What's the problem with my tach?
 

So here's the situation:
I started up my car on monday, and the idle (which is normally 900-1000RPM) was registering at 1600RPM. My ears perked up a bit and I said to myself, "what the......"
Having changed the belts the day before, I figured that I must have done something to cause a vacum leak and the result was that my car was idling fast. However, just as I have a "seat of the pants" accelerometer built in, I also have a fairly well tuned "tone of my ear" tach that I developed over several years of driving a 73 VW beetle with no tach. My ear tach disagreed with what I was seeing on my physical tach.
So I took the car out on the road, and noticed something else: normally, I hit 80 mph in 5th gear with my tach at about 3900 RPM. On this day, however, I was at 80mph in 5th and my tach was close to 5000RPM.
Now here's the strange part...after driving around for about 15 minutes the tach went back to normal, I was once again at 3900RPM doing 80mph, and my idle was back where it should be. Having driven the car daily since then, every day but thursday (which was a bit warmer) I had the same problem. So it seems to me that something in my tach system is misbehaving until the car warms up. The question is, what should I be checking? I am handy with a wrench and passable with a multimeter, but I don't really know much about how the tach gets its input, or whether this is a common problem. Any recommendations or suggestions? Thanks guys!

OK, an update.....I just drove the car for about 30 minutes. Same problem, but fixed itself after about 10 minutes. However, a new development: I've been tracking my mileage because I just installed the new belts, and filled the tank when there was 73 miles on the belts/odometer. So when I filled up the tank tonight, I was at 274 miles, or 200 miles since I filled the tank. Normally I get about 25-26mpg, so I was expecting a fillup of about 8 gallons. Well, 11.2 gallons later, I am really befuddled. I guess my tach readings must tell the engine what mixture etc? Well anyway, my mileage has obviously plummeted. Nobody knows what might cause this? C'mon guys, I'm up for random guesses here. I hate it when my beautiful 944 is feeling sick....really bums me out.

check voltage at battery when running.

do late cars have different gear ratios? im only at 3100 at 80 mph

3100 at 80? i think is little rpm...Need to check it in the manual,but i think i have the same ratios and i should have longest.

If you have your manual,check it,i can send you a pic of the late´s manual with the gear ratios.

I'm showing 12.3, that's within acceptable range right?

After hearing this, I checked closely on the highway while my tach was in a good mood, and at 80mph my tach was reading just about 3850rpm.

should be 13.7 or so.

Do you think I should check out my battery or my alternator? Would the lower running voltage affect the DME and the fuel injection and other stuff like that? And would it affect the function of the tach?

low voltage can affect almost everything electric.

dude at the autoparts store told me that aleternator puts out different voltage at different times. like, it should be closer to 14V if you go drive around, come back and turn on the lights, radio, wipers etc.

and Manute: i dont have a manual :(

I can send you the pre-85 owners manual in french,and the post-85 (mine´s) in german,all in a zip.At least the gear ratios chart are universal. :)

if you wouldnt mind that would be great. i know a little french and german so maybe i can figure some of it out.
thanks :)

Ok! I´ll send you later!

So do you think my low voltage is causing both the tach prob and the mileage prob, or just the tach?