I follow 'Hoovies Garage' on YouTube, and today he bought an '04 Ram 2500 4X4 Diesel with (supposedly) just shy of 1 MILLION miles on the engine. Says the Carfax backs up the claim.

BUT, in the video, he points out the factory computer that shows MPG is VERY optimistic. It showed 30 MPG average at his first fill up when in actuality it was getting 18 MPG.

My question is, could the odometer be reading 'optimistic' like the MPG is? Just curious.

Aftermarket computer controlled power adders feed fuel outside the factory computer range so it's never accounted for via the onboard fuel economy (lie-o-meter) gauge.

I had an '06 and the overhead was never accurate. It would say 20 when in actuality it was 14. I believe these used software to generate a presumed number much like the oil pressure gauge.

Funny, my '01 was reasonably close until I added the TST powermax and later edge w/ attitude.

Here's my suspicion of how the factory calculates fuel economy. I_think_due to emission requirements they have FULL BSFC numbers at all throttle positions? And the computer calculates your mileage from these tables?

FWIW, 40+ years ago I had a Zemco driving computer on my '80 Mirada. It used a fuel flow sensor you added in the fuel line, the computer allowed you to "calibrate" it with your hand calculated numbers. I'd calibrate it every fillup, it was pretty consistently two MPG off plus or minus. Back then it seemed really cool to watch the instant fuel economy numbers.

total junk

One would have to wonder how much R&R was done on the rest of the truck at even half that mileage.

Definitely a little bit of "Washington's Axe" going on--clearly a lot of things have been reworked and replaced. But still impressive for any vehicle to make it that far--especially in as good shape as it appears to be for its age.

Gee, 1 Million miles ?!?!?! . . . guess I have LONG LONG LONG way to go with my 02 Cummins . . .only have 80k miles on it now !! Have owned since new . . .

FWIW, 40+ years ago I had a Zemco driving computer on my '80 Mirada. It used a fuel flow sensor you added in the fuel line, the computer allowed you to "calibrate" it with your hand calculated numbers. I'd calibrate it every fillup, it was pretty consistently two MPG off plus or minus. Back then it seemed really cool to watch the instant fuel economy numbers

Back in '83 my 16 yr old self wanted one of these soooooo bad, but my part time minimum wage job wouldn't allow it.

Back to the truck.......

The truck is in incredible shape for the mileage, but I seriously have my doubts that 'just' maintenance has kept it looking/working so good.

My old '01 was pretty accurate on the MPG calculator also. I checked it many times.

My 04 CTD Dually which is currently down for some fuel related issues. Has 209,000 on it. Mine say average 17.8 in town/freeway driving.

My '04 with 24 valve has 310,000. ALWAYS some little electrical problem going on with it though. Has not stranded me anywhere - but annoying, and keeps me from using it for long trips.

I've known of a few high miler Cummins powered Dodge trucks.
Our company equipment repair guy had a '93 with over 700,000 on it before he parked it the last time. The entire truck was dented, scratched, dirty and just worn out. The engine was still running but not well.
Another one was a 2006 that had almost 600,000 when the company closed the doors and sold off all the assets.
If I could bring myself to tolerate the smell, I would have bought a diesel truck long ago.

My '92 is over a quarter of the way there at 311,000. It's had a rear main seal and 2 valve adjustments. The VE pump was resealed, not rebuilt and I just recently pulled the front cover off to check and secure the killer dowel pin. Not because there was a problem, but just for piece of mind. It was firmly nestled in it's bore though. It doesn't leak or use oil but I do think the VE pump is getting a little worn.

I suppose its possible one of those rare vehicles where everything was assembled perfectly, and had an owner that really took care of it rides around for lots and lots of miles.

Back in the early 70s I worked at a gas station and one of the customers had a 68 Chevy Impala. The guy was an on the road salesman and that Impala saw a lot of road miles every day. Somewhere around 1975, the old Impala met its death with somewhere near 350,000 miles. It had a 327 and still had the factory installed timing chain. The car was involved in a minor accident, but it was pretty rough by then, so that was its end.
We also had a customer with a 64 Chrysler New Yorker that went past 250,000 miles, it was also very wore out when the guy traded it in on a new Chrysler.

My experience with high mileage vehicles that were still original is that everything gets wore out. If this truck actually has close to a million miles on it, there have been a lot of things "maintained" over the years. The term "regular maintenance" has a lot of variation built into it. To some people, regular maintenance means you replace things as they wear out, but to other people it means you change the fluids on a regular interval. On this truck, I suspect things were replaced as they wore out, and that included nearly everything on the truck at some point as the miles kept adding up. Gene

to OP, I saw this question pop up in the comments too.

Basic common sense thinking; which is more likely, Having the odometer which is a pretty strong legal matter be off a lot OR the MPG on the OH console be inaccurate?
Sure, speedos tend to be off a bit (often optimistic compared to calibrated ones), but it would be pretty stupid for the mfgr to have the vehicle rack up miles faster than 5-10% over.
OE and aftermarket leasing, rental companies, insurance and book values would all be keeping a close eye on such things.

I had a '92 CC 5.2 Dakota with 450,000 on it when I sold it, still running and driving. Diff was never opened, engine had intake and head gaskets, a radiator, 3 or 4 fuel pumps and a couple trans rebuilds. Not bad for a lemon buyback vehicle.

04 Would be a common rail, without lots of injector changing over the years it would never make a million miles! My 04 and 05 both had injector problems in the 260K range, worse injectors ever designed...

I ❤️ my 97 12 valve

Taller or shorter tires then OEM could alter both mileage and the computer's mpg numbers. I suspect the truck has seen lots of different tires with that many miles....
What happens after the truck left the Dodge dealership the first time is beyond Dodge's control. Dodge can only base warranty on the mileage the computer says the truck has. The computer has no idea what size the tires are.