My car came with 15" wheels with 205/65R15 tires on 15X6 steel wheels with hub caps. I upgraded to 205/60R16 tires on 16X7 alloy wheels
Larger diameter and larger width typically means more weight, but alloy wheels weigh less than steel wheels for the same size wheel
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- By keeping the tire diameter the same, I was able to maintain the speedometer accuracy.
- By keeping the tire width the same, I was able to avoid any additional weight/rolling resistance from a wider tread.
In the end, I was able to keep the weight of each tire/wheel the same. No effect on fuel economy. The car rides as about as smooth as it did before, but handles a bit sharper.
The alloy wheels I bought were fairly cheap (Fondmetal 6700) and not very light for the size. If I had bought more expensive alloy wheels that weighed less, each tire/wheel would have experienced a new weight reduction compared to stock.
For a given wheel, the weight will vary greatly by manufacturer/model.
Take a look at this link and you'll get an idea how much wheels weigh.
http://www.wheelweights.net/
Wheel weight is definitely a factor in fuel economy, but tire selection plays a role as well. Even for a same size tire, some tires have less rolling resistance, while some may have a lot more.
When I had my Maxima, I used to have 205/65R15 92S Michelin XH4 tires. S-speed rated (112 MPH) long wearing tires with extremely hard compound. Grip left a lot to be desired, but I got over 70K miles from those tires. I switched to Falken ZE-327 V-rated tired (149 MPH rated) tires of the same size 205/65R15 92V. Much much better grip, but lost 1-2 MPG after I changed the tires. Lasted 40K Miles.
http://www.tirerack.com