I used to own a Magnum that I was getting 19 mpg in town with on stock 18" wheels, when I switched to 20's my mileage went to 14...the 20's are killing your mileage and IMO just to big for the IS anyway

 

I have just over 2000 on my car. I've never reset the trip B counter so it's reflecting my mpg since I purchased the car with 56 miles.(1950+miles to date) the mpg has gone up from 20.1 to 22.5 as of today. As for the trip A counter I reset it after each fill up. Presently it shows 23.6 MPG and it's almost all city driving with some freeway driving thrown in.
On the only trip I've taken (San Diego, 79 miles) It was showing 34 MPG driving 70/75 mph. About what I would expect. Remember my car is a IS250...

 

I drive a lot of freeway. I have 16k miles since July. California gasoline is the worst for good mileage, but I set my cruise at 80 mph on my trip to New Mexico over Christmas and got 28.3 mpg indicated on the tank once I got the California fuel out of the system. As soon as I got back to California, the same speed under similar conditions got me 25.1 mpg.

I have seen as high as 29.0 mpg on California gas, but they've recently done something to the fuel (probably added more alcohol to it) and mileage dropped considerably. Many of my friends have noticed the same thing.

Finally, I've never reset the AVG MPG. I've seen it as high as 24.9, but it is now closer to 24.7. I drive a LOT of freeway miles with the cruise control on.

 

I just drove from Ft Lauderdale to Key Largo yesterday and managed about 28 mpg. Now mind you, this was all highway miles with the exception of sitting in traffic for about 15 minutes getting onto US1 entering the Keys.

I usually get about 21-22 mpg. My commute is about 90% city and 10% highway.

I have the stock 18's.

 

I don't know if someone mentioned this....but have you checked the air pressure in your tires?

I run ~37psi in my tires and average 22-24 MPG in town. I have a 25 mile roundtrip commute daily that is mostly steady driving.

James

 

I haven't reset the avg mpg since buying the car and its at 24.5 mpg. which is correct. my tank average using the old fashion pen and paper method has been from 23 mpg to 27 mpg. i only got 27 mpg once and never got that ever again. it usually hovers around 24.5 most of the time. 80% freeway/20 city.

 

Something is wrong... You should not be getting 16mpg highway... low 20s is about as bad as it should get with "normal" highway driving...

I get around 15 city, 28 highway

 

Whats considered city and highway for you? In Hawaii, I drive about 3 miles to work. so for 5 out of 7 days of the week, I dont go far. On the weekends, I may go about 10 miles one way and 10 miles back on the freeway. For me this gives about ~17mpg.
I have a 2007 GS350

 

Yeah. Check your tire pressures. Use premium gas and a good brand of gas. Also make sure you're in sixth gear on the highway, set the ECT to normal, and close your windows. Those are just general things to conserve gas.

Also, are your 20" rims and tires heavier than the stock ones? And is the circumference of your 20" tires bigger than the stock ones? Those could make a big difference.

 

I live in LA and drove to SD over the weekend. I was pretty chill driving down there not exceeding 70 mph. I averaged 28 mpg for the entire trip. Though my normal driving is more like 19-21 mpg.