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I have a 2008 S crew cab, 4.8 with 67,000 miles that has always got good mileage, 16/22 mpg. Now it's only getting 12 mpg around town! I don't know why because it's running smooth, not skipping, no hesitations. I have started to get a little whiff of a catalytic converter smell occasionally but not bad. Does anyone have any ideas? TIA!
 

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2000 Silverado Z71 4x4 5.3L 480k+ miles w/ GM rebuilt motor and trans
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I saw something where they were going to allow more ethanol in gas because of the "crisis", but never followed up on it to find out if it was true...
 

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Most gas contains 10% ethanol however my understanding is some places are allowing 15% ethanol now. You can purchase a ethanol testing kit on Amazon for around $12 and actually test the gas to see how much is in it.

More ethanol = less gas mileage.

Another thing I would do is check live data and make sure everything is within acceptable range.
 

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2000 Silverado Z71 4x4 5.3L 480k+ miles w/ GM rebuilt motor and trans
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Thanks but mine does not have the capability of checking live data, I check it manually. I have been putting in 10%, but I always have. It just drastically changed
I think what he means as "live data" is your fuel trims and such. This will require a scan tool or a Bluetooth dongle and phone app.
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2000 Silverado Z71 4x4 5.3L 480k+ miles w/ GM rebuilt motor and trans
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I use the Torque Pro app (paid ~$5) and a cheap Bluetooth OBD2 dongle from Amazon. There is a Wiki for it and some online guides. You will need to install your vehicle specific PID's once inside the app. Torque Pro does some brain work on its own, so the "not supported" PID's still work. I prefer this app.

I also use BlueDriver from Amazon, but it doesn't work completely on my model year vehicles. I think it would work fully on your model year, but I'm not 100% certain. It depends on what sensors and PID's GM programed in and supported.
 

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I have actual scan tools but I have used the Piston OBDII app which is free with a obdii dongle it was about $10 on Amazon for me a few years back called Vgate Scan. You have to be careful though there is a lot of fakes out there, the first one I purchased prior to the Vgate scan ended up simply being a little circuit board and a LED light.

BlueDriver as mentioned above is a good choice for most people it is $99 but the app is free and it allows you to access mode 6 which can be very useful.
 

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2000 Silverado Z71 4x4 5.3L 480k+ miles w/ GM rebuilt motor and trans
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BlueDriver as mentioned above is a good choice for most people it is $99 but the app is free and it allows you to access mode 6 which can be very useful.
I originally bought BlueDriver for the Mode 6 data, but it would not show me the misfire data for my 2000 Silverado. I read somewhere the really good Mode 6 data starts on GM vehicles somewhere around 2008. I was able to get basic misfire counters from Torque Pro with the GM PID's add-on (free in-app).

If I had it to do over again, I would not have bought the BlueDriver. None of my personal vehicles are new enough to fully use it.
 

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I have a 2008 S crew cab, 4.8 with 67,000 miles that has always got good mileage, 16/22 mpg. Now it's only getting 12 mpg around town! I don't know why because it's running smooth, not skipping, no hesitations. I have started to get a little whiff of a catalytic converter smell occasionally but not bad. Does anyone have any ideas? TIA!
Try running some Cataclean in it if you suspect smelling the cat, but have your tried running codes? Just because there's no check engine light doesn't mean it won't throw codes. I check mine weekly on my 8.1 Sierra, and have caught things like my MAF needing cleaning, or throttle body etc, before it was bad enough for the check engine light.

I live in the hi desert, so around once a month I clean my air filter, and while i'm at it, i clean the throttle body and every two months i clean the MAF. I'm not showing a check engine light but I know my TPS is bad based on the symptoms, and sure enough when I checked, there was a code for it.
 
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