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Discussion Starter · #1 ·
by mounting my bike on the back end of the car? I bought a trunk mt bike rack today and mounted my bike up since I have some riding to do this weekend and during hwy driving I was around 34 mpg today. I just made my 2nd fill up but I have not yet got this low on any single trip. Car was going from green to blue with cruise on 65 mph. Could there be that much added drag? The rack does sit high up on the car
 

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I would expect a 5 to 10% decrease in fuel economy as a result of the increased drag of a roof rack or bike rack. On a 15 mpg truck this would be 0.75 to 1.5 mpg, but on a 35 mpg CRZ, it would be 1.75 to 3.5 mpg decrease.
 

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It won't have a % impact on mpg, but it will add l/km (i.e on a 15mpg truck, the effect is not noticable, but with a 40mph car, the effect will be significant). With my Rx7, top speed dropped from 150 mph to about 130 with either a (folding) bike on a boot carrier, or the roof down. Wasn't looking at mpg's or the price of filling up :)
 

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Discussion Starter · #8 ·
After 2 days of driving around with the bike on the car I am at less than 34mpg on this tank. Since I am done with my charity ride for the weekend It is now off the car and We will see how much it goes up. I guess the other variable that goes into this is that I filled up at the same time I put the bike on the car. Still on the same tank of gas so We will see if it immediately goes back up.
 

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Discussion Starter · #9 ·
Well, Bike rack off the car and just made a 70 mile round trip. 40.2 mpg on that trip so I guess the bike on the car was about the only explanation. I knew it was gonna hurt the mileage a little but did not think it was going to be that much.
 

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Discussion Starter · #14 ·
The deal with the bike is the rack I found that fit snug enough holds the bike very high and actually the top tube, handlebars, and seat of the bike are actually above the car. It also appears that the bike is longer than the car is wide so each wheel is catching some wind coming down the sides of the car. Putting the bike in the back of the car was a very tight fit even with the front wheel off so I had to find a way to mount it. I talked to a couple of places about a hitch receiver for a bike rack and no one wanted to put one on it.
 

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Would the factory spoiler add additional drag and reduce milage? I was thinking of adding it, but am interested how much it will affect milage.
Aerodynamics are a very subtle thing. Something like a spoiler, applied at the end of a hatch could potentially help or hurt aero. But you would not know unless you tested in a wind tunnel. But I don't think that the factory style spoiler would have much of an effect one way or the other. OTOH, someting as non aerodynamic as a bike rack or roof rack is going to have a pretty significant impact.

Keep in mind the amount that areo effects your fuel economy will be mostly based on how fast you drive. If all your driving is at speeds of 50 mph or less, you may see little to no difference from hanging a bike rack or roof rack on your car. But if you drive interstates at 75 mph+, you might see as much as a 25% reduction in fuel economy.
 

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Discussion Starter · #16 ·
Aerodynamics are a very subtle thing. Something like a spoiler, applied at the end of a hatch could potentially help or hurt aero. But you would not know unless you tested in a wind tunnel. But I don't think that the factory style spoiler would have much of an effect one way or the other. OTOH, someting as non aerodynamic as a bike rack or roof rack is going to have a pretty significant impact.

Keep in mind the amount that areo effects your fuel economy will be mostly based on how fast you drive. If all your driving is at speeds of 50 mph or less, you may see little to no difference from hanging a bike rack or roof rack on your car. But if you drive interstates at 75 mph+, you might see as much as a 25% reduction in fuel economy.
mine was all at hwy speed. 65-70 mph. I am sure around town there would have been little affect
 
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