resistance than thinner tires (less sidewall flex). Unfortunately this was after I switched to thinner tires...

Taller tires allow more air under the car and boosting up the rear end will make the windshield steeper in relation to the airstream.

When I put taller tires on an 87 Acura integra, I actually noticed a degrease in MPG (even after I corrected for odometer discrepancy).

That's my 2 cents- but others may have a different opinion.

[theholycow]

Here's some of the aerodynamic considerations I can think of...

- "Cali lean" (rear end lower) would expose the undercarriage as some NASTY frontal area.

- Some "Texas rake" (rear end higher) should come out about the same as level, with a slight effect of more rear clearance allowing more air under. That is, unless more rear clearance doesn't bring in air, and the air underneath is effectively pulled down away from the undercarriage, which would be good...but I doubt it. For that effect, you probably need an extended air dam and maybe side skirts.

- The side of the car is convex, and changing the angle of attack means the air will travel over the hump instead of straight parallel to it.

- The rear window may benefit. Others have posted better info on this, but the optimum angle for the rear of the car is something like 11 degrees. Your SL2 is more of a notchback shape, sloping sharply down and then flattening.

Also, thinner tires may help aerodynamically but they may have worse rolling resistance. There's a link in my sig about tire width where I've written at length about it. Data on it is inconclusive but does cause me to question my own thoughts on tire width. I think that in reality the difference between one model of tire and the next is larger than the difference in width.

[s1120]

Cool, thanks for the info!!!! I think once I get the weight out of the trunk. [I have a garden plow in there now!!!! ] I will do a good eyeball on the stock rake. I may put the bigger tires on any ways. I have them, and there not THAT much taller. [195/65r15 vs 195/60r15]

[Philip1]

My advice is a non raked drop in ride height this will reduce frontal area and reduce that air that gets underneath. I've also found that the underside of a saturn could use some help. Cover as much as you can it helps enough that you don't have to work at milage real hard. Have a look at my wagon and feel free to steal any ideas you think might work on yours.

[s1120]

Thanks. I will give a good look under the car and see where my trubble spots are.

[Improbcat]

Lowering the whole car overall can help, it effectively reduces your frontal area. I'm noticing that since I lowered my xB about 1.5" I am seeing a (slight) improvement in MPG, mostly by virtue of not having to work as hard to maintain the MPG I was getting.

Though I will admit, that wasn't why I put the lower springs in...

[Mayhim]

I've noticed in the truck that a few hundred pounds in the back on a highway trip will bring better mileage. This brought the truck to a more level attitude.

I can only think the trucks lower rear has somehow helped since there were no other differences.

[kamesama980]

Originally Posted by Mr Incredible

I've noticed in the truck that a few hundred pounds in the back on a highway trip will bring better mileage. This brought the truck to a more level attitude.

I can only think the trucks lower rear has somehow helped since there were no other differences.

It changes how hard the engine works. tiny increase in throttle plate opening may bring the engine into a slightly more efficient loading range. Kind of like my truck getting better mpg the faster I go, more weight, and harder I drive on the highway despite the lack of overdrive gears. as far as I can tell, the higher revs are more efficient combustion wise (since it's very short stroked for the bore) and it's 1/2-3/4 throttle to cruise has pretty low pumping losses AND it's throttle body injected so increased airflow makes the mixture much finer and more even. 60 mph steady cruise with tonneau: 27-28 mpg. 70-80 mph, carrying 600 lbs without tonneau, driving crazy in/out/with traffic is a solid 29-30 mpg

back to the original post... 60 to 65 sidewall is a 3% difference as far as rpms go. enough to mess up your ABS if you have it. however the total height difference is only about .67" so you'll be changing the car's height about 1/3"... NOT going to make a difference IMO. I'd put them on the drive axle and adjust your odometer readings and see how you do.