better gas mileage with the fuel neck plum full?

[Pu241]

Bingo!
roycehof got it!
MPG is dependent upon a lot of thing:
driving habits, speed, terrain, and how reproducibly you fill your tank.

I fill mine all the way to to the top of the filled tube every time!
200K miles and doesn't leak yet!
At this rate, even if it started leaking tomorrow I wouldn't attribute it to filling the tank up to the top of the fill tube.
As far as weight, yes, lighter is better, but the difference between a full tank and a half full tank MPG wise is easy offset by driving habits, speed, traffic, etc!
Now if your talking a couple hundred pounds we might be talking a measurable difference!

[quaddawg]

It makes a SLIGHT measureable difference to fill up when it's cold out, like in the morning, as when it gets warm out, gasoline expands.. it is a TINY difference, almost negligible, it is just an example of the little things that add up.

Fuel quality also makes a difference. I was getting 33 at one particular station, I changed to a better brand and am getting consistant measurements of about 34-36 now, with the same basic driving habits. Every now and then I get something ridiculously different, like 27 or so.. or 38 really makes me wonder about pump calibrations at different stations. as I doubt I could get that big of a difference from minor driving changes.

I always fill my tank up to where it almost pours out, when I am checking mileage, so I will know I am putting the same amount in each time.. I know it's not good for the enviorment, but I don't check mileage all that often anymore.

You also have to take into account any changes in tire size. I use the miata.net calculator to apply the correct conversion factor for undersize or oversize tires.

You also need to take into account odometer error, in my current case (blue festy) the oversize tires I have actually CORRECT the odometer, according to my GPS. Most speedos read a tad fast, but the odometers are supposed to be correct. I find that they are NOT correct in most cases, but actually show a tad LESS miles than actual.

Human error in measuring fuel mileage is HUGE with small tanks, and you really need to run 1000 miles and keep accurate gas records, along with filling up the tank completely to start, and once more when you are finished. THIS is the only way to get a very accurate AVERAGE mpg for your car.

[rain406]

roycehof, how in the world do you get mid-40s gas mileage?? that's awesome.. I get about 32.8

[Pu241]

roycehof, how in the world do you get mid-40s gas mileage?? that's awesome.. I get about 32.8

Probably how I get high 40's low 50's!
Slow down! Use mid-grade fuel, increase presssure in tires, do not idel excessively, coast when you can!

[packrat427]

^^ Yep

I get 48 when I want , I have got 51.5 as a best .
I use 92 or 93 octane most of the time , I have always got better with the bigger numbers .

If I remember B3's are 10.0-1 comp , they deserve premium .

I have a real hard time driving normal :lol:

[Gomez]

11:1 - 11.5:1 is where you need to start thinking race fuel of about 98 octane or higher. WOW I didn't realize the 'ole 1.3 was 10:1 I guess that is good info to know. Yes for that 92-94 would be perfect. Hmmmmmmmmmmmmm

[NovaSS]

Static compression ratios are only one effect on the octain needed, piston top and combustion chamber design have a large effect.

Do a google for dynamic compression ratio, these are more realistic and use cam timing to calculate actual cylinder pressures. an Engine with 11 to 1 Static CR may only have a 8-1 CR due to cam events like overlap.


You can 14-1 on 93 IF you have the cam timing to bleed off some of the pressure

[Gomez]

OK, I guess I could have elaborated a little more. At least you need higher oct fuel when you run 11:1 in a 2 cycle engine. Fat top pistons and no valves to bleed off. I was making a general statement at the same time as realizing that it was something to think about. Lets not hijack this anymore than it already has, Sorry.