Will a lift kit effect my gas mileage?
A lift kit and wider tires will have no noticeable difference. These things may create some resistance but not enough to ever notice.
(this is not hard to do and the part is not expensive but you need to know how to figure out what gear to purchase)
- The tire height may affect mpg greatly or it may not depending on if the gearing forces the engine to work much harder
- Tire height can also affect speedometer and odometer which can be corrected by figuring the final gear ratio and tire by replacing the gear on the cable that connects to the transmission.
Taller tires will almost always make the speedometer/odometer work slower meaning you are traveling faster and farther than it shows. which can cause confusion on your mileage leading you to believe it has dropped a lot when the truth you have actually traveled farther than it shows. This can indirectly lead to speeding tickets unless you properly calibrate the speedometer/odometer by installing the proper gear to compensate for the larger tire.
Rim size has nothing to do with mpg.
Total tire height does and that is determined by width, ratio (series) and rim size.
As an example
You could put 18 inch rims with a shorter profile on your vehicle and they could be shorter than the originals. If you did not compensate for them being shorter you would believe that fuel mileage has increased because odometer would register more miles than you actually travel and speedometer would show that you are going faster than you actually are possibly keeping you under the speed limit and preventing tickets.
Don't worry about lifting the vehicle because you won't notice a small amount of loss.
Possible solutions to try
If you know someone who has the same size tires you wish to purchase ask if they would swap them with yours for a couple weeks and figure out how much your speedometer/odometer would be off so that you can compensate. Figure out how far you actually traveled that way you can figure the correct fuel mileage.
This way you can find out exactly what effect it will have on your fuel economy prior to putting on your lift kit