speedometer calibration

[jr4115]

Changed from stock rims to 17"'s on my 1996. Had, in hand, a chart which indicated changing from the stock size to a lower profile (55's) would make less than a 2% difference in my speedometer indicated and my GPS real time speed.  Well this is not the case. At 60 on the GPS, needle reads 40 in the dash. Think that's a tad bit more than 2%. Speedometer doesen't even start moving till about 15 mph.

In reading on-line, I thought the transmission, with the  analog dash, still reads pulses thru the cpu but was told that this is not the case and, although, accessing the "brain" and actually (with the proper scanner) putting in the current tire size, adjustments can be brought back into line. According to a 20 year Toyota/Lexus tech, this cannot be done this way.

20 mph off is a big difference. Anyone who has had a similar problem and have come up with a solution and could pass along the best possible fix would be greatly appreciated, thanks.

[Love-lexus]

Wish I have the solution for this... Will contact my autogeek friend to come answer this.

[sha4000]

It's not the tires, your speedometer is most likely off.