ORIGINAL: duckstu

Newbie here,...first post.

I just completed a test on the gas guage,...for mileage.

Had the 2006 H3 serviced,..dealer replaced an exhaust cam position sensor,..changed the oil. 15,000 miles on the odo.

I had the alignment checked and carefully setat zero toe.

Topped off the gas with midgrade BP/Ammoco and started normal driving. I'd say 70% city, 30 highway. But 1/2 of the city driving was cruizing on 50 mph main roads with well timed lights at fairly steady speeds.

So going from;
full to 3/4 tank took 90 miles
from 3/4 to 1/2 took 90 miles
1/2 to 1/4 took 60
1/4 to "low fuel" indicator took 71
from there to needle fully in the red another 9
ran out of gas at 360.5 miles and coasted another .2 miles (had 3 gallons in the car with me).
(note,...1/4 tank to empty was 120.5 miles)

MPG average was 15.84 when I filled up.

From completely topped off, it went to
3/4 tank at 90 miles
1/2 tankat 180
1/4 tank at 240
"low fuel" at 311
needle fully in the red at 320
came to a stop at 360

So basically the guage is off a chunk at 1/4 tank. Each 1/4 tank should be 90 miles,..and itIS, except for at 1/4 where it reads 240 instead of 270. If I were to draw a new 1/4 full marker on the gauge, about 3/16 farther down from where it is now,....the gauge would read correctly.

I added what it took to fill up the car to the 3 gallons I put in it when I ran out to find both the fuel mileage,..and the tank capacity. (included mileage to the station in MPG calc).

With the car parked leaning slightly to the right,..I was able to get 19.82 gallons in it,..that with the 3 gives me a capacity of 22.8 gallons to gas-cap full.(all the way up the gas filler neck). So if it really does get 20 mpg on the highway,...I should be able to get 456 miles out of a tank before it stops.

I'm going on a road trip next week with it,...so I'll let you know. 75 mph for 1,400 miles should get a good solid idea of what it does.

Interestingly,..the speedometer is right. In most cars it is optimistic,.. I checked it with a hand-held GPS. With the speedo at an indicated 80,...the GPS read 79.5.

Also,..I had a slight tinkle sound from the engine when new (detonation). The dealer said GM was aware of the problem but didn't have a fix. (They said they thought it might be a problem with the counter-ballance shafts) They noted it in my records. (whoopie)

I started using mid-grade and it went away (guess it wasn't the counter-ballance shafts huh?). A friend borowed the car for a few weeks recently. (He's an engineer with a GM supplier and has a laptop with ecu software etc). He said it was pulling timing with regular grade,..and not with midgrade. No difference going from mid-grade to 93 octane. (Gets better mileage with mid-grade). He also found .3 grams more on the mass-flow sensor with the airbox lid off. (I found no gain from that HP wise on the ole butt-dyno,..and the honking made my head hurt,..so lid stays on).