Sc400 Gross Speedo Error

[STORMINORMAN]

1993 SC400, 95K miles, starts, idles, runs, drives & shifts perfectly (IMHO). Standard sized 225/55-16 tires, no CEL's (check engine light comes on and goes off at starting, as it is supposed to).

Speedo consistantly registers 10-15 mph LESS THAN ACTUAL SPEED!

I'm advised that with OBD I no CEL = "no stored codes". Dealership wants $120 to hook up to OBD I scanner, but if there is no CEL there wouldn't be any cdes to read, would there?

I can purchase an OBD 2/OBD I compliant scanner with accessory plugs for Toyota/LEXUS, Ford, GM, etc. for $199, but what good is that going to do me?

Any ideas or help would be greatly appreciated.

p.s Does anyone have an accurate RPM to mph chart, assuming 4th gear and normal freeway speeds? This would greatly prevent a repeat of Friday's conversation which began with "I clocked you as going 71 mph (in a 55 zone!) plus or minus 2 mph..." when my speedo was registering between 55 and 60!

[1990LS400]

If you want to verify your true speed, consider buying an inexpensive portable GPS - Garmin, Magellan, Tom-Tom. The speed display on all the one's we've owned have been more accurate than car speedometers. I probably pay more attention to the speed shown on my Garmin than the speedometer in my LS.

[STORMINORMAN]

Great suggestion, BTW! Yeah, I bought a COSTCO Magellan unit with free forever mapping updates, AAA links (which included an upgrade on the warranty from 1 yr to 2 yrs!) for $89.95 shipped to my home. Hooked up a smartphone/GPS app and an indicated 20 mph is 10 mph low, @ an indicated 70 it's closer to 84-85 mph actual!

I'm still wondering how I can not be throwing codes: there is no indication that the odometer is off (still about 180 miles between our homes) and gas mileage is still in mid 20's depending upon how much of a tank is local vs. primarily freeway speeds.

On another forum someone suggested there was a possibility of a "worn speedo motor" actually driving the needle? Could this be some reference to the VSS1 in the dash?

I'm stumped, but not likely to get another ticket by being unaware...