My 2005 fuel gauge will read 3/4 full when I fill it until the gas pump cuts off. At that point, the gas tank is almost full to the brim. If I top the tank off all the way to the brim (just barely below the lip of the filler) it will then read 4/4 full; I may have added 0.2 gallons more.

But that's not the biggest problem.
After riding for 75 -90 miles, it will show very low (reserve), so I will gas it up but only have to add 2.5 gallons!
What gives?

Any ideas?

 

That sounds normal to me. When my gauge blinks red on the bottom bar, usually about 90 to 100 miles, it doesn't take more than 2.5 to maybe 2.8 gallons when filling it on the sidestand. I think maybe once, I got 3 gallons in there, filling it to the bottom of the lip. But fuel gauge readings can vary from bike to bike, especially if somebody jammed the gas nozzle down the filler and bent something.

I run it down to the blinking red anytime I'm gong to pull the tank off, then it's light as a feather.

 

Doesn't it seem odd to you that the tank is a 4.2 gallon tank with a 0.7 reserve and the gauge registers low when only 2.5 gallons are used?

The math is wrong.

2.5 + 0.7 = 3.2 gallons NOT 4.2 gallons

So why does it register low/reserve when there are 1.7 gallons left?

 

The gauge has 4 bars... But it has 5 levels... 4 bars is full or just under 4.2 gallons... 3 bars means roughly 3.2 gallons... 2 bars means roughly 2.4 gallons, 1 bar would then mean roughly 1.6 gallon...

Ie where you go and fill it up... But the gauge goes from 1 bar to a blinking bar, which means reserve... And that means 0.7 gallons... Just like it's supposed to...

Obviously, there are slight variations, and if someone bent the float, it's off... But that's why you get the math wrong... You are never on reserve when you fill up...

 

My '05 shows all 6 bars when filled on the sidestand to the lower lip on the filler neck. I fill up at least twice a week.

To get full capacity you would have to have it standing straight up, and overfill it.

 

05Titan thank you for posting this. I assumed my sending unit in the tank was AFU. My gauge works just like yours.