Cool weather -- mileage, and tire pressure warning

As the weather has turned colder I've encountered the following with my 2010 Jetta TDI sedan (has about 1,500 miles on it):

- On short(ish) trips, my fuel mileage has decreased. My commute is about 7 miles one-way, and when it was warmer I typically got about 38 mpg on my mixed city/highway path. With the colder weather, I'm now averaging about 33 mpg in similar traffic. I'm guessing this is temperature related as 1) the car is only just hitting normal operating temperature as I finish my commute, and 2) on 60 mile roundtrip freeway drive on this same tank of fuel I got the best mileage I've seen yet -- averaged just over 50mpg. (Mileage is based on the trip computer, which has proved to be pretty accurate when I calculate actual mileage after a refill.)

- I'm getting a low tire pressure warning. This started when it first dropped below 40 degrees and has been on constantly since. My tire pressures, checked with a good gage, were showing at 35-36psi when cold. I added about one PSI to each tire (so now all four are at 36-37psi) and the warning hasn't gone away. I'll keep experimenting with higher pressures, but is there any way for me to determine if I've got a bad sensor?

Fuel mileage info: http://forums.tdiclub.com/showpost.php?p=3199667&postcount=145

Do you have a VDCS to scan and see if it's something other than low tire pressure?

ajm80031 said:

I'm getting a low tire pressure warning. This started when it first dropped below 40 degrees and has been on constantly since. My tire pressures, checked with a good gage, were showing at 35-36psi when cold. I added about one PSI to each tire (so now all four are at 36-37psi) and the warning hasn't gone away. I'll keep experimenting with higher pressures, but is there any way for me to determine if I've got a bad sensor?

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It's normal. Using relative numbers, tires decrease 1 PSI for every 10° temperature drop. The TPMS low range is somewhere between 38-41 PSI (at least for the 2006, 16" Biolines).

If you want to increase MPGs for your 7 mile commute I recommend a plug-in coolant heater, since it's the increased warm-up time that lowers MPGs -- and winterized fuel to a lesser degree. I've been thinking about designing a similar heater that runs off an aux battery, but not sure it's worth the cost of parts/mods -- let alone distaste of a trunk mounted battery.

If that aux battery will be recharged via the car's electrical system, you will loose overall and the mileage will be less, not more.

Don't forget to check the spare tire. I found mine at 45psi filled it to 60psi and light went away.

OP, the mileage hit is normal. Also, with a 7 mile commute, you're nowhere near operating temp. The TDI takes much longer than that to be fully warm (no matter what your temp gauge says!)

I love my Frostheaters, on a timer from 5:00a-about 10am, anytime we're below 40f. Coolant at 105-115 at start-up.

tpms light

How do you turn off the TPMS light?

Vag com can disable the system.

Also there is no TPMS in you spare, at least mine doesn't have it.

If the valve stem is rubber there's not TPMS in it. Also the TPMS works and is turned on in each rim when the tire rolls. I found that out when switching over to my winter tires with no TMPS, I had the biolines in the car sitting still, and the system went off, indicating I had a problem.

Interms of the TPMS, there was an update to correct false low readings. If you haven't had that done at the dealer, you may want to. Of course, tire pressure will fall as temps fall (PV=nRT). So you'll need to monitor your tire pressure. In terms of the coolant heaters and improved mpg, this makes sense. I wonder if the improvement in mpg offsets the electricity used to warm up your engine block.