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Discussion Starter #1
18' 3500 here. My def gauge was at 75% full. I added 2.5gal thinking that would almost top it off but it was spilling over once I was at about 2.2gal. I thought the total tank was 5 gal and the gauge was at 2.5gal or less? Any help would be greatly appreciated.
 

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Was it actually full or was it just not able to vent fast enough as you filled it? I've had mine spillover when I know there's still lots of room in the tank, just had to go slower.
 

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18' 3500 here. My def gauge was at 75% full. I added 2.5gal thinking that would almost top it off but it was spilling over once I was at about 2.2gal. I thought the total tank was 5 gal and the gauge was at 2.5gal or less? Any help would be greatly appreciated.
If the tank size is 5gal and your gauge read 75% full, you should have room for 1.25 gals. 2.5 gals won’t fit. I think the def tank is between 5 and 5.5 gals. Pretty sure the Chassis cabs are larger. The gauge stays on full for a long time, they’re not real accurate. Next time wait til the tank is less than 50% then add the 2.5 gals. I wait til it’s near empty then add 2.5 gals, trying to use up the def. Others have chimed about def going bad being in the tanks so long especially in the heat. There is a u tube by Too Tall on how to empty the tank if necessary.
 

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gauge is not accurate at all, the tank holds about 2 gallons above what the full gauge says. I let mine get into the red, poor a 2.5 gallon jug in and it takes it back to 100%. I could easily poor in 2 more gallons, but I do not go through the DEF fast enough, and in this summer heat it goes bad in the tank over time.
 

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DEF tank is 5.5 gal. When close to "full", I've gone upwards of 3k miles before the gauge even moves. I've added a 2.5 gal jug when it was showing around 3/4 full without it sounding like it was topped off. Perhaps it was a case like Lins stated in post #2.
 

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I usually just had 2.5 gallons at a time. That will have my gauge show full when I fill it up when it hits the red. I don’t go through enough to dump 5 gallons in.
 

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Discussion Starter #7
Thanks for all your tips. Yes, I will wait until it's at half or less next time and not even worry about the extra 2 or so gallons. I'm in FL so that'll suck if the fluid goes bad from the heat.

I pull a 5th wheel every other month, does def fluid burn faster when towing heavy loads? About 13k lbs
 

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Thanks for all your tips. Yes, I will wait until it's at half or less next time and not even worry about the extra 2 or so gallons. I'm in FL so that'll suck if the fluid goes bad from the heat.

I pull a 5th wheel every other month, does def fluid burn faster when towing heavy loads? About 13k lbs
Yes it uses more def when towing, but not a huge difference. Have also heard others say if the def is weaker due to heat or age it will use more also. Some of us that don’t put many miles on need the Chemists to come up with DEF stabilizer like what Stabil does for gasoline. Maybe it already exists, anybody?:stirpot:
 

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please watch this informative video on how your DEF gauge works

https://youtu.be/czJZuc8ThhI
Thank you Learjet, that explains it clearly. So we have approx. 3.5 gals when the gauge starts to move off of full. Good for about 3500 miles if we get 1000mpg as the video says. Ballpark range is 5500 miles per full tank of def. great information.....
 

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if you wait until the gauge gets down to 1/2...you can dump 2.5 gallons in.
 

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The gauge works a lot like most fuel gauges work, too. Stays on Full for a lot longer than when the tank is actually full. Goes down a lot faster once it gets below half.

I run mine down until the warning messages actually come on the EVIC. There is a truck stop 15 minutes way that has DEF at the pump; works great to fill up exactly to the top, no need to store DEF that will eventually go bad.

If you do use old / expired / bad DEF, your truck will know, display error messages, and trip fault codes.
 

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When my DEF gauge needle touches the red, I add 2 1/2 gallons which pushes the gauge to full. I don't think the gauge is accurate or linear.
 

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This explains how the gauge works and why it is not linear.
Wow, that explains it. Thanks for posting the link.

RAM should have since they are using a non-traditional fluid level detector, either included a paragraph in the owners' manual that says something like the gauge indicates full and only starts to track when the fluid level once it falls to 2 1/2 gallons or used a different notification on the cluster. I guess one could argue no one reads the owners' manual but there are exceptions.
 
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