How significantly do soft-sided roof luggage carriers affect mpg? I am looking for specific numbers if anyone has them. How about “aerodynamic” hard shell carriers?
The effect is noticeable but not huge. There is no universal estimate. Different cars different carriers, enclosed or open all make differences. Then there is the question of being able to remove the carrie when not in use, your personal driving habits. specific numbers will only apply to the specific car the specific speed of the car, load in/on the carrier etc.
Back in the 70’s when I had a carrier on my VW Bug, the empty carrier did produce some wind noise, but no noticeable mileage change, I seem to remember about 1 mpg on my car that averaged just under 30 mpg.
It depends upon the vehicle and how you are driving. A roof rack on a Hummer driving around town will not decrease mileage by much. On a Prius driving cross country at Interstate speeds will.
They both reduce mpg by increasing drag. Buy the one that you find easiest to put on and take off. Just put it on when you need it, then take it off. That is the best way to keep getting decent mpg.
If you buy a hard shell carrier and don’t have a place to store it you’ll keep it on the car empty. That is the worst option since you lose mpg all the time.
For what it’s worth, I drove from Mississippi to San Francisco with 2 bikes hanging on a trunk mount and returned without them. With a GPS keeping track of all the details the Toyota Camry got less than 28 mpg with the bikes from stop to stop and average mpg going west but 33 mpg without the bides going east. And the bikes were mounted well below the roof line of the car. We traveled west on 40 and returned on 10 and 20 but averaged speeds were much higher on the return, more than 5 mph faster. The drag of the bike’s handle bars and seats just above the trunk lid was apparently very costly.