Matching your categories: As an individual I spend $120 on groceries, $60 on gas(fuel), and $40 on extras and in recent years $70-100 for projects. I already have $500 saved for emergency situations and that goes no higher.
Unfortunately, income, location, # of family members, type of vehicle, lifestyle choice, etc, all distort the numbers too much to make relevant comparisons.
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If I may look at your situation from a different perspective, you have $850 to use on 4 categories per month and the rest is Savings for the new truck. I am going to make the assumption that the truck is somewhat vital or handy to your daily life, and so you want to save as much as possible in order to get a new truck as quickly as possible. From Most to Least flexible:
1. Extras
2. Unexpected expenses/farm projects
3. Groceries
4. Gas
$200 - Gas savings are likely to not be worth any extended effort. If you have to travel less for errands or do less post office/grocery runs, that's a small win I guess. I make a trip into town once a month and that's it, but compare that to driving to work everyday and it's not saving a lot.
$160? (80%) - Groceries are basically up to your comfort level and only you can decide that. My average food budget would probably be $120 a month, but I'm saving up for investments aswell, so I'm currently at only spending $72(60%) per month. 60% seems to be a sort of thresh-hold where I can still buy enough ingredients to create fulfilling meals. I don't use any
milk or
coffee and hardly any condiments, so it's not too hard for me.
$150 - Unexpected Expenses/Farm Projects I can't really say much about without knowing more details, except that currently I'd assume you have more time than money, so doing extra preventative check-ups on machines might help reduce this expense. I usually try to take about 10 minutes per machine a week just to check them out a bit. One time after a busy week, I found my boss' front-end loader had lost 2 out of 4 of the bolts holding the engine mounted. He was very relieved to have that pending disaster discovered and cheaply repaired.
$105? (70%) - Extras also goes along with Miscellaneous, and are usually the one where lots of cost cutting can be done. I can guess many items could be reduced in use or used more efficiently.
If the above is applicable, then that could put your Savings at $235.
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Even though it's not included in the OP, you can likely save more by changing habits in electrical and heating usage. I did some experimenting this winter and managed to reduce heating by 15-20% and cut electrical by at least 20% and didn't have to sacrifice much comfort.
More about expanding income than savings, but using some of your farm equipment, even something small like a chainsaw, to get extra work once in awhlie on the weekends would likely be the most efficient way to expand your savings.