Those must have been faulty to be off by that much. I've run mine down till it said 0 miles and there were NO flower petal lights on the gauge anymore.

It was fine.

 

The approximate range should change based on how you are driving. Hence it goes up on the highway and drops in stop and go traffic....

 

FWIW my R53 behaves in the same manner

normally - using the analog guage

Full to 3/4 - 160 miles
3/4 to 1/2 - 120 miles
1/2 to 1/4 - 90 miles
1/4 to Nil - 70 miles

Approximate numbers, but you get the point...

I've a theory, which is based on a lot of half assed assumptions, and centers on the fuel tank sending unit.

What if the sending unit is affixed to the top of the tank, and consists of an arm connected to a float. If you imagine this, and imaging that arm traveling from dead level (full) to fully depressed (empty) you might note that the amount of rotation required by the pivot - where I imagine the sensor to be - varies in a non-linear manner.

Now I'm too damned lazy to whip out my trig book and refresh, but I reckon a sine function would closely approximate the relationship between actual and indicated fuel level, which if i recall correctly is what happens when you map a curve onto a linear axis.

But mebbe I'm full of it - it's happened before...

 

Yes, you definitely have a point about it mattering more about knowing when you're close to empty. However, this flies against people's intuitive view of how it works (or at least how it should work). Hence all the complaints!

I've had two past Audi's and both were perfectly linear... pretty much to the km.

 

Normally I just wait til about 3 corns and fill, but yesterday I found a deal - $4.199/gal - so I topped off. I only had 4 corns off, so I wasn't expecting to put in much. It ended up taking 8.1 gallons. I was going to report them to weights & measures until I read this post here.

 

Here's how I interpret the fuel gauge:
10 petals: full tank
9 petals: 3 gallons burned/11 gallons left
8 petals: 5 gallons burned/9 gallons left
7 petals: 7 gallons burned/7 gallons left (half a tank remaining)
6 petals: 8 gallons burned/6 gallons left
5 petals: 9 gallons burned/5 gallons left
4 petals: 10 gallons burned/4 gallons left
3 petals: 11 gallons burned/3 gallons left
2 petals: 12 gallons burned/2 gallons left
1 petal: 13 gallons burned/1 gallon left

In other words the 1st petal is 3 gallons, the next 2 petals are 2 gallons each and the 7 remaining petals are 1 gallon each.

 

The reason fuel gauges aren't linear is because the tanks aren't a perfect square, rectangle or ball. The float sits in the tank and as the fuel is used up the float gets lower. Picture a bowl from your kitchen. It's much wider at the top than at the bottom. If you were to deplete a liquid in that bowl down to the mid point of its height your gauge would show half full/half empty. The remaining liquid would not be half of total capacity. Fuel tanks have similar characteristic albeit they are not like a soup bowl. They are shaped in general so that when your gauge shows half full there is less than half of the fuel left, that's why you get fewer miles from the second "half" of the tank.

 

This is understandable, but with a digital dial of 10 segments and known tank dimensions would it have been that big of a feat to have it read a bit more accurately? It is not like there is a string attached to the float that runs directly to the needle of your fuel gauge.

 

Excellent point and I tend to wonder that myself. I also wonder if it had something to do with cost. Ya know the bean counters in the auto industry try to cut corners every place they can and making the instrument convert from analog to digital may have been too "expensive." Who knows?

 

I make it a habit to calibrate and define the truth from what is shown on a fuel quantity gauge, this looks like an excellent way to read a MINI Gas Gauge
Since MINI did not mark for [F ¾ ½ ¼ E] like most gas gauges,
just remember 'three dark= ½ gone' and appreciate the accuracy for the remaining ½ tank now marked at one gallon per petal.

 

My Acura's guage is linear, or at least it seems linear. When it's at 1/4 empty, i fill 2-3 gal. 1/2 empty, i get 6-7 to fill the tank. 3/4 empty and i'll get about 9-10 gal.
--

You could be on to something, i had 2 petals left and filled 11+ gallons. I'll test this when i next fill up.

 

I like the breakdown. But at 2 petals (2 gallons) I usually show about 70 or so miles on the OBC for range. I wouldn't trust that I would get 35 mpg at the end of a tank! I try to fill up at 3 petals just to be safe.

 

My Scangauge gave me slightly different numbers, also the petals make more sense in litres, and the tank holds a little more than the rated 50 litres.

 

DanF, I like your analysis. I drive many miles each week and I've checked my last two fillups and your scale is right on the money. Your "spreadsheet" is a testament to the shape of a fuel tank. Thanks for taking the time do work it out.

 

I love this forum. After I posted, I started thinking, "I oughta suggest that we start taking notes & post the data from fillups." And POOF, someone already did the work. Thanks! I think I'm gonna print this up on a little card and keep it in Bimini so I can check her numbers against it. Maybe an added page in the owners manual.

And back to my post and the reason I came here - it looks like I did get ripped off! I'm gonna report that SOB.

 

I only 'fill' to the auto-cutoff of the nozzle.

My first 'fill' was 7 miles past 2 red corns and took 13.4 Gallons, second 'fill' was 10 miles past 1 red corn-cob and took 13.7 Gallons.

I have never had an Accura, Audi or Honda, but have noticed that most auto gas-gauges I have seen are not linear. MINI may be the least linear

Prefer that capacity is greater than expected, found out the hard way that my LeBaron's 14 Gallon (in the OM) tank would only hold 12 gallons

The Only Time you can believe an Aircraft's Fuel Gauge is when it shows empty and you better have another tank left to switch to.

 

This is more accurate I am sure but who would remember all the fractional amounts. The numbers I put up for each petal are easy to remember and close enough for me.

I took a road trip last weekend to NJ. Filled up in New Hampshire. MPG gauge was averaging around 44-45, so I figure the actual mpg was around 42-43. 1st light went off at 134 miles, 2nd went off at 210. So no, what I put up is not totally accurate but it is better than assuming that at 5 petals left you have 1/2 tank of gas left.

I filled the tank about 10 miles after the last 2 petals went red. It took 12.8 gallons to fill after 545 miles.

3/2/2/1/1.............etc. Very easy to remember.

 

Great info for those of us who don't operate in gallons. Saved off a copy for future reference. Thanks!

 

Update: I just filled up. 10 miles into four petals remaining (8 miles hwy, 2 city). I guessed it would be 9.5 gal based on the breakdown DanF posted, and the total fueled was 9.72 gal.

 

What I don't understand is why they don't calibrate the software (ESPECIALLY since our gauges aren't needles... they're little lights) to compensate for the shape of the tank.

Say if the float has gone down 1cm. Make this represent exactly how much fuel has been used. At the next cm, due to shape, the car may have used a different amount, so represent that. This would not be difficult on their part when designing the electronics. I'm sure there's already a microcontroller in there somewhere in the car with some extra bandwidth they could spare for some simple math functions.

I also know, however, that all companies do what they do now to compensate for silly people who like to drive as close to empty as possible

 

But it has just the opposite effect when "silly people" look at "Half-Tank" and think they can drive another 200 miles, when 100 miles would be all you have.

Many areas of this USA can have far more than 100 miles between gas stations.

 

Does the SGII show anything but fuel used?

 

It shows lots and lots of things. See http://scangauge.com/ Mine's currently showing MPG for the current trip, fuel left, corrected speed and coolant temperature.

Theoretically its possible to show anything the ECU can tell you, if you can work out the right command (I haven't managed that yet).

 

I don't trust my fuel gauge at all. The other day it showed two pedals lit then finally the low fuel light came on. It went to fill up right away and the tank took 13.5 gallons. The manual says the tank is 13.2 gal (aprox). I know that the plastic tanks swell a liitle bit but obviously it was down to less than a few tenths. Since running out of gas is so bad for fuel pumps you should never let it get below 1 gallon. There are a lot of quirks I can tolerate or chuckle at on this car but this is definately not one of them.

 

Are fuel left and corrected speed done via X-Gauge? If so, would you mind sharing how you did them?