Figuring out gas gauge inaccuracy

Hi guys. Now that my bike has been running solid for a week im focusing on the next issue. The gas gauge is way off. I did some math and calculated its coming on 50km sooner than it should. My notes:

• Filled up tank to top and reset trip meter
• Did 304.4 kms
• Calculated mileage (km-age?) = 5.98 L/100km
• Noticed gauge float resting on fuel at fillup and lots of fuel remaining
• Filled up to top using 18.2 litres
• Fuel tank capacity for 5th gen = 20.8 L (4.6 imp gal; 5.5 US gal)
• 20.8 - 18.2 = 2.6 litres remaining after 304.4km
• From the manual 3.5 litres is when the light is supposed to come on
• Difference of 3.5 - 2.6 = 0.9 litres
• Noticed last fuel bar came on at a certain landmark
• Math to calculate distance from landmark to gas station

34.1 + 30.8 = 64.9 km
304.4 - 64.9 = 239.5 km

calculating where 3.5 l actually was during last trip

5.9 0.9
— x —
100 x

solve for x
=15.25
= (-)15km at fill up when light should have come on
= 304.4 - 15.25 = 289.15
= last fuel bar should have come on at 289.15​

289.15 - 239.5 = 49.65

tldr:
Last bar came on almost 50 kms early​

My two questions are:

1. Does anyone know the angle of the wire on the fuel float sending unit (37010-MBG-000) should be at so I can verify it? I did mess with it and may have slightly bent it.
2. Or is the problem with the floaty part at the end after 20 years and should that be replaced (probably whole unit)

 

This kind of fuel gauge/sender is at best a rough estimate, at the best of times. If you want it to be spot on, your best bet is to throw it away and install something better.

That being said, mine is pretty reliable. It drops a line for every 50-55 kms. I fill up at 300-330 kms so I don't know how the last line behaves. Also means I never have to put in more than 17'ish liters, so not sure about the real capacity (there is always fuel you won't get out). I should really try to run it dry just to find out.

You really have nothing to worry about here, unless it starts fluctuating wildly. You will never get all the fuel out. You will never get it to be 100% in line with the factory specs. Those are at best, estimates.

Also; Unless you were riding real hard, you might want to check up on your fuel consumption. Looks like you are getting a full 3 km/l less than I do and that is not a small difference. We are talking something like 15%

 

Ill just buy another sender unit. Its definitely off. Got lots of bikes and havent run into something this inaccurate. The last bar/fuel reserve light shoudlt be this far out so something is definitely up. eg: first bar on my vfr lasts maybe 5km. And yes Ive been pushing it pretty hard to find its weaknesses while testing it.

http://www.fuelly.com/motorcycle/honda/vfr800_interceptor/1999

Im not far from the average of 5.75 and should get way better L/100km once im in cruise mode.

 

Even 5.75 is pretty damn bad. Even pushing it to the limit, the absolute worst I'm getting is around 5.40. And with mixed riding (still giving it plenty of wrist), it's more like 5.00.

I keep VERY good track of fuel consumption because it is often the first obvious place to spot problems. And yours is how enough that I would be pulling it apart to find out WTH is going on. Small problems become big problems real fast if ignored.

 

Yeah i know. I spreadsheet every one of my bikes for all maintenance and gas. Thats how i got my mileage figure. I am pushing it pretty hard to find suspension weaknesses. I race bikes at the club level. Going to need emulators.

 

I use an app called “Fuelly” on my iPhone. Keeps track of every thing and calculates your milage after each fill-up. Categories for tires, oil changes and other maintenance. You can have multiple vehicles, but I don’t bother with my old SUV!

 

Yep posted a link to fuelly above to reference fuel economy from other people. I used road trip in the past on my iphone and it was good. I found that I like having a spreadsheet (google sheets) because I like to have more fields to edit for my maintenances/notes and can use a ton of formulas for customization.