I did a test this weekend, 10 miles of flat hiway with cruise set at 80mph. Dash reading was 20.4 mph @ 79.6mph for that 10 miles. Sounds low to me, I thought I was getting around 24MPG before I put on the wider tires and new rims. Once rolling the heavier rims won't affect mpg, but maybe my tires have high rolling resistance? 245's on all 4 wheels. 35psi.

 

More weight, more friction to overcome, diff overall tire dia will all affect mpg

 

But weight is only 5lbs each wheel, no more than a couple 12 packs on the trunk. I understand acceleration is hindered with added wheel weight, but once cruising, weight is just added car weight just like adding passengers. My mileage is the same with just me, or with 4 passengers. And overall diameter is the same as stock 18's.

 

I think rotational weight is much more penalizing then static weight.

 

I think its not overall weight, but weight distribution. Adding 20 lbs to the wheels definitely affects MPG and acceleration more than 20lbs distributed evenly around the car. The heavier the wheels are, the more the engine has to work literally just to get them started and keep them going. As for drag, I don't know for sure, but as with anything its a give and take. More tire on the ground means more traction, but also means more work that the engine has to put out to get those things spinning. All this equates to using more gas.

That's my two cents at least...

 

This would be the problem- this statement is 100% correct

 

"An object in motion tends to stay in motion". So once you're at a fixed speed, rotational weight is now static unsprung weight. When it comes time to accelerate or decelerate, rotational weight is taken in account and keeps you from speeding up or slowing down. So once you set the cruise, one should assume rotational weight just adds static weight. Take a gyroscope for example, a heavier one will want to stay spinning longer that a lighter one. It takes more energy to get to speed, but once there it will spin longer. Any thoughts?

 

Did you get 24MPG under the same conditions? Same stretch of road and same environmental conditions?

I have also noticed a slight difference since going to 20" wheels, but I would still say the difference is negligible overall.

Technically speaking, the rotational inertia is greater on a 20" wheel.

 

added weight of rims, added weight of tires, profile of tires, weather conditions (wind), etc etc... all could be a factor...

 

That law holds true only in the absence of an external force.
In this case you have to overcome friction, drag etc etc - so that law is not applicable in its stated form.

 

^^ Correct. I'm just comparing lighter wheels to heavier wheels. Once in motion and at a fixed velocity there would be no more drag with heavier wheels than lighter ones. Heavier wheels only affect change of velocity, resulting in slower acceleration and slower deceleration. Are we talking the same?

 

all the above notes are true... but -4mpg sounds a little too extreme. My previous car had upgraded 15"s to 18"s (195->215). That penalty was about 2mpg.

At any rate, you shouldn't feel too bad about it. There are some of us who are barely getting 20mpg on pure highway with stock 17s.

 

dont forget the road is not level smooth from point A to B. u're constantly accelerating here and there.

 

I've read somewhere that some tires have less friction than others and result in 1-2 mpg increase, so maybe stepping up to wider 245's in front and having tires with greater friction could result in a combined decrease of 4mpg? Maybe? Who knows, just making sure my mileage is on par with others here on the board.

 

the thing about the car being in motion, it still needs energy (gas) to maintain that speed, so the added weight of the rims/tires still have an effect

profile of tires (if it's smaller in overall diameter) at that speed, needs more fuel too...

time to step up to forged rims, lol