Speedometer/Vehicle Speed Sensor Low Reading

[Loaf]

Hey everyone. Like a lot of people, I've been lurking for a while, but finally got around to registering for this problem.

I have a 2008 Aveo LS with a manual transmission. My speedometer recently started displaying my speed as significantly lower than my actual speed (saying 25 when I was going 70 mph). I could still judge my speed using the tachometer and other cars around me, at least. At first it would still read correctly as long as I was in gear and I was pressing the accelerator (but not when coasting, even in gear), but then it stopped reading correctly all the time. For instance, if I was driving at 60 mph, the speedometer might show 20 mph, but it would bounce around like it sometimes wanted to read higher or lower (e.g. bouncing between 10 and 40 mph).

The odometer is also reading my miles traveled as less than what I'm actually driving now (saying I've gone about 150 miles on one tank of gas).

I figured it must be the VSS (on the rear side of the transmission), so I bought the part from the dealer and swapped it out. Now the speedometer no longer bounces and keeps a steady reading, but it still reads my speed about 3/5th of my actual speed (saying I'm doing 30 mph when I'm going about 50 mph). The new part is identical in every way as the old part, even the serial number on the sticker.

All fuses that I could think to check look perfect. I have no check engine light. I want to use my multimeter to check the wiring, but I don't know what values to look for. Anyone have any ideas?

Thanks.

[petrified.rabbit]

It might need the values in the ECM to be set by the dealer.

[2010AveoLT]

are you running stock Size tires? (I'm running a 185/70 Series Tire, and my Speedo is off about 3MPH because of it) another reason it might be off is the actual stepper motor that runs the speedometer might be giving up because of being driven by the bad VSS for so long. (also, do you have a sedan or hatch?)

[Loaf]

are you running stock Size tires? (I'm running a 185/70 Series Tire, and my Speedo is off about 3MPH because of it) another reason it might be off is the actual stepper motor that runs the speedometer might be giving up because of being driven by the bad VSS for so long. (also, do you have a sedan or hatch?)

Stock wheels and tires. I'd have to have monster truck tires to make my speedometer be off by 30 mph, haha. I doubt it's the stepper motor because I changed the VSS within a couple of days of the problem starting (unless both malfunctioned during the same trip), but that is worth looking into. Car is the sedan version.

[Pickles]

While we are on this topic, what do I do if I go to a much smaller tire? Pay the dealer an outrageous amount of money to tune it for the new tire size?
Was gonna get a set of 165/45r15

[petrified.rabbit]

just figure it out in your head.

[misslomenzo]

Did you find out what the problem is? I'm having the same issue.

[marketwatcher]

That much off it can't be tires