It's just rough math. Here in TN, Diesel is about 20% more expensive than Regular unleaded. So to account for the difference in price, I just dropped the mileage a bit on the Dmax to get the like/like spend.
That's not an accurate method. It's far easier just to put in the actual fuel prices and run the actual numbers based on combined mpg's and annual mileage.
Depends on some other factors that may need to be considered. Oil changes for my F350 at the dealer is $140. Oil changes for my wife's 4Runner is $50. Add the 15k fuel filter changes and you are at another $150 service. And in some cases, with the sensitivity of the new fuel systems, NOT doing regular filter changes is risky WRT warranty work.
Dealership maintenance is hit or miss and often varies from one shop to the next. Ford dealers in particular are known to charge high prices for their services. And moreover, the F350 is a bigger truck with a bigger engine, so maintenance will cost more relative to the 2.8l duramax Colorado. To make the comparison as apples-to-apples as possible, exclude dealership labor costs (these jobs are easy DIY's anyhow) and look at the pricing for the actual parts and consumables.
My 4runner requires at least 2, if not 3 (when going by the exact OEM recommendations), oil changes over 15k miles of annual driving. That's ~$100-$110 of DIY maintenance.
Over the same mileage, the Colorado with the Duramax requires DEF, 2 oil and filter changes, and fuel filter change only happens every 30k-37k miles (assume half that cost for 15k miles of driving). You're looking at about $160-$170 worth of parts and consumables.
The difference in annual maintenance costs (which everyone tends to blow out of proportion) is ~$60-$70, depending on the prices you find for parts. My numbers are derived from looking at actual prices for these things and accounting for OEM maintenance intervals and reported DEF usage.
I have a hard time with the idea that it will pay for itself... I just don't think that math works in this case. I might be wrong - often am - but I challenge the idea that spending an additional $10k to get into the diesel, plus the maintenance and mileage that is likely only 20-25% better than the current truck... will pay for itself. I know my f350 is no where near paying for itself vs. my 2015 Tundra... but it sure as hell makes towing a much nicer experience.
I did agree that the OP would be better off sticking with his Tacoma since it is paid off. If he wants better mpg, he needs to take some of the modifications off.
When I said that the 2.8l Duramax "pays for itself" in the long term, I meant that over a 5-7 year horizon the better fuel efficiency will pay for the $3,700 MSRP cost of the diesel, even in spite of the greater fuel prices and maintenance associated with that engine option. I wouldn't expect a F350 to "pay for itself" when compared to a Tundra; you're talking about completely different categories of trucks. Comparing a Tundra to a Ram 1500 ecodiesel or the upcoming F-150 diesel might be a different story. But you have to run the numbers to confirm or disprove that instead of relying on lazy math.
With all that said, the OP did say he is looking to do a lot of long highway drives in the near future. If he were to sell his Tacoma and buy a diesel Colorado and leave it stock, the better fuel efficiency could very well pay for the cost of transition. I don't know that for sure, but he could run the numbers and check that himself.
