As the title asks, (for the IS200). Which set of axles does the speedo take its readings from?

its off the rear

cheers Rob, had a feeling it was as its RWD, would my logic be correct for all/most cars?

The sensor is almost certain to be on the output side of the gearbox. In the "good-old-days" there was a flexible drive shaft driven by a gear on the output side of the gearbox, that drove a slipping clutch mechanism inside the speedo head. Nowadays the connection is electronic, but it still makes sense to take the reading from the gearbox output shaft.

Agree with Sagitar, it's comes from the gearbox on manuals

Just a word of causion here people,most cars with T.R.A.C and V.S.C use all four wheel speed sensors to calculate the cars speed.The analog signal is changed in-to digital for the e.c.u to read

Interesting - thank you - I wasn't aware of that. The last vehicle on which I had a speedo problem was a Camry (with traction control and cruise control). The problem was cured by changing the vehicle speed sensor on the transmission. Hence my comments. However that vehicle was, from memory, registered in 2001 so it is some time ago.

I had assumed that vehicles with sophisticated traction control systems would still retain the speed sensor on the transmission to provide a basis for comparison with individual wheel speeds etc. and as a relatively simple basis for cruise control. Do Lexus not now fit a transmission mounted vehicle speed sensor?

So if the rolling radius was different on the two axles would it take an average?

on the manual the speedo reading is taken from an inducer located at the rear side of the gearbox, on an automatiic the speed reading is taken from the right front ABS sensor