Long ago, you told us about diminishing MPGs depending on cargo. I asked my husband not to drive around with a full cooler, cases of wine and beer, bags of cat food and bird seed, but to unload when he buys them, but he says it’s no big deal. I remember you saying it made a difference, but can’t remember the ratio, fuel loss, per lbs of stuff. I’d love to quote you, and “Nanny nanny boo boo” to him.
Tell him to take a five mile bike ride (ten miles if he’s in shape). Tell him to then tie 30 or 40 pounds of any load onto the bike and take the ride again. Ask him if it took more energy to haul the load.
There’s a reason expensive bikes typically under 20 pounds take a lot less energy than dime-store bikes weighing typically 40 pounds. That reason is primarily the difference in weight.
Tractor pull competitions are an extreme example of the difference weight makes. The winner is the tractor that can pull the most weight the minimum distance… because weight matters.
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It makes a small difference, but I consider it a crime to drive around in hot weather with good wine in the trunk! Not sure how beer is affected. Wine does not like heat!!
The mileage loss would be small compared to the improvement made by adopting careful driving habits and using the brakes as little as necessary and not accelerating fast!
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Heat skunks the beer! Just say NO, on that reason alone!
As for how much weight hurts mileage - It Depends… on how heavy the empty car is, where he drives, how he drives, ect. but is DOES hurt mileage.
@the_same_mountainbik has the best teaching example… make him push that extra weight with his own legs… that’l learn him!
If he has light beer, it’s pre-skunked at the factory. Heat won’t adversely affect it at all. 
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While extra weight will effect miles per gallon the real question is why he is too lazy to unload the car.
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What’s hubby drive, anyway? If it’s a Ford 450 Superduty crew cab with 4WD, or a Bugatti Chiron, the amount of extra weight you describe won’t even be noticeable. If it’s a Smart car (a true misnomer IMHO) well, he’d be better to listen to you.
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Indeed!
Shouldn’t the Truth In Advertising statutes prohibit that type of blatant mis-labeling?
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It has obviously been a few years, but at one time when I was younger, I was what is called a Class A bicyclist.
There were people who rode across the USA, I was not one of them. They almost always had relatively heavy bikes. They were more durable, and less energy was lost in flexing as the lighter ones might do.
Some of them also did not have the very high pressure tires. They went for larger, modest pressure tires (90) which withstood better the heavy loads they were carrying.
Rolling resistance was, however, critical. I had a Schwinn Super Le Tour, which was noted for low rolling resistance. I used to take up to half an hour just to get the wheel bearings adjusted perfectly. And, I do mean perfectly. Other models in those days were not machined sufficiently well to allow perfect adjustment. I wasn’t too fast uphill, but I really smoked them going down. 
I have no idea what is currently being practiced.
If it’s a Smart car he’s not gonna be able to fit 1/4 of that stuff in there. 
With many hundreds of thousands of miles experience with vans hauling cargo I can with some certainty say that from empty to 1,000 lbs there is little difference in fuel mileage. Of course a car may be quite different. And the husband may be hauling all that junk just to be prepared for… well, something.
Re those wheel bearings being adjusted, I have for many years hung the wheel on the frame and with the race on one side locked incrementally tightened the race on the other side until there was no discernible side to side play at the rim and then spun the wheel expecting it to continue spinning until I got bored. And I have found that 90W120 GL3 performs better than even the high dollar synthetic bicycle greases. Also my mountain bike coasts as easily with the larger 45psi tires as it does with 90psi tires when riding on coarse pavement. What has been your best experience with tires, pressures and lubricants?
While I am not trying to set any records I do try to keep up with friends on the trails and most are a few years younger and a few pounds lighter than me so I need all the help I can get.
We are too old for biking. There are narrow country roads, and minimal sidewalks, if any. Fast pickups, trucks or farm machinery. Too dangerous even if we could.
I didn’t say he ‘drives around’ with wine or beer, just sometimes, after a 16 our day and a lot of pain, he won’t always unload the trunk for a couple of days. When I wrote this, it was freezing, and we only had bird seed and cat food in there.
When he drives, unless just for shopping, his work is often over a 100 miles each way.
He has back pain and works long hours. Not lazy, just old!
As a senior citizen, former cyclist, and fellow back pain sufferer, my best suggestion is that this isn’t important enough to worry about.
Does extra weight affect milieage? Yes.
How much? depends entirely on the vehicle and the weight.
Is some “junk in the trunk” worth worrying about? IMHO not at all.
If this is an important enough problem in your life to write about, than you are truly blessed. I personally have far greater challenges to struggle with. 