Scott,
Your brother's 'uneven' markings on his fuel gauge probably had less to do with
inherent inaccuracy than it did with a lack of linearity. Gas gauges in early
VW bugs, for example, had that the half-tank mark way over to the right and the
last half of the tank took up three quarters of the rest of the gauge. Whatever
mechanical or electrical method was used was not perfectly linear in
registering on the gauge. The first gallon out of a ten gallon tank did not
make the needle move ten percent, nor did the last gallon out of the tank make
it move ten percent. The first gallon maybe moved it three percent, or maybe
not even noticeable, and the last gallon maybe moves it seventeen percent. A
lot of it may have had to do with the shape of the tank, the arc that the float
arm on the sender unit swings through, and so forth. If you were a real
stickler for accuracy, you would need a gauge or meter that was fully
adjustable for Span (where the meter starts and ends measuring), Offset (the
starting point for measuring, usually something other than 'zero') and
Linearity (each unit along the range of measurement being the same). For a
sending unit which changes resistance, it would be possible to put pre-scaling
and bias resistance in the circiut to change it. Most people don't bother
trying to do anything differently from whatever the carmaker did in the first
place. Your meter may vary.
Jon Larsen (79 MGB, with a gauge that never reads full and never reads empty)
>>> "Scott Gardner" <gardner7@pilot.infi.net> 01/13 4:23 PM >>>
> I read an article a while ago in Car & Driver that explained that fuel
> gauges are built intentionally inaccurate. In a modern American car,
> the gauge will read full until the tank is a few gallons low. The gauge
> will read empty when there are a few gallons left in the tank.
>
<<SNIP>>
> >>> Douglas Gaither <gaither@ix.netcom.com> 01/12/98 03:41pm >>>
Douglas,
I'm not positive, but I thought that there was actually a fixed
number of gallons that HAD to be in the tank when the needle read
"empty", courtesy of Ralph Nader, et al. It was really a pain in the
ass on my old Prelude (~12 gallon tank), because I think mine had
something like 2.5 gallons left when it read empty, which was a
quarter-tank. I wish that they would just make the gauge accurate,
and leave the risk-taking to us.
On a related topic, I think there may be some inherent inaccuracy in
gas gauges. My brother's BMW had the 1/4-tank marks spaced unevenly
along the face of the gauge, but they were accurate. Perhaps this
was to compensate for problems with the gauge?
Scott
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