Fuel Mixture Math Question
 

I've got a gallon (actually more but to keep it simple we'll say a gallon) of fuel that is mixed with oil for a 24:1 mixture. If one needed to make this fuel a 40:1 mixture how much fresh gas (no oil) would you add to make it a 40:1 mixture?

Some facts for mixing a gallon:

a 24:1 mixture requires 5.3 ounces of oil

a 40:1 mixture requires 3.2 ounces of oil

In other words what would be the ratio of premixed 24:1 gas/oil together with plain gas to get a ratio of 40:1.

Thanks in advance for your input.

Once you figure out the actual amount of oil in your 24:1 mixture, you can easily figure out how much total gasoline you need, and then figure out how much gasoline you should add.

Let's talk in volumes.

24:1 is 1/25 oil, 24/25 gasoline (at least in my head). Likewise, 40:1 is 1/41 oil 40/41 gasoline.

We can use a little algebra to set up some equations for the individual liquids (gas, then oil).

40/41 * (Total Liquid) = 24/25 * (1 Gallon) + 1 * (Amount gasoline to add)
1/41 * (Total Liquid) = 1/25 * (1 Gallon)

Since you aren't adding any oil, this is pretty simple. You can use either equation to determine how much gasoline to add (if you were combining two mixes, it'd be more interesting, as you would have to substitute equations).

The second equation is pretty simple (as the Total Liquid = 1 gallon + how much you add)

41/25 = 1.64 Gallons (total), meaning, you add 0.64 Gallons of gasoline for every gallon of mixture you have.

The other equation can be arranged for the same result.

40/41 + 40/41 * gas added = 24/25 + 1*gas added
gas added = 40 - (41*24)/25 = 0.64 gallons


I hope I didn't make this too confusing :)

Same thing but a little simpler perhaps:

24:1 = 4% oil
40:1 = 2.44% oil

(1 gal)(4% oil) = x gal(2.44% oil)
x = 1.64 gal

Therefore dilute with gasoline to 1.64 gal.

I get the same answer, but I think a simpler approach:

You currently have 25 "parts" in your gallon: 1 "part" of oil and 24 "parts" of gas.

You need to add 16 more "parts" of gas. By our definition a "part" is 1/25 of a gallon. You need to add 16 of these, so 16*1/25 = 16/25 = .64 gal.

Thanks for the quick replys. In my mind I was (quickly) roughly figuring to add a half gallon of fresh gas to the current 24:1 mixture to come up with the 40:1 mixture so I'll add a little more.

Now that the sailing season is over my brother-in-law didn't want to transport the left over gas/oil mixture for the dingy motor back home to Minnesota from the Chesapeake, I said I would take it and use it in my leaf blower. Hence the dilema.

I love this forum, you get quick square answers. You guys are great.

Thanks again.

280SE Guy.

I like that approach.

Hehe, no problem. We're pretty diverse, so, you can get a wide variety of answers on just about any topic.

I do admit my method is a little over the top (I should have just KISS). I would probably give the same level of response with 3 or 4 fluids and multiple concentrations of concern :)