Huge loss in fuel economy with wheels

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Your tires probably have higher rolling resistance. There's also Tire Tech Information - Tire Rolling Resistance Part 3: Changes to Expect When Switching from Worn-Out to New Tires and I also wonder about odometer error w/these non-stock sizes.

 

You're SOL really. The upgrade to 17" wheels will cause you to lose mpg no matter what you do. That includes using light weight wheels like the RPF1.

The best you can do is stick with a light weight 17x7" and wrap it with a 205/50/17 Ecopia EP422 or PureContact. The 205 is the narrowest 17" tire you can effectively run. Even then you'll lose a few mpg vs. a good 15" tire.

The speedo should be close to accurate with the 205/50/17. It runs 1.5-2mph too fast with the OE 195/65/15.

 

Ohh snap. I just realized you have a GenII instead of a GenIII.

The 205/50/17 is much too large in diameter for a GenII. You need a 215/45/17 to more properly match the diameter of the 185/65/15.

You'll average a 4-8mpg loss with most 17" combos on a GenII as the hit is larger than on the GenIII. I've done the swap dozens of times and it was always there despite trying different tires and having 15lb wheels.

 

I would suggest using an app that mimics your odometer or other GPS tracking tool to see how far off your odometer is now reading. Your fuel economy will be artificially lower than it is in reality because of the odometer error. Figure out what that error percentage is and you can calculate that back into your mpg number. Your 205/50/17 is quite a bit larger than your OE tire so it will be off by a good amount.

 

We almost have the same setup except you have a bigger wheel and tire. Fancy a lightweight 16x7 instead? Didn't really affect my consumption even if my tires aren't LRR. tire is 205/50/16.

 

Note that if larger tires threw off the odometer, they also changed the speedometer a similar amount. So if you drive at a set speed on the dashboard, you are actually traveling faster on these new tires than on the old. This also consumes more fuel.

So your computed MPG is taking at least a triple hit:
(1) greater tire drag loss (real loss);
(2) less indicated distance (apparent mpg loss only, not real);
(3) greater speed (real loss).

 

without thinking about it, wouldn't the equation to adjust MPG be
MPG (actual) = MPG (observed ) x (MPH (actual)/MPH (observed))

 

No because odometer and speedometer errors are different.

 

I'm fairing ok with my 17" factory wheels with Toyo's. MPG has been going up ever since I bought the car.. started with 42mpg and now I think I'm around 47-48mpg! This improvement is due to break in and learning proper fuel efficient techniques.

 

I keep thinking that if the mod's were not done in the first place, chances are the mileage would not have dropped. But that is water over the dam so to speak.

I don't want to sound rude, condescending or anything like that. The bottom line is you made the changes, and due to those changes the MPG suffered. It really is as simple as that. Since you don't want to go back to the "ugly stock wheels", you pretty much have to accept what you are getting.

As F8L pointed out above, you are pretty much up the creek without a paddle.

 

do note what tires you're running.

 

I honestly don't see wheel weight as a big issue. While it might change suspension dynamics and perhaps slightly reduce your acceleration capability it shouldn't make a big dent in the mileage.

You're looking at about a 14% hit in mileage. If you were driving a 22 mpg car that would be the equivalent of a 3 mpg drop.
Some of it might even out after the tires break in a little more.

 

Have you actually measured your new, lower mileage or is this from the car display? It could be that it hasn't been as hard a hit has it's seemed.

Good luck!