I almost never look at my DEF level. When I'm at a station that sells DEF at the Pump. I top off. I've only seen the 500 miles to empty message Once on this 2015 truck in 80,000 miles and only a couple of times on my 2011 truck with 150,000 miles. I just don't think it makes much difference whether I top of with 1 gallon or 4 gallons. I stopped buying the jugs and just pump at the stations and top off what ever it needs.

 

Everyone's been talking around the answer, but it's pretty simple: You can't rely on the accuracy of the truck's DEF gauges, either 500 or 200 mile. Nor is there a consistent rate of usage. If it runs dry, it'll sense that at another point than the in-tank gauges and will put you in limp mode or turn your engine off entirely. So you're better off filling the tank every 3000 miles or so and not worrying about it after that. I do an oil change every 5K miles and top off DEF at every 2.5K interval. It's easy to remember. There's no reason to let levels get unnecessarily low. You can always get a handle on usage by measuring how much you put in every 2.5K miles. That's about as useful as how many miles before the 200 mi light goes on.

 

Well. Since I began this thread I might as well end it too or at least keep up with it. Just yesterday I got another 500 mile warning. This one popped up right about the mileage I thought it should be. Something set the first one off prematurely which caused me to generate this inquiry. I suspect all's well with my warning system and I'll monitor it and fill it when I get the 200 mile indicator. So, by the time I get the 200 mile I project that I will be in the neighborhood of 4000 miles since my last re-fill.

ThanX all for your discussion and input. At least I think I understand my DEF consumption a little better now.