| I have had my 2500HD with Duramax and 5 speed Allison since it was new. Got 43,000 on it now. I always use a good grade of 15W40 just as the book recommends. I took it back to the dealer for the first oil change at their recommended mileage. They will check everything else while you're in so you will have fewer problems. These trucks all have 3:73 gears - even the 3500s. They all should get around 17 -19 COMBINED city and highway. I get up to 21 on long trips unloaded. Because I live about 10 miles from the city I don't know what a good average would be for city driving. The only things that make a big difference in fuel mileage in these trucks is #1- your driving habits. If you are like I was at first you won't be able to resist quick starts away from lights and strong accellerations up the freeway onramps. These things excellerate amazingly well and are F-U-N to drive. So, you get what you can expect. #2-The other main thing may not have so much effect because the torque is there in all of the Durmaxes. That would be the gross weight of your truck. The lightest one, a 2500 HD extended cab short box is about 1000 lbs less than the 3500 HD crew cab long box 4wd. If you tow a camper or other trailer with high wind resistance expect your mileage to drop accordingly. Terrain, flat or hills, will dictate too.
If I were you I wouldn't change to synthetic oil until about 30 to 40 thousand miles. The reason is that diesels wear a lot slower than gas burners do. You want to make sure that you get it good and broke in before you switch or else you open up a new can of worms all together. Generally, it's better to stay with the same grade and brand all the way. |