hello,
i recently put in a K&N drop in air filter in my car and i now belive that my engine is opperating at a slightly lower rpms if i rember where it used to be correctly.
those of you with a stock IS 250rwd could you please chime in on what ur rpms are at idle (warm engine)...and 70mph (flat surface)....

thank you, just want to see if im just seeing things...

 

If your car is idling at a different RPM, then it is more likely to be caused by a change in weather. The RPMs on the highway are determined by gearing, so the only way to change the RPM's at 70 MPH is to put on different sized tires, or swap out parts in your transmission or rear end.

 

I did just get new tires as well...but same size as the last set of tires...perhpas theres a slight change...

 

The change in tire size will change the true speed of the car slightly, but the relationship between the speedometer and RPM while cruising will not change.