An owner-operator cross-shopping a Freightliner Cascadia, a Kenworth T680, and a Peterbilt 579 usually thinks the decision is about the trucks. On the spec sheets that matter most to your wallet, it frequently is not, because all three can be ordered with the same Cummins 15L engine. Same X15 on newer builds, same ISX15 on older ones. Same fuel pump. Same EGR carbon problem. Same rebuild kit part numbers.
That is the useful thing to know before you compare anything else, and it is also the only part of this comparison our evidence lets us state with confidence. The rest of this article is about being straight with you regarding which is which.
The engine risk is one story told three times
Order any of these three with Cummins power and you inherit the same documented issues. The fuel pump is the headline complaint on the platform. Ceramic plungers from 2010, fixed around the 2016-17 window with an added piston ring for the carbon-packing problem, though a mechanic in our record filming after 2021 says pump complaints ran "from 17 all the way to 21" anyway. EGR valves sticking open from carbon buildup at low idle. DPF ash cleaning every 250,000 to 400,000 miles. On high-mileage examples, spun main bearings and broken cylinder head bolts.
The repair economics travel with the engine, not the badge. As of a 2026-08-19 check at dieselrebuildkits.com, the X15 complete overhaul rebuild kit is $3,995, the upper gasket set $795, the head bolt kit $395. One documented in-frame job on an X15 ran a $4,600 add-on in parts and labor for intake valve reseating, valve springs, and cam bearings, with the presenter explicit that the figure covers those items and not the whole job. None of those numbers change based on which cab the engine is bolted into. The block does not know what badge is on the hood.
So the first move is not picking a chassis. It is separating engine risk from chassis bet, then pricing the engine risk identically across all three candidates and negotiating on it.
The engine option that changes the math
Two of these three can be ordered with a non-Cummins engine, and here is where we have to stop and flag something.
The T680 and the 579 both offer the PACCAR MX-13. Our research on that engine is thin, and we are not going to pretend otherwise. When we went looking for MX-13 failure-mode evidence, the video sources we pulled for both the T680 and 579 MX-13 records turned out to contain essentially no MX-13 content. One is a generic fault-code clearing walkthrough with no faults found. One is uncaptioned. Two others were Cummins rebuild videos that got miscategorized. We could not locate a usable MX-13 engine-family write-up either.
That means we cannot tell you whether an MX-13 T680 is a better or worse bet than an X15 T680. Anyone who tells you confidently either way, on the strength of the same public sources we searched, is guessing. If you are cross-shopping an MX-13 truck, do the inspection work yourself and weight a documented service history much more heavily than usual, because there is no body of evidence standing behind it for you.
What the listings actually show
Here is where we have to be blunt about the limits of what we pulled. Our 2026-08-19 price fetch got aggregate range banners from TruckPaper for two of these three chassis, and nothing usable for the third.
Kenworth T680 sleeper trucks: an aggregate range of $12,000 to $599,950. Peterbilt 579 day cabs: an aggregate range of $9,000 to $527,950. Both of those bands span every configuration and both engine options, they are not per-unit, and they are not engine-specific. A band that wide is a category descriptor, not a comp. It tells you the used market for these chassis runs from a worn-out high-miler to a nearly new specialty spec, which you already knew.
For the Cascadia, no aggregate band appeared on either the day cab or sleeper page we fetched, and commercialtrucktrader.com returned a CAPTCHA wall. We got year and mileage points but no asking prices: a 2014 at 425,000 miles, a 2013 at 320,000, a 2019 Cascadia 126 at 942,052, a 2018 Cascadia 125 at 510,150, plus roughly two dozen more spanning 290,079 to 1,004,943 miles with no year tag. No engine tag was visible on any of them, so even that is chassis-wide, not X15-specific.
Which means we cannot give you a Cascadia-versus-T680-versus-579 price comparison. We do not have matched comps. Building one out of these three fetches would require inventing the parts that are missing, and inventing them is how the rest of the internet produces the confident comparison tables you have already read.
The things everybody claims and nobody sources
You will find articles asserting that Freightliner has the widest dealer footprint, that T680s hold value because operators keep them longer, that 579s are hardest to move used, and that each manufacturer's transmission tuning gives the truck a distinct feel. Some of that may well be true. We did not verify any of it, we have no data behind it, and we are not going to launder plausible industry folklore into a buying recommendation by writing it in a confident voice.
Dealer proximity is the one item on that list you can verify yourself in about ten minutes, and it is worth doing. Map the nearest dealer for each brand against your home terminal and your regular lanes. A fuel pump failure on a Friday night is a very different event forty miles from a service bay than two hundred. That is your data to collect, and it is specific to your operation in a way no article can be.
The decision you can actually defend
Price the engine first, because that is the part we can document and the part that is identical across all three. Establish the build year, get the fuel-pump service history, pull the fault codes, and check the DPF ash-cleaning records against that 250,000 to 400,000 mile interval. Then set your rebuild contingency using the kit prices and the documented $4,600 add-on case, and hold that contingency constant across all three trucks, because the engine does not care about the cab.
Then pick the chassis on things you can verify with your own eyes and your own map: how the cab fits you, what condition this specific truck is in, and how far you are from a dealer that can work on it. Not on a resale percentage somebody made up.
Per-chassis records with the full evidence trail: Cascadia X15, T680 X15, 579 X15. The thin-evidence engine records, flagged as such: T680 MX-13 and 579 MX-13. Purchase math: the used ISX15 and X15 buying guide.