I went from 16's to 19's

225/55/16 to 225/40/19 f and 245/40/19 r

and I noticed my gas millage go from around 18-19 to 16. driving is short fwy distances, mostly street. does this make sense or could it be something else?

 

Makes perfect sense,

1. More tire = more friction (better traction, worse mileage)
2. Large diameter rim = more rotational force required compared to smaller counterpart
3. Large rim size = increased weight (relative to tire weight)

Think about it, if you drew a circle on the floor with chalk and another one double the size and ran around the perimeter of each, which one would take longer?

 

I run 245 275 19" on my SC300 and I get about 16mpg or less.

 

The inertia at the wheels determines how hard the engine has to work to spin the wheels. Inertia is the tendancy for still objects to remain still, and thus, breaking the inertia barrier is necessary for the vehicle to get moving. If you raise inertia, you lower economy because the engine has to work harder to rotate the wheels.

Rotational inertia (the type of intertia you are worried about at the wheels) is proportional to weight -obviously- and the radius of the center of rotating mass squared.

Perhaps the term "radius of the center of rotating mass" is not clear. Imagine you could cut the wheel and tire combination lengthwise, along the tread of the tire, right down the middle. Now, imagine you took one of those halves (shaped kind of like a frisbee) and then cut it like a pizza, into ten slices. How far away you would need to place your hand from the center of the wheel to balance one of those pizza slices is the "radius of the center of rotating mass". Larger diameter wheels, by nature, move this radius outward. Changes in this radius will affect rotational inertia exponentially.

Larger wheels are often heavier than stock wheels and larger tires are foten at least as heavy as stock tires. From a mass argument, rotational inertia is affected proportionately. More importantly, the extension of the rotating center of mass outward has a dramatic effect on the rotational inertia at the wheels. As a consequence, larger diameter wheels do indeed affect economy because the engine has to work harder to accelerate the wheels.

With a given wheel style and diameter, the best defense you have against poor fuel economy is to buy the lightest tires possible in the size that you are looking for. Every pound saved in tire weight has a similar effect to saving three pounds in wheel weight because the tires are located farther from the center of rotation.

 

great point. I'm sure what weight is off the stock lexus wheels but I Rays forged wheels I have weigh about 19 and 21 lbs.

I check the tire pressure today and it was 26psi so I raised it 35psi.

 

Filled it up today. added about 17.8 gallons, last tank lasted about 268 miles.

so thats 15mpg!!!!

Since then I added 35psi to all the tires and changed the air filter it was pretty old.

I don't know what else to do. Plugs are new but I could do plug wires and cap and rotor, and I was told the 02 sensor would throw a CEL

 

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