The math isn't quite obvious to me, but if you go out camping and get 5 MPG on the way to the campground and turn around and get 10 MGP on the way back, your average MPG won't be 7.5 as you would expect. It's actually 6.67.In other words, you used two thirds of your fuel on the way up and one third on the way back.Which gets me to thinking, why don't we simply use fuel consumption (i.e. gallons per hundred miles) instead of miles per gallon? It would be much easier to compare mileage if we did so.On edit: I'm not complaining about fuel mileage at all. This is a theoretical situation, not an actual report of MPGs. I'm just commenting on how odd it is that you can't average MPGs together.On second edit: Some people seem to be thinking that I'm complaining about fuel mileage. I'm not. Really, I'm not complaining about fuel mileage. I agree, you shouldn't complain about fuel mileage and I really, truly would never complain about fuel mileage. I'm talking about math here and how you can't average MPG averages. It's a quirk of math only. That's my only point. Really, I swear. I am not complaining about actual fuel consumption of any recreational vehicle.

It's only confusing when you phrase it the way you did. If you say you drove 20 miles and used 3 gallons of fuel, what would your mileage be. Nothing confusing about it really, just if you phrase it to be confusing. Mileage is miles driven divided by fuel used. Nothing could be simpler.

We are used to MPG in America, while liters per 100 KM is common in Europe.I like MPG much better.If you are driving a $100,000+ vehicle, why worry about the $300 to fill the tank? You probably can save more or spend more by finding another insurance company. Do you complain about the $1,200 spent on insurance? Or only the amount spent on fuel?Camping RV's are a pure luxury. While I was living in my RV, it was more like a low cost housing system. I could live in my RV and stay in a RV park twice a month for much less money than I spent on living in my prior home. There I had a $265 homeowners association fee, and utilities around $85 a month. My motorhome was paid for, and my only expense was insurance, gas, and the occasional oil change. Propane was about 10 gallons every 6 weeks in the summer and 10 gallons every 3-4 weeks in the winter. I spent a lot of time in one location to keep mileage down. MPG was improved a lot by driving at 55 MPG, as I no longer needed to arrive by 5 pm, in order to get a good nights sleep and go to work the next morning.So I kept my expenses below $800 a month that I had spent on my old home mortgage. Good luck,Fred.

50 miles one way5 mpg going....used 10 gallons10 mpg coming back......used 5 gallons15 gallons used.....100 miles traveled------6.67 mpg round tripWhere/how do you come up with 7.5 mpg??Your math & thought process is flawed

He got the average of 7.5 because he was only addressing the numbers 5 and 10 rather than the actual fuel mileage.

Taking an average of averages is mathematically incorrect and almost always leads to a non-senssical answer.So the (5 MPG + 10 MPG)/2 giving the 7.75MPG is wrong. As OldBiscuit pointed out you need to back out the average to get to the original data sets and combine them to correctly calculate the true average.It's a bit like that old math puzzle: 3 men go to a hotel and rent a room for $30 where they each pay $10. The desk clerk realizes the room is only $25 so he sends the bellboy to their room with a $5 refund. The bellboy realizes he can't split the $5 evenly, so he pockets $2 and give each man $1 back. Now the men have each paid $9 for the room ($10 - the $1 they got back), for a total of $27 and the bellboy pocketed $2 for a grand total of $29. Where did the other $1 go?

What's the average of 5 MPG and 10 MPG? | Good Sam Community

70 Replies

monkey44
Nomad II
Jul 19, 2015
bob_nestor wrote:
Taking an average of averages is mathematically incorrect and almost always leads to a non-senssical answer.

So the (5 MPG + 10 MPG)/2 giving the 7.75MPG is wrong. As OldBiscuit pointed out you need to back out the average to get to the original data sets and combine them to correctly calculate the true average.

It's a bit like that old math puzzle: 3 men go to a hotel and rent a room for $30 where they each pay $10. The desk clerk realizes the room is only $25 so he sends the bellboy to their room with a $5 refund. The bellboy realizes he can't split the $5 evenly, so he pockets $2 and give each man $1 back. Now the men have each paid $9 for the room ($10 - the $1 they got back), for a total of $27 and the bellboy pocketed $2 for a grand total of $29. Where did the other $1 go?

They each paid $8.33 to get to $25 ... so each man actually paid $9.33, not $9.00 ... OR, one paid $9.00 and the other two paid $8 each ...
monkey44
Nomad II
Jul 19, 2015
bob_nestor wrote:
Taking an average of averages is mathematically incorrect and almost always leads to a non-senssical answer.

So the (5 MPG + 10 MPG)/2 giving the 7.75MPG is wrong. As OldBiscuit pointed out you need to back out the average to get to the original data sets and combine them to correctly calculate the true average.

It's a bit like that old math puzzle: 3 men go to a hotel and rent a room for $30 where they each pay $10. The desk clerk realizes the room is only $25 so he sends the bellboy to their room with a $5 refund. The bellboy realizes he can't split the $5 evenly, so he pockets $2 and give each man $1 back. Now the men have each paid $9 for the room ($10 - the $1 they got back), for a total of $27 and the bellboy pocketed $2 for a grand total of $29. Where did the other $1 go?

They each paid $8.33 to get to $25 ... so each man actually paid $9.33, not $9.00 ... OR, one paid $9.00 and the other two paid $8 each ...
cameronpatentla
Explorer
Jul 19, 2015
Actually, to be precise, we'd have to use calculus and take the derivative of a function at the time you are measuring the fuel consumption to determine the rate at which you are burning fuel. That function must take into account a lot of factors--wind vector, vehicle weight (which itself is continuously changing as fuel is burned), coefficient of friction between road and tires (which is also continuously changing based on road surface characteristics such as tire wear), road inclination or declination, moon location, as if it is overhead, it's gravity will slightly pull the coach up affecting the force of gravity from the earth pulling the coach down. The weight of Cheetos in the coach should not change the formula too much if they are eaten, except to the extent weight moves from the cabinet over the back wheels to the drivers' seat over the left front wheel. Physicists refer to the foregoing as Winnebago's equation. Cheers,
Mike era70x.com
LadyRVer
Explorer II
Jul 19, 2015
I calculate the mpg at each fill up, varies greatly. BUT, then I take total miles of trip and total gallons used to get an all around mpg. Just for fun, doesn't really matter. I enjoy the rv, enjoy the trip and most of all the liestyle.
SoundGuy
Explorer
Jul 19, 2015
holstein13 wrote:
... why don't we simply use fuel consumption (i.e. gallons per hundred miles) instead of miles per gallon? It would be much easier to compare mileage if we did so.

Sorry, don't agree at all ... here in Canada we were forced years ago into the metric system so vehicles are rated for fuel mileage in litres per hundred kilometres. :h Fortunately my Avalanche still reads in litres per kilometer :h or if I want to really understand how bad my mileage is when towing :E I can switch it to read using the only reference that makes sense to me - miles to the US gallon. :B OR, I can simply ignore the issue entirely, fill the tank, and just go camping. As has been said many times before, if you're going to start worrying about fuel mileage when towing a trailer then perhaps this sport isn't for you. :R
bob_nestor
Explorer III
Jul 19, 2015
Taking an average of averages is mathematically incorrect and almost always leads to a non-senssical answer.

So the (5 MPG + 10 MPG)/2 giving the 7.75MPG is wrong. As OldBiscuit pointed out you need to back out the average to get to the original data sets and combine them to correctly calculate the true average.

It's a bit like that old math puzzle: 3 men go to a hotel and rent a room for $30 where they each pay $10. The desk clerk realizes the room is only $25 so he sends the bellboy to their room with a $5 refund. The bellboy realizes he can't split the $5 evenly, so he pockets $2 and give each man $1 back. Now the men have each paid $9 for the room ($10 - the $1 they got back), for a total of $27 and the bellboy pocketed $2 for a grand total of $29. Where did the other $1 go?
toedtoes
Explorer III
Jul 19, 2015
He got the average of 7.5 because he was only addressing the numbers 5 and 10 rather than the actual fuel mileage.
Old-Biscuit
Explorer III
Jul 19, 2015
50 miles one way

5 mpg going....used 10 gallons

10 mpg coming back......used 5 gallons

15 gallons used.....100 miles traveled------6.67 mpg round trip

Where/how do you come up with 7.5 mpg??

Your math & thought process is flawed
Golden_HVAC
Explorer
Jul 19, 2015
We are used to MPG in America, while liters per 100 KM is common in Europe.

I like MPG much better.

If you are driving a $100,000+ vehicle, why worry about the $300 to fill the tank? You probably can save more or spend more by finding another insurance company. Do you complain about the $1,200 spent on insurance? Or only the amount spent on fuel?

Camping RV's are a pure luxury. While I was living in my RV, it was more like a low cost housing system. I could live in my RV and stay in a RV park twice a month for much less money than I spent on living in my prior home. There I had a $265 homeowners association fee, and utilities around $85 a month.

My motorhome was paid for, and my only expense was insurance, gas, and the occasional oil change. Propane was about 10 gallons every 6 weeks in the summer and 10 gallons every 3-4 weeks in the winter. I spent a lot of time in one location to keep mileage down. MPG was improved a lot by driving at 55 MPG, as I no longer needed to arrive by 5 pm, in order to get a good nights sleep and go to work the next morning.

So I kept my expenses below $800 a month that I had spent on my old home mortgage.

Good luck,

Fred.
fj12ryder
Explorer III
Jul 19, 2015
It's only confusing when you phrase it the way you did. If you say you drove 20 miles and used 3 gallons of fuel, what would your mileage be. Nothing confusing about it really, just if you phrase it to be confusing.

Mileage is miles driven divided by fuel used. Nothing could be simpler.

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What's the average of 5 MPG and 10 MPG?

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