Well after trying a very basic experiment on my helmet the only thing I figured out is that I need better numbers
I picked up a Schwinn 12 Function Cycle Computer from Walmart for $10. It uses a magnet and sensor to pick up speed from the front wheel. I wedged the magnet into a hole in the brake disk and zip tied the sensor to the fork. I measured the diameter of the wheel for a base number, and on my way to work today I stopped at mile markers zeroed the trip odo and went a mile then adjusted the wheel diameter till I got it right on the mark. Now with that trip computer I have average speed, top speed, and some other random data. This gets my distance measurements to 3 decimal places in the trip odometer. It won't be that accurate I'm sure but it is still a bit more accurate than the stocker and a lot more useful data than I had.
I have been trying to think up ways to get a more accurate measure of how much gas I put in. I finally realized I was being goofy and I had the numbers all along. The pump only reads to 0.1 gallon but the cost gives a much higher precision. So instead of saying 1.8 gallons of gas I can say gas is $2.899 a gallon and I put in $5.27. Dividing that out I get 1.8178 gallons. $5.28 is 1.8213 gallons. I talked to the gas station owner and he said the pumps should be accurate to the penny easily but even if I give it a +/- of 1 cent that is still way more accurate than the gallons indicator.
Now with my new setup I will ride for another week and see how this setup actually works. The bike trip comp is very simple and I could prob mount one in my car without much effort. I don't think it would be that useful in a car but it would be a lot less weight than a normal speedometer
Ha ha. The thought of a bicycle speedo gives me that "I could have had a V8!" feeling. AT first I was worried about maximum speed, but it says it's good up to 99.9 MPH, which appears to be based on the width of the display.
Well I have it calibrated pretty good. GPS log shows 92.08 miles and the odometer reads 92.881. Original odometer read 96 miles.
Average speed was 51.2mph max speed 76mph. Gas was $5.23 @ $2.859/gal which gives me 1.829 gallons. 50.782mpg, Temperature today was 75 degrees, small amount of rain.
I will drive a few more days next week keeping all the information possible and see what I can get for consistency before coming to any conclusions. If nothing else I might build up a good database of mileage numbers that can be compared for interesting things like avg speed/temperature vs mileage. I might get decent numbers or I might find that even though I am trying to go for at least a 0.1mpg accuracy it might not be much better than what I was doing before. But 10 bucks for the bicycle speedo that can give me average and max speed is kind of handy so I might end up sticking one in my car also.
Well I found a small problem I need to work on my magnet and sensor location a bit, Over ~75mph the speedo misses pulses and gives me the wrong speed. I tried running wide open to see how close to 100mph the bike can get and noticed I was showing 65mph on the speedo. Stock one was at 105. I messed around and pretty much decided over about 75 it starts getting erratic and I have not been able to get it to show 80mph yet.
So knowing that here is what I got today. AVG 49.2mph, max 76mph, 115.93 miles, $firstname.lastname@example.org/gal for a mileage of 45.54mpg I know the top speed was closer to 100 and the average should be in excess of 60mph considering I ran a huge portion of my 4 lane commute running wide open.
Overall I think this will actually work once I get the sensor to read a bit more reliably at higher speeds. I hope it is just needing a bit of adjusting on how I have the sensor and not a limitation of the speedo from not sampling the pulses fast enough to actually read over 75.
BTW: The 66cc 2 stroke bicycle gets 90 to 120 mpg depending on how I ride. The first test of the Lifan 200cc dual sport showed 82.6 mpg.
Once that Lifan gets broke in good I bet you can get a 90mpg tank out of it easy With a few minor mods and some easy driving that thing should be an easy 100mpg bike. My buddy with his has quit tracking his mileage so I can't say much about what he is currently getting but I know it is pretty far up there still. He can go forever on a tank of gas it seems like.
Today I stayed at ~75mph on the 4 lane and here are my numbers
96.906 miles distance
Gives me 45.368mpg
I tried not speeding over 75 and just tried maintaining a constant speed. The distance seems reasonable so if I went over 76 it wasn't for long and not by much. Also maybe a mile of a dirt hillside shortcut to get around an accident. The tank actually was full on my fill mark at $5.80 but I went ahead and put the extra bit in to save from bothering with change. Would have gotten me 46.93mpg without the extra gas. I guess I could deduct .20 from this fill and add it to the next one to make it right. Either way I am pretty confident that this is giving me enough resolution on my mileage now that I might actually be able to test some things and get good numbers. Too bad the riding season is probably drawing to a close unless the weather stays like last year.
So for accurate measurements get one of those bicycle computers mounted, fill your tank to the exact same spot every time and you should be good using this method. Now to try mounting one of those trip computers to my car
If you need more high speed accuracy, try a Sigma bicycle computer. I've tested one on a display stand up to 225 mph. One drawback is that Sigma computers show speed in whole mph increments, no decimals. Cateye computers show speed in .1 mph/kph increments and are also high quality, though I've never tested it at high speeds.