Using reserve light to check MPG
 

How accurate would you say the reserve light is? It's a 21.1 gal tank and the reserve light is suppose to come on with 2.9 gal left. It's a fixed position float switch on the bottom of the sender and my guess is that as long as it is on level ground it should be accurate and repeatable.

I just came back from a long trip where I drove 500 miles on VO before the reserve light came on. That works out to 27.47 mpg! I used a GPS to keep track and the average speed was 50 mph. Some of it was stop and go, most of the back roads taken was 55 to 65 mph and 30 to 45 mph.

Reason I have to check MPG this way is I use the main tank for VO and have a small 5 gallon tank for diesel and have no accurate way to measure the VO I put in (non metered pump).

The real question is how accurate is the fillup. Unless you patiently wait for the foam to dissipate every time.

So how are your reporting your quantities so that you can "legally" use vo?

:facepalm:

The reserve light is actuated by a contact that rides on a wire. It is a fixed point. On a wagon, its probably worthless as the fuel tank is on it's side, but on a sedan with the tank standing straight up, it will be more accurate. I don't know how the thickness of VO will interact with that float and contact.

The reserve light in my 83 doesn't work, so I use the tripometer to tell me how much further I have, but I have to know my mpg for that to be accurate.

I guess you'd have to check the repeatability / reliability for your self.

If you live in a relatively flat part of the world you could carry a dip stick with you and check the level in the tank every time your warning light flickers - if I were you I'd check it through the sender hole in the sedan tank... alternatively you could undo the pipe under the tank - drain it and measure! (Very handy solution I know)

I'm guessing you are going to have to be the pioneer on this one - and as said above, you need to find an equally reliable way of making sure you know how much fuel goes in. A dip stick might be handy there too.

(You'll also look pretty cool wandering about with a 46mm spanner in your back pocket)

well, since the EPA has not authorized vo as a "fuel" there is no way to "legally" use wvo/svo...as a fuel... but many states (the only authority that's interested in fuel use...) do not require individuals to report vo/wvo/bio use...
so it's kind of a moot point.

Me thinks I'll move this to the alternative fuel forum...:rolleyes:

I've always found the reserve lights to be rather irregular. Maybe a brand new one will be consistant. But the 30 year old senders in most of our rigs are pretty wild. Using it to determine mileage would at least give you a rough idea of what your getting, though I would rely on it too heavily.

Foamng is with diesel only. VO does not foam, at least not with the pump that I use. I fill it with the car in the same spot, to the brim of the filler neck so the tank is 100% filled. Good idea draining and measuring what comes out when the reserve light is on and using a dip stick through the sender hole. I will do both. I have 2 drains on the tank, my fuel outlet (strainer removed) was also turned into a drain with 5/16 hoses about 12" long with a plugs at the end so it would be very easy to drain.

Great stuff - I'd be interested to hear how consistent your reserve light turns out to be - will you post results? (Please!)

As a matter of interest - do you know how clean your sender is? Have you taken it apart recently? If you've never done it you might be in for a shock:-

http://www.peachparts.com/shopforum/2480958-post19.html

How does this help the OP with his question in any way?

Army, the sender and reserve switch is clean. I can take it apart in my sleep now, with so much practice! :D

Next time the reserve light comes on, I will drain and measure and report the results. I have drained the tank before to see what comes out (always clean). It is quite slow being 5/16 hose.. My guess is it'll take 1 to 2 hours for 2.9 gal depending on how cold ambient is.

wow. that's thick. how much preheating do you do with your fuel?

I'm trying to follow the process:

Fill up with VO*
Start mileage count on GPS
Drive till light comes on and record miles driven, hoping you catch it at that moment?

In practice my light comes and goes: left turn = light on, right turn = light off. Slight glow, then full bright later. It's not on/off.

Then assume 21.1-2.9 for consumption when the light is on steady? (you could sub whatever measured value for 2.9 into the equation, but I still think it's unreliable.)


What if you consider both tanks as a unit: the tank in your car and the non-metered fill tank, I assume in your garage. Start with a full tank in the garage, fill up the car, then measure how many gallons it takes to refill the tank in the garage. Or put a dipstick into the fill tank in the garage at start and again at finish. You'd have to figure a graduated metric on the stick. The utility of this method may depend on the size/type/shared use of the filler tank.

Or average mileage over several tanks if you can't refill the "filler" tank immediately after filling the car. As long as you can track the miles driven from one "fill" of the filler tank to the next fill, your mpg will be more/less accurate.

Trying to account for the amount of fuel that goes out of (or in to) the filler rather than relying on always being able to go from full-to-low-light in the car for that fixed quantity seems a more practical method. It also lets you refill at levels above the low light so you don't have to wait until you're on E to fill up.

*Note: VO used as a mileage enhancing additive to the trace amount of diesel that actually burns as fuel, just to be legal in most states...

On VO, the reserve light and fuel gauge reacts much slower than on diesel because of the viscoisty difference, even after drilling out the drain/fill hole in the sender to a bigger size. I may drill it out bigger to make it react faster. On mine, when the reserve light comes on, it stays on (this is on the highway, no sharp turns etc).

Just realized that when the light comes on, I probably will not be at a location where it is convenient to drain the tank.

Plan B:

On level ground, when tank is near reserve (light not on), slowly drain tank till reserve light comes on, then continue draining into another (empty) container and measure what comes out.