Last night I had my cruise control on (set on 65) and started down a long steep hill. The hill was about three-quarters of a mile long. My IS downshifted from 6th to 4th gear to keep the speed at 65 mph. Every other vehicle I have driven and used the cruise control on has let the car coast up to whatever speed that gravity would allow or until I hit the brake. It’s not that big of a deal to me but I was paying close attention to the fuel mileage and when the car downshifted into 4th the RPMs went up to 4500 and the mileage reading immediately went downward.

Have anyone else experienced this?

 

Yep. I actually like it as it helps keep my ticket count down. It would be especially handy if you had done the nav. hack and were busy watching a movie.

 

grrr, don't even get me started on that behavior.

 

I'm impressed it has that much control.

 

Every other car (with an automatic) that I've driven has felt like it was coasting as I brake for a light or a stop sign. The IS actually feels like it is downshifting, an odd sensation, but in this case I'll often see my tank mileage increase by a tenth of an MPG.

 

That's a good thing when it downshifts. Means that they programmed it to be in the proper gear for re-takeoff, in the event that you don't have to stop. Being in the proper gear is safer (which is why I'm a strong advocate of manual transmissions), and means that there will be less shock to the drivetrain as you plant your foot into the gas pedal.

 

I'm a little confused by this. Your gas mileage should be the same regardless of engine RPM if you're foot is off the accelerator. If you're coasting at 65, the mileage gauge should have been at 90+ in either 4th or 6th. There may be some torque converter issues that I'm not aware of but gas mileage shouldn't drop as you mentioned.

 

I think the gas milage computations take in to account the amount of fuel going to the engine and engine RPM. At 65 mph the 250 engine is turning 2000 RPMs in 6th gear. Shift down to 4th at the same speed and the RPMs jump to 4500. More RPMs require more fuel regardless of how far down the gas pedal is pushed. I had the instrument panel display set to the average MPG screen when this happened. The display takes a reading at timed intervals and computes the average with the previous readings. It showed I was averaging 33.1 MPG before the downshift and 32.4 MPG at the bottom of the hill. I then drove on mostly flat roads for another 20 miles (at 65 MPH in 6th gear) and the average rose back up to 32.9.

I'm not too concered with it though. Although at one point in my approximately 150 mile drive last night, about 45 miles in, the display showed I was averaging 35.6 MPG!!