chikoo

The IS250 comes with rear Stock Tire - 225/45R17
Optional upgrade - 255/40R18

Speedometer Difference: 4.242% too slow with the optional 18".


Question: The speedo is connected to the front wheels?
If so, it is ok.

sktn77a

iTrader: (2)

Older cars had mechanical speedometers connected to a front wheel. Most current cars have electronic speedometers based on engine speed and gear position (ie rear wheel speed).

chikoo

ouch...so how does this work out when the car comes with 18" optional tires?
Does the factory calibrate the speedo for the 18" differently than the 17" standard?

ScKcBc

iTrader: (2)

rwd is250's have 245/45/17 in the rear, not 225.

And the sensor is in the front, not the rear.

225/45/17 is what comes stock on the front. Compare that to the stock 18" tire size, which is 225/40/18 and they're only about 1/10th of an inch different from each other

Sango

iTrader: (10)

For the Lexus vehicles, it is already calibrated from the factory (assuming proper size spec tires used as indicated), such that the speedometer will always show at above or at speed and never below. It is not reprogrammable/recalibrated.