truspeed by superlift (
http://www.superlift.com/accessories/truspeed.asp )
this unit is made for toyota trucks; when installed on solara, it was inconsistent from when you first start driving to 20 minutes later, however directions were not followed on install so results may be wrong as instead of hooking up to wire at transmission, wire behind gauge cluster was used, also wires for signal and 12v supply were bundled together and i believe they are supposed to be separated.
I could get this thing perfectly accurate, let the car sit over night and the next moring it would be -10 mph off at 35mph (gauge says 35 when doin 45). But after driving the car for over 20 minutes this would slowly change and get closer and closer to accurate. biggest problem is even when splicing into the wire directly behind the guages, shift points are changed as well. it seems that the signal hits the gauge and then goes to the computer. when the speedometer reads less than actual speed it will hold gears longer and rev into redline, obviously the opposite happens when the speedometer reads more than actual speed... the shifts come too soon.
I agree it sux that when the speedometer reads higher than actual speed the odometer says your car has more miles then it really does. my car was 4-5mph over actual speed with stock wheels. Readings on car speedometers is not always accurate from the factory.