Hi All, Pardon my ignorance, but I am totally lost as to what to do about my extremely erroneous speedometer/odometer on my 1978 308GTS. Both the speed and miles register about 18% high. I drive the stallion frequently, and have checked the odo against mile markers and have timed the distance travelled; every 10 miles on the markers I register 11.8 miles, and at an indicated 70 MPH travel an actual 10 miles in 6 minutes and 10 seconds. The car is very stock with Toyo Proxes 215/60R 14 tires installed. Even accounting for the reduced radius tires, I think it should only be about 5% off at the max. As far as I know, the gearbox is standard and feels much the same ratio-wise as other 308's I have driven. Is there a reasonable fix, or am I destined to rack miles at an excellerated rate? I only have service & maintainence history on about the last 20K of the 60K showing. I am totally in love with the old girl, and would like to get this sorted out if possible. Thanks in advance for any hints. Cheers, Warren in Phoenix
My mechanic told me that the contacts needed cleaning. So, I told him to do what it needed. It fixed it along with the bouncing fo the needle as well.
I have same issue with my 83 308. I have discussed this with a few other owners and have determined that it is common to all of our cars. I can estimate the speed, but the over mileage is hurting value. Dirty contacts would indicate a voltage drop which would cause a jumping in the guage, but also would result in a lower speed and recorded mileage as the signal intermittently dropped out. (My guess - no real knowlege of problem) I do not have a jumping guage, and there is no zero offset, but I read about 15% high with the original 390 wheels and TRX tires. If you find the cure please post and let us know.
It is not uncommon for the sending units to go bad on these cars. Mine has crapped out twice in 20 years.
I would agree with Tommy, try replacing the speedo sender. While usually they just stop (not read high). It is possible that is the cause. They only cost $125 or so...and if it is not the cause, worth having a spare, since they go every few years anyway. Otherwise, do what others said, send it to a speedo shop and get it properly calibrated.
Gentlemen, Your advice and observations are much appreciated. In reply to the other owners with similar problems, the speedo needle is rock steady, and swing is smooth and consistent; I agree about living with the correction, but the registered mileage is most important. The only fun part about it is when giving short test rides to friends and acquaintances; the zero to 100MPH times are dazzling, and the risk of being tagged in a 75MPH limit zone while thrilling you passenger with such seemingly brisk performance is minimal! I think my first approach will be the sender - both because it is a known issue that is neither expensive nor puts my car out of commission, and second because the smooth operation of the gauge does not match the hop some have experienced. If the sender is not to blame, then a parcel to Palo Alto will probably be soon to follow. Thanks again for the informative responses; this is a great group! Cheers, Warren
Hi Based on my reverse engineering of a 308GT4 speedo, the only calibration possible is for the speed (the needle part). The odometer is driven by pulses (divide # of pulses from sender by 32). I don't see how there would be any way to adjust that. Pulses from sender should be marked on the sender, for 308GT4's and early 308GTS/GTB it is 4 pulses/rev.
