2011 BMW 535 - Fuel gauge

bmw

#1

Why are car manufacturers unable to make an accurate fuel gauge? In this age of sensors and off the shelf technology it seems absolutely impossible not to have something that tells the amount of gallons remaining in the tank. This applies to every vehicle I have driven.
Thanks
PS. feel free to edit after the first sentence. Miss you guys on the radio


#2

Well, if you do like most people and just refill when the level reaches 1/4 mark then it does not matter how many gallons are left. Also so many vehicles now read out ( miles to empty ) that is fairly accurate seeing as how it only applies to current fuel mileage.


#3

Because accurate fuel gauges cost more money than the “close enough for 99% of people” gauges they use now.

I owned an '84 Corvette that had the most accurate trip meter I’ve ever seen. When it said you are on reserve there was exactly 2 gallons of gas in the tank - Exactly. As an engineer, I thought that was cool. As an automobile owner, it didn’t matter at all.

Really, what IS the problem? When the gauge gets low, or the yellow low fuel light comes on, fill it and get on with your life.


#4

Every modern BMW I have tested does have an accurate readout of the amount of miles left to travel before the vehicle is out of fuel. Every car of every brand actually. The automakers use miles rather than gallons, but the math is easy if you want the volume left.


#5

Good question. Here’s some of what makes the fuel gauge inaccurate

  • The calculation (made electronically) between measured float arm angle and tank fuel level isn’t accurate
  • The gauge needle movement (vs voltage/current input) isn’t accurate
  • The variable resistor which senses the angle of the float arm isn’t accurate
  • The float arm is bent
  • The float is not shaped the same as other floats for that car
  • The fuel tank is not shaped the same as other tanks for that car

Even if all that stuff above could be made super-accurate (which it could if cost were no object), the fuel level still changes with acceleration/deceleration, uphill/downhill leaning, left/right leaning, and bumps in the road. A really smart gauge could compensate for acceleration and leaning, but hard to compensate for bumps in the road, so that’s probably the limiting factor if cost were no object. But even if you knew the exact amount of fuel left in the tank, you still don’t know how many miles that will last b/c it depends on traffic, terrain, and driving style.


#6

Funny you should mention that. I had an '89 and had the same reaction to how well it tracked fuel level and consumption.


#7

Thanks so much for the detailed response. I still believe that just like checking oil on a warmed up engine,
on a flat surface it would be possible to determine an exact amount under certain circumstances. As for the other factors, I believe most

car enthusiasts [and “normal” people] know the mpg of their car after using it for several months. That’s

assuming they monitor that number.

Thanks again,

Lew