Originally Posted by mk5tdi
Hey guys. I have a 2010 TDI Jetta and I swear that after I hit 10k miles my mileage improved about 2-3 mpg. I monitor it religiously. My wife has a 2011 JSW TDI which just hit 10k miles last week. We carpool to work and my drive is 99% highway - about 22 miles. The car hit 49.9mpg today - the highest amount I've seen on either of our cars for the trip to work. Her car normally averages about 43-44mpg. Any ideas if there is significance on the 10k? My friend is an engineer at GM and told me that on some of the older GM cars (he was specifically referring to his 96 Imapala SS) the car was programmed to put out an extra 10hp after the 10k mile break in. This sounds like total BS to me. Any input is appreciated.
Yes. Not to burst the bubble, but typical. There are a series of little things that in effect, work together or conspire if you will, to give you the 2-3 mpg improvements. They each have their percentages. However as you will probably concede, the individual ones are hard to measure. As the car approaches 30,000 to 60,000 miles, you can expect even better mpg (1 to 3 total) improvements due to diesel break in mileage.
An easy one worth app .5 to 1 mpg is that new tires get worse fuel mileage. Just in terms of fuel mileage, it will get better the closer to bald you are. So for example if the tires start off at 11/32 in, 3/32 in should be the BEST.
Another is the engine wastes less and less heat energy due to break in. It is not yet however at full compression (range of full compression is app 30,000 to 60,000 miles)
You can further do little things to improve those numbers. There used to be heated discussion about the obvious increased tire pressure effects. However even VW has come out in the (2012 Touareg TDI's) owners manual, saying that higher than oem recommendation psi (3 psi more) will yield better mpg. Since the recommendations are 33 front 38 rears or 64.7%F 74.5% of max side wall pressure of 51 PSI, 3 psi more would be 36/41 PSI or 70.6 % F/ 80.4% R. of maximum side wall pressure (51 psi) So for a Jetta TDI I started at 85% of max side wall pressure or 43 psi and adjust according to SOTP's. So for example some folks like a softer ride. Over 175,000 or so miles I found that 38 to 34 psi is a good balance between tread longevity (i.e. slightly less wear) and ride.
Glad you have having good luck and pleasure with the two TDI's.