I'm really missing something here on why there's all this confusion determining fuel mileage.
Step 1. Ensure your odometer is properly calibrated (or determine the percentage error of your odometer). Never mind your speedometer, just focus on your odometer. On your car that will be in miles. For us on the prairies it's quite simple to determine odometer calibration, as there's a grid road every mile going east / west, and every 2 miles going north / south. The 66' that was set aside for the roads is part of the mile.
Step 2. Keep track of your fuel purchases and mileage. Your fuel purchases are in litres, there's 4.546 litres per Imperial Gallon. Divide the number of litres by 4.546 to get fuel in imperial gallons.
Step 3. Divide the number of miles driven (from your odometer) by the number of gallons used.
I sent a note off to Gas Buddy and here is there reply
It appears that I will have to do some calculations to get an accurate figure
It is interesting to note that they say the calculations are not entirely valid for Canada due to the size of the gallon and kph.
Don(GasBuddy)
Oct 5, 12:59 PM CST
Hi Cliff,
Thanks for your email.
I'm sorry for any confusion I'm about to cause, this is typically a hard to answer from time to time as the units conversion can vary, depending on what website you're using, especially when it comes to the differences between modern and older Canadian vehicle converting to gallons.
This is because, and should go without saying, newer cars measure fuel economy in L/100 KM, older vehicles pre-metrification used MPH speedometers and miles on the odometers. Older cars were built with gallons in mind (because back in the day, gasoline was priced and purchased in gallons - in Canada specifically, the Imperial gallon was used).
The Fuel Logbook doesn't necessarily care about the size of the fuel tank, more so it looks for the amount of fuel used between complete fill ups (from empty to full is most recommended for the Logbook's use).
On a Canadian site, the units of measurement are in metric units, prices are input as c/L, and fuel economy can be measured in L/100 KM or MPG, odometer readings are input as KM. The question is whether this MPG is referring to US gallons or Imperial gallons, and to be honest, this isn't a frequent question, but I'm assuming that the MPG on a Canadian site uses an Imperial gallon. An Imperial gallon might do some auto-calculation with the provided KM odometer reading, but I can't say that for certain. The US sites allow for odometer readings to be input with miles, prices are input using $/Gal, but calculated fuel economy in MPG presumably with US Gallons. Canada doesn't do things in a similar fashion anymore, unfortunately.
The tricky part is a Canadian website shows the odometer readings as KM regardless of what fuel economy rating you jump back and forth between, which makes figuring out miles per gallon harder to calculate or be visually represented, when the distance being input is used as a KM and not miles. Have you tried converting the odometer readings to KM (if being input as miles), taking the L/100 KM fuel economy calculation and manually converting between US Gal and Imperial Gal?
I'm not sure if there is going to be an easy way to answer this, or if the fuel log can be helpful for your Impala. The last thing I want to say is you might need to do your own math, but, you might need to in order to get an accurate representation of your Impala's fuel economy, I don't think the Logbook will be 100% accurate for your use-case
PS - Would you happen to have a picture of your Impala? Younger generation with a shared passion, responding (I drive a newer Buick Regal "Gran Sport" ) - would love to see a picture!
Well if you are feeling good about it I really shouldn't say anything to take away that feeling. Think there is a problem though. Just a quick look and I think you have made a conversion away from your miles on your odometer to kilometres. That isn't necessary as all you have to do is convert your litres to gallons. Taken from the first chart you put up you will end up in the 16 to 18 mpg range which is more realistic. I think you will have to be going down hill all the way to average 24 with combined city and highway.
Here's what I get. I'm not sure how you're calculating your average, but I suspect you've just added up the calculated MPG's and divided by 19. Unfortunately that provides unfair weighting.
I took the total number of miles driven, divided by the total number of gallons consumed. That's the true average.
im now redoing carbs, everyone ive seen so far is set way to rich for the engine, that goes for holley, carter , or quads. ive managed to sqeeze a steady 14 out of my 406 and its a fulltime 4x4. the quads are a pain to set up because of all the drilling required but very reliable once set up .
I used your numbers and calculated mpg's and I get 12 to 18 miles per imperial gallon. Still much lower than I would expect.
Is your choke stuck? Once it's warmed up, take the air cleaner off and look into the carb. The choke plate should be fully open.
As mentioned, is the spark advance working? GM HEI distributors are well known for having the mechanical advance weights seize and not advance as RPM's come up.
Any indication you've got brakes dragging (hot to the touch after a drive with little braking)?
Is the inside of your exhaust pipe full of soot? A little is normal, but if it's building up, it's a good indication you're too rich.
