In a “traditional” business model you could look at it this way:-
I have made the figures simpler than they would be in real life, but the principle is correct.
Truck Cost = $100,000
10% interest over 5 years = $10,000 x 5 = $50,000
so your monthly truck payments would be capital of $1667.00 + interest $833.35 = $2500.35 (in real life it would be less than this but never mind).
but don’t forget about depreciation, this is tax deductible, but it is also a real cost and you should treat it as such:
Depreciation @ 25%(say) over 5 years =
$25,000 year 1
$18,740 year 2
$14,062.5 year 3
$10,546.88 year 4
$7,910.15 year 5
Total depreciation = $76,258.00 = $1271.00 per month
You must include this when calculating your true mileage cost.
So if you open a savings account and put in $1271.00 per month for those 5 years, when it comes to replacing the truck you will have $76,260 + any interest earned + the trade in value of the truck you bought 5 years ago.
That way you should be able to fund your second truck and after that always have enough to replace with new after 5 years.
That’s the theory anyhow.