Madsci625

If I switch from 18" to 17" OEM wheels/tires combo for winter driving, will my speedo be screwed up? Is there a fix? Optimally, I would like to have the AWD sized tires (symmetric) all around for easy rotating (225/45/17).

lobuxracer

iTrader: (2)

Using the AWD set up will screw with the calibration. The front and rear on the 350 are staggered (different rolling diameters) so the skid control ECU is expecting staggered wheels, not the same size on all four corners. The wheel speed input is to the skid control ECU then the display.

n0th1ng

iTrader: (9)

i threw off my speedo by about 3 mph by putting wider and smaller tires... from 255/35 to 275/30...
not a big deal tho...

Madsci625

hmmm, so is there any way to recalibrate it? I wold be nice to have the same winter tire all around.

Kurtz

iTrader: (1)

Couple mph on the spedo is pretty minor... of greater concern might be the fact your odometer will be increasingly wrong too

Gernby

iTrader: (1)

There is enough tolerance in the skid control ECU to run non-staggered tires. I've been running rear tires that are supposedly .1" shorter than the fronts for years without any issues. As far as the calibration goes, you just need to use a GPS to determine the current speedometer error (mine was about 3 MPH off at 60 MPH), then do some calculations using the outer diameters of your current tires to find the "optimum" outer diameters of your future tires. You could easily increase the outer diameter of your tires even with a smaller diameter wheel.

Gernby

iTrader: (1)

Going by the specs on TireRack.com, it looks like your speedometer and odometer would read about 8% higher if you swap out the OEM 18" tires with 225/45-17's on all 4 wheels. It seems like a better choice would be to use 225/50-17's, which would probably make the speedometer more acurate than it is now, or 235/45-17's, which would probably keep the same speedo error you have now.

Madsci625

Which wheel is the speedo based on? Rear? Therefore, I should try and find a tire gives me the same circumference as the stock 255/40/18? Maybe I should just say F-it and remount winters on my stock 18's...it would be nice to have the same tire all around though..

as3.0cl

iTrader: (2)

what is 3mph really? when i went from stock 17's to staggard 19's my GPS and speedo where then right on the money with each other.

honestly pick up a set of 17's and throw some winter rubber on it, your not going to hurt anything.

lobuxracer

iTrader: (2)

It's not based on a wheel. It's based on an aggregate of all four wheels. That's why it's a problem.

Gernby

iTrader: (1)

I haven't done any serious testing to figure out the ECU's logic for determining the speed, but it does know the speed of each wheel since there are 4 distinct speeds on the CAN bus. However, I believe it will use the rear wheels only if the front wheels are stationary (like on a dyno). While driving down the road, I suspect it averages the 4 wheels speeds.

Gernby

iTrader: (1)

I'm interested in knowing why you think this is a problem. As I said earlier, I've been using rear tires that are the same outer diameter as the fronts for about 35K miles, and haven't noticed the slightest problem even on black ice.

lobuxracer

iTrader: (2)

You're misunderstanding what I mean by a problem. It's a problem for speed determination because the speed is determined by using all four inputs and it's based on having the stock sized tires on the car. It's not necessarily a problem for the skid control ECU.

At the same time, you've not been able to fully test the skid control ECU's response under all conditions with both set ups, so you can't say anything except it hasn't done anything scary with your set up and the conditions you've encountered.

Gernby

iTrader: (1)

As you know, I really respect your opinion and knowledge, but I can't believe that you could be right on this. I would bet the largest of my 3 ********* that the non-stupid ECU developers at Toyota are aware that people deviate from OEM sized tires, and designed their software for it. If their ECU's operate in closed loop to adjust fuel and timing trims, why wouldn't the ECU also monitor difference in rotation speeds while cruising to figure out exactly what the stagger is?

As you also know, I'm a hardware / software guy myself, and I know how easy that would be to do. I would fire an engineer that would hard code a stagger for anything important.

Gernby

iTrader: (1)

Oh ... I forgot to mention that I really don't know of any other driving scenarios that I haven't "tested" my non-staggered fitment. We don't get "powder" snow in north Texas, but we've gotten everything else while I've had this setup.