rooftop cargo mpg

Last week I mounted a 20 cu ft. cargo carrier on my roof expecting my gas mileage to suffer. Instead it went from 16 mpg to 17.5 mpg. ???
Is the aerodynamic shape reducing wind drag or did they just reformulate the gas for the summer? What am I missing?

I would guess that the reformulation helped along with a change in driving habits (a little lighter on the gas pedal, more highway driving).

Exactly my thoughts, I bet you drove a little more on the easy side worrying about the rooftop cargo carrier and therefore increased your mileage by not being so harsh on the peddle ;)

Actually, 17.5MPG on the highway is not good.

MPG's FROM 16??? That's not too good. My 4.3 98 Jimmy gets 19.5, plus a little, on the highway.
Only California, I think, has the RFG reformulation.
Alcohol-containing gas supposedly gets LESS mileage by a very small margin.

Did you put bigger rubber on the corners? This will increase your mpg's, but NOTNOTNOT when re-calculated by the new mileage showing on the odo. Of course, you lose a little off the line cause your end-ratio changes, but who's draggin' their blazer?
In the dirt, the lost 'dig' due to more rubber is recouped by 4HI!

I'm guessin' your old mileage was strictly city, and you were doin' the highway thing when you had the tote up top

I do about an even mix of city/highway driving. Stock tires. My old blazer was a 2 door with the 4.3Z motor and I got about 18 city and 22 highway. This one is a 4 door 4.3 W motor with 40,000 miles. I bought it as Certified Used from the dealer last August with 27k. At 35k I did the 4X4 service and switched to synthetic gear oil. I'm thinking of changing the cap, rotor and plugs reguardless of what the maintenance schedule says.

True, but remember, the lower MPG was probably caused in part by the carrier.

In the original post I stated that my mileage improved with the carrier.

In the original post I stated that my mileage improved with the carrier.

Do you really think a carrier will improve fuel economy?

Apparently it has. I'll use up a few more tanks full and then remove the carrier and check it again. Ill let you know what I find. I've seen wind deflectors attached to the roof of the cabs on big rigs, maybe this is causing a similar effect.

Apparently it has. I'll use up a few more tanks full and then remove the carrier and check it again. Ill let you know what I find. I've seen wind deflectors attached to the roof of the cabs on big rigs, maybe this is causing a similar effect.

Nope, you have a calculation problem, or the gas has been changed in your area.

The carrier ADDED more resistance to your truck.

I think you'd better do the #'s again, hunter. I don't think you can add anything that is gonna add to the mileage of the vehicle.
The heavies - 18 wheelers - add an air dam because of what's BEHIND the cab - namely the trailer.
It's like a bullet. The only place you're gonna reduce wind resistance is by modifying what's at the beginning, or at the end. Not at the middle.
You give a bullet a 'pointier' point, or a 'boattail', which is a taper at the end.
Now, if your cargo carrier has an SRB, all bets are off.

SRB = solid rocket booster;)

Thats an interesting mod, I'll have to think about that.