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I went from stock 15x6.5 (195/60R15) steels to 16x7 (205/50R16) alloys on my 2005 LX and lost about 1.5mpg avg.

Overall I noticed a more stable, smoother ride and better handling. It's hard to tie that to the new rims alone tho as new higher quality tires we installed at the same time.

That and the improved appearance was well worth the loss in fuel economy.

I waited until I needed new tires to minimize the impact of the cost.

 
Rep Power: 117

The larger the wheels are, the lower the speedometer will read. So your loss in gas mileage is likely mostly due to the odometer being inaccurate and reading less miles between fill up's. Look into a speedometer corrector if you plan on keeping this setup. You may lose a tiny bit of gas mileage from heavier rims and wider tires, but probably not 1.5 MPG.

 
Rep Power: 517

Tire circumference/radius/diameter (revolutions per mile) is the factor. Not wheel size.




My stock wheels were 15 inch.
I'm on 18 inch wheels now.
My speedometer error is literally zero because I chose a tire size that matched the original diameter.


My fuel economy dropped as much as 20% the moment I bolted them on.
Can you tell me why? It's not due to miscalculations because of speedometer or odometer error at all.

 
Rep Power: 242

The type of tire you choose can make a difference. Low Rolling Resistance tires are best for MPG. Overall weight too.

 
Rep Power: 117

Well, what width tires are you running? My best guess would be that you put wider tires on that have more rolling resistance, as well as more aerodynamic drag since the tires are wider. The aerodynamics of the new wheels may also be different. Also, your larger rims with lower aspect ratio tires are most likely heavier than the stock wheels and tires since metal is heavier than rubber. Rotational mass, such as heavier wheels and tires, can have a large effect on gas mileage. All those things combined could easily add up to a 20% reduction in gas mileage.

 
Rep Power: 517

Good job!


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Stock tires were 195/65/15 Firestone LRR
Currently on 225/40/18 Michelin Pilot AS




Still wanting to try the 19" wheels from a 15 Accord sport, but don't want to pay for 35 series tires until these Michelins are wasted LOL

 
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I thought by choosing a lower pro tire I was coming pretty close to maintaining original circumference/diameter. It's not exact but pretty dang close. Also the new wheels are alloy which even tho larger and wider by 1" and 0.5" respectively should be lighter than the steels...lower pro tires should weigh less as well.

I chalked it up to more wind-drag from wider tires and the offset. Perhaps rolling resistance as you mentioned as well.

 
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Man, those 19"s look sweet--I'm too afraid of potholes tho, they are pretty bad here.

 
Rep Power: 517

I ran your tire sizes through the calculator, difference is minimal and speedo error is minimal.. Your change in fuel economy should be due to the other factors mentioned... rolling resistance/aerodynamics/weight.



I forgot to mention the 18" wheels and tires on my car are a lot heavier than the stock steel wheels with Firestones. I never weighed them to see exactly how much heavier, but it's significant.
I also put the original Firestone LRR tires on wheels from a Civic Hybrid a couple years ago, for weight and aerodynamics....but so far I've only run them in the winter when I expect a lot of snow.