lobuxracer

iTrader: (2)

^^that's why I said not necessarily. It would be fun to put the skid control on a bench and feed it inputs to see what it really does and where it gets really upset and confused. It would also be more work than I care to engage when I have a million other things I could be doing unless there was something tangible to be gained.

Madsci625

I would think that the dealer could enter the new effective wheel size into the computer so the ODO/SPEEDO/skid control/etc doesn't get screwed up. I doubt that they have different ECU for cars that get the 18" from the factory. Maybe I'll give them a call...

Phil888

iTrader: (1)

Funny thing i just remember.. WHen i had my 06' Lincoln Navigator, the factory wheels were 18"s and i put some 24's on, and it totally threw off my speed by 10 mph. I got pulled over twice for speeding cause my guage is reading 70 mph when its suppose to be reading 80 mph. You just got to be cautious about thing like this.

Gernby

iTrader: (1)

I wouldn't even consider calling the dealer about it. Send Carchitect a PM about it. He's a master tech for Lexus, and has an IS350.

lobuxracer

iTrader: (2)

No entry in anything for wheel size. If you check the wheels you'll see all the optional wheels are the same rolling diameter despite the rim sizes. The only thing that uses wheel size is the nav's dead reckoning and you need to tell it when you replace the tires so it can take the larger diameter of a brand new tire into account.

Madsci625

Well, there is a slight difference in tire circumference for OEM tires, 1.4% for the rear and 0.5% for the front, 18"s vs 17"s respectively. Obviously, using the same circumference (size) tires all around will have much larger errors, which is what I originally wanted to be able to have.

I'm sure it is possible to change this parameter in the computer it is just a question of willingness of the tech at the dealer. Seems like a pain in the *** regardless. Further, I'm not willing to let my speedo/ODO deviate from the actual mileage.

It looks like I'll have to keep with a staggered set up whether on 18"s or 17"s. The tire size for the 17"s would probably work better than 18"s for a snow tire set up though...

lobuxracer

iTrader: (2)

Let's try this again - THERE IS NO WAY TO ENTER ANY DATA ABOUT THE WHEELS. I have full access to Toyota/Lexus/Scion documentation. IT IS NOT POSSIBLE.

The wheels are staggered from the factory on the RWD and they are not on the AWD. The 17" standard wheels are staggered and the 18" wheels are equally staggered. The 17" wheels and the 18" wheels have the same rolling diameters so there is NOTHING TO CHANGE in the ECM or the skid control ECU to tell the car which wheels are on it. The standard and the optional wheel rotate the same number of revolutions per mile regardless of the rim diameter.

Gernby

iTrader: (1)

Is this an OCD thing? The car comes from the factory with speedometer eror. You could make the speedo even more correct by changing the tire diameters...

lobuxracer

iTrader: (2)

DOT allows +/-3%, and most speedos read faster than you are actually going. I've only owned one car with a dead accurate speedo - my Supra - as measured using cruise control, mile markers, and a stop watch over 5, 10, 50 and 100 miles. Most indicate 60 mph, but you're really only going 58.

Madsci625

Thanks for the info. Nice Supra BTW. Is it stock? I always wanted to put a nice single turbo and meth kit in mine but graduate school stipend didn't allow for such luxuries.

Well, I want to have a true ODO reading on the car, I don't car as much about speed. So are you saying all IS350's have a systematic error in the speedo from the factory? What is it?