Rim weight for better momentum going down the highway

[junkyardjeff]

The previous 4 times I drove my 55 sunliner to Florida it had aluminum wheels and it got between 14 and 16 miles to the gallon on the trip,before I went the last time I put the steel rims back on and it got 17 to 18. The only thing I done to the motor was to go from ported vacumn on the distributor to full vacumn which could have had a effect but I was also wondering if heavier wheels that would keep the momentum up help too,I do not know if the heavier wheels had a effect on the stopping since I also added a brake booster before the trip too which helped a bunch going through Atlanta.

[moose]

Were the tire diameters the same?

[chimp koose]

were the rim widths the same ? loaded radius changes with rim width . Drag car will see more rpm at top end with a wider rim on the same tire . If your steelies were narrower you could see an increase in loaded tire radius and a reduction in rpm at the same actual (not speedo indicated ) speed . Think of it like a slight gear ratio change .

[chimp koose]

I could also see the manifold vacuum helping the mileage as it would possibly increase the timing at a speed lower than when the ported vacuum would start to work . I would guess that your mileage may get even better if the alloy wheels went back on from the lower rotating mass .That is , as long as the rim widths are the same and you use the same tires .

[enjenjo]

At a constant speed the weight of the wheels changes nothing, inertia does not apply.  When accelerating or decelerating inertia can have a effect. I would be more inclined to think it it a difference in tire roll out, or rolling resistance.

[junkyardjeff]

The aluminum wheels were 6 inch wide where the front steels are 5 or 5.5 and the rears are 7 which are too wide but that is another story,its most likely the change to full vacumn that got the mileage up but I was wondering if heavier wheels would help keep the momentum up on the highway lessening the demand on the motor. I used the same tires that were on the aluminum wheels.

[kb426]

My exp. with heavy wheels or tires has been the opposite of what you are questioning. I'm not saying this is the defacto standard but all rotating items take hp. to run them. The heavier the object, the more power to bring it up to speed and the more energy to stop the motion. All my history has been heavier items take more energy to move and are less efficient . My 2000 Dakota Rt had a wheel and tire combo that weighted 66lbs each. The driveability was poor in suspension and braking. I replaced that combo with standard Dakota parts and removed 20 lbs. per wheel. It made a huge difference in driveability but the truck still has poor fuel mileage.