Older heavier car gets better gas mileage WHY?

[John1397]

I have two cars one is a rear wheel drive weighing 3300 pounds getting 28 mpg highway the other is front wheel drive weighing 2800 pounds getting 20 mpg. Both V-6.
The lighter vehicle is running over 14:1 fuel ratio it has power steering air pump and air conditioner not running when tested. I bought this car used air pump was not connected to catalytic converter was wondering if exhaust back pressure could be causing loss of fuel economy or does power steering take a lot of power to run or even front wheel drive versus rear wheel drive?

John

[Stealthee]

Its not all about weight. Gearing, horsepower, emissions equipment, technology, etc all affect gas mileage.

[John1397]

Originally Posted by John1397

I have two cars one is a rear wheel drive weighing 3300 pounds getting 28 mpg highway the other is front wheel drive weighing 2800 pounds getting 20 mpg. Both V-6.
The lighter vehicle is running over 14:1 fuel ratio it has power steering air pump and air conditioner not running when tested. I bought this car used air pump was not connected to catalytic converter was wondering if exhaust back pressure could be causing loss of fuel economy or does power steering take a lot of power to run or even front wheel drive versus rear wheel drive?

John

I took off catalytic converter all I got out was a screen 17" long 3" diameter took it to junk yard they seem to think it is stainless steel can look right thru the converter from one end to other, does the precious metal burn up and go out with the exhaust?

John

[Tech II]

What are the two cars you are comparing? Year, make, model, engine, transmission, mileage?

[Blue Bowtie]

Aerodynamics also plays a substantial role in highway fuel mileage. A well-tuned and reasonably efficient engine will only produce the power necessary to maintain vehicle speed (overcome air resistance) and thus will only use the fuel necessary to accomplish that.

For example, a lifetime ago when I was shopping for a new car I compared identical models - A Caprice Classic with the L99 (265 CID) V-8 and the same body with the 350 CID LT1. The city mileage rating was 16 for the 265 and 15 for the 350. The highway mileage was identical for both at 26 MPG, and both engines used the SAME type fuel injection system. Despites its size and weight, the frontal area and CD are both relatively low for that body. The BSFC is determined by kinetics, not the power plant.

Comparing a Corvette seven-speed LS6 to a Lexus LS-460 (with 100 fewer cubic inches in its V-8), the Chevy makes gobs more power but gets better highway mileage at 29 MPG than the Toyota's weenie V-8 at 24 MPG. The Toyota punches a bigger hole in the air, and thus wastes more power to overcome air pressure.

Incidentally, the LS-460 lists at $75K while the Corvette is only $60K. Let's just talk mileage, and not even consider reliability, substance, and resale. For $15K less with the Corvette you even get a real frame under that sleek body. Granted, the Lexus is about 800 pounds heavier than the Corvette, but is almost 1,000 pounds lighter than my 20 year old Impala which still gets 26 MPG - TWO miles better than the brand new, "high-tech" and no-frame Toyota.

Weight is important in stop-and-go driving since it requires more power to accelerate a greater mass (plain physics) but is almost inconsequential in steady-state highway driving, even considering hills, since the weight that penalizes you while climbing also benefits you while descending.

In short, don't put too mush weight in weight as the only factor in determining mileage. It's really only a smaller factor of those factors involved.

[Blue Bowtie]

Incidentally, you MAY want to reconnect the air injection pump on your troublesome vehicle. Injected air may help the cats light off sooner and the downstream O² sensors stay active, thus trimming fuel more effectively.

[edwinn]

Just a few comments: my 97 Riv gets slightly over 29 MPG in good weather, across the country in the mid-west, with a full trunk and tank fill-ups. Fuel efficiency drops to 27+ MPG if the trip is over mountainous areas, or with heavy AC use.


The point is.. at the Buick dealer last week, I saw fuel efficiencies on New-car stickers of 27 and 28 MPG. There might have been a 29 in there, but can't say for sure.

So the 97 Riv is getting comparable or better mileage to the 2014 Buicks. Sorry don't have empty weight numbers to post. Now.. another shameless plug for the G-platform Rivieras. They're GREAT for cross-country trips with plenty of trunk space, long wheelbase and comfy interior. And they still turn heads!

-Ed