Ghost Rider

just got back from a roadtrip over to St. Louis were my Boxster S averaged more than 27mpg with the cruise typically set at 78 and the temperature in the low 90's meaning that the A/C was on set to Auto and 72 degrees. I made this run with the hardtop on, so I'm not sure but I might have squeezed a couple of extra MPG with it on. This is with an EVO intake, modified ECU and TTP headers so for those worried about gas mileage after modifying their S, fah-get-about-it...

One problem of note, however, from about 75mph to 95mph I have a vibration that cause a bit of a drone sound to echo in the hardtop and the steering wheel to shimmy a little back and forth. I had my tires and wheels balanced recently but it seems to still be occuring. Any ideas what might be causing this? The effect begins about 70mph but really becomes noticeable by 75mph and escalates up to about 82-85mph where it then starts to tail off to where it becomes almost "normal" by 95mph.

Tires? Wheels? Suspension? Suggestions on how to fix please!

Thanks,
Ken

Sean

Ken, I'm not sure if this is the problem, but you might consider the possibility of a bent rim. I'm told that this is happening more often these days with 18" wheels & low profile tires. I know because I am having this high speed vibration problem, and this was the diagnosis given me.

Who knows when it happened. It could have been done by a careless valet.

I suspect that this problem is relatively rare compared to tire balancing / alignment issues. But a good tire shop should be able to give you some answers.