Quote:
Originally Posted by 66Submarine
Only 19 from that combo going 55MPH? Huh. Lots of real steep hills, maybe? When my father was driving my '65 C10 locomotivated by a shot-out 250 with 3.73 gears and a three speed (30.5" tall tire) the truck got 17-20MPG and seemed to average something like 18MPG most of the time.
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Please allow me to make a comment about calculation of MPG. I'm not saying anyone here is calculating it wrong. But then again, they might not see that their method is potentially flawed.
In the example above, I presume that with a taller tire, the actual odometer reading (mileage) was re-calculated due to the fact that taller tires will throw off the accuracy of the true mileage driven.
Two things MUST be accurate for the MPG calculations to be valid. Exact amount of FUEL and exact mileage. But all too often, we mess one of those up (or both).
Examples of inaccurate calculation of mileage: I have a small 4 cylinder Nissan RWD truck. I have slightly bigger tires on it than the factory installed. Using a GPS, I have figured out that my speed is off by 5%. So 50 MPH is really 52.5 MPH and 60 is really 63 MPH and so on.
Take home message = different tire sizes will make your speedometer inaccurate and throw off the calculated MPG at the pump.
Another example: I recently drove my old FORD classic on the highway for a 4 hour trip. I wasn't sure if my speedometer was accurate so I started counting mile markers. I found out that mile markers on the highway are not spread out at exactly 1 mile. Pretty close, but not close enough to be accurate for exactly 1 mile.
Take home message = If you use an aftermarket programmable speedometer - the kind that has you press a button and then drive exactly 1 mile and then press the button again to set the speedometer - well, it might not be as accurate as you think if you used a mile marker on the highway.
About calculating the amount of fuel used - Most of us calculate it at the pump. Hopefully we don't use the gas gauge on the dash.
I'm reminded of a Mythbusters episode. They used a fuel cell and calculated exactly how much fuel they used by weighing the fuel cell. Weight change was equal to amount of fuel consumed. Just that simple. Very accurate but I certainly can't do it that way.
Most of us calculate MPG at the pump. We see how many miles we went since the last fill up. Then use simple division to calculate MPG. Then we hit the trip meter on the dash and click off the miles until the next fill up. However, errors can be made using this method. Still . . . it's about the best I can come up with and stay practical.
Examples of potentially inaccurate fuel calculation:
All of these will cause only a small mistake in calculating fuel use.
1. Using the dash gauge is highly inaccurate. Too often folks think that when the gauge is at the half mark that they have used 1/2 the tank. Rarely true.
2. Filling the tank using a different pump. The different pump may have a different shut off pressure and may not fill up the tank the same.
3. At a different pump, the tank may fill differently due to truck sitting at a different angle.
4. The last person who filled up didn't top it off the same way you do.
Having said all this, what is best? First, make sure you have a way to accurately calculate mileage. You must verify that your odometer is accurate and if not, then what percent is it off by? I suggest that you follow someone with a modern vehicle that has factory installed tires. Measure at least 10 or 20 miles by their odometer and see what yours shows. If possible, use a GPS to confirm the distance. That should give you a way to mathematically calculate your true mileage in the future.
For fuel, calculate the MPG by using the combined gallons from 3 or 4 fill ups. The small variances will average out.
After reading my words above - I see how truly dull and boring I can be.
So I'm gonna go to bed.