Odometer accuracy and MPG

[Awesomeness]

This has been touched on in a number of other threads.

ODOMETER:

I did a recent test of my odometer accuracy against my GPS's odometer. This represents a small sample from this test, but I believe it to be consistent with my previous use. I drove 42.9 miles (on the odometer) in one continuous trip, during the day, and the GPS recorded 44 miles (it doesn't display decimals). Then I did the return trip at night (with the lights on), the odometer recorded 41.0 miles, and the GPS read 46 miles. Interestingly, the GPS and the speedometer often disagree by +/- 3MPH, and at night when the lights are on it's more like +/- 6MPH. Overall it looks like the difference between odometer and GPS mileage is about 2.5-11% off.

MPG:

I track all my gas mileage in an app, since I got the truck. I try to always put the fuel fill nozzle into the tank the same way, so that it stops at about the same fuel level, and I generally fill up at the same gas station (which only has 2 diesel pumps, so I'm usually literally using the same pump/nozzle). There will be some variation on fuel mileage of a particular tank full, based on how full the tank is when you stop filling, but this should average out across two or more tanks (e.g. if you really only filled it up 1/2 way to get to "full", it will assume that you only used a 1/2 tank to get a full tank's worth of miles so it will be high, but the next fill you'll have to fill that back in and you'll record a really low MPG tank to balance out). If I haven't driven the truck in several days, I'll often start it up and let it run 5-10 minutes, especially in winter, and in winter you obviously have to let the truck warm up for sometimes as much as 30+ minutes.

During the lifetime of my ownership of it, my average is 5.19 MPG. The first couple years I had it, I averaged slightly higher, around 5.50 MPG, but there is a very obvious decline in my graph when I installed the 290HP 3116 engine last summer. My best tank ever was 6.44 MPG (which I happen to know was during a long straight summer highway drive, with no canvas on the back), and the worst was 4.10 MPG (November 2016, so perhaps lots of idling and warming up, and back then my fan was always stuck on) - but please remember the note about good and bad tanks balancing out too.

I have been running the Goodyear tires the whole time I've owned it. My CTIS works, and maintains a propper ~55PSI inflation. I'm in Denver, "mile high city", so 5280ft elevation.

CONCLUSIONS / THOUGHTS:

1. It's curious why the odometer is more incorrect at night. Does the voltage drop with the lights on impact it somehow? Is this just a problem with my odometer, or is this seen by others too? I do most (95%+) of my driving during the day.
2. Even at it's worst, the odometer doesn't seem to be more than about 10% off, but considering that most of my driving is during the day, the majority of the time it is probably in the 2-3% incorrect range. That means MPG recording should be reasonably accurate.
3. I can see trending in the graphs after certain major changes to the truck.
   1. When I repaired the fan clutch so that it could turn on and off again, my mileage seemed to improve ~0.1 MPG thereafter (2-3% net improvement). One of the military reports specifies the anticipated power consumption of the fan to be around 23 kW (I think), which is 30.8HP.
   2. Switching to the 290HP 3116 was an obvious negative effect, of about 0.5 MPG (10% net decline).
   3. Running with the canvas off seems to have a small ~0.1 MPG improvement (+ 2-3%), which I would have thought it would have been more, but maybe while it increases wind resistance in the front it is also decreasing it on the sides too?
   4. Very recently, I put in the Hot Shot's Secret - Stiction Eliminator ( https://www.hotshotsecret.com/stiction-eliminator/ ) just for fun. I'm generally very skeptical of this kind of stuff, but I kept the receipt and plan to mail in for the "100% guarantee", so it should be a fun snake oil experiment. The last two tanks of diesel since I put it in have actually shown a small improvement, in that same ~0.1 MPG (+ 2-3%) range, though this could have been due to that driving being more continuous distance driving, instead of stop-n-go.
4. The military specifications call for the 58 gallon tank of diesel to give the truck an operating distance of around 300 miles. That equates to 5.17 MPG. My numbers seem to be in line with that. The military's number is also probably a "worst case", so under more ideal civilian conditions I would anticipate getter better than that.
5. I'm really curious about the people who are claiming 8-12MPG. Frankly, I don't think it's physically possible, without major modifications (e.g. air dams and other aerodynamic drag modifications, changes to the 4WD system to eliminate spinning the front drivetrain unnecessarily, modifying turbo boost, etc.). High speed gears may help some, if you keep going the same 50-55 MPH, but even that drop in RPM is only going to give you a <5% increase to MPG, not a 100% increase. Obviously, my truck is a sample size of one though, so I'm open to it. I suspect that there is some other obvious cause of the discrepancy (e.g. extreme odometer inaccuracy, they aren't actually tracking MPG and are just doing "mental math" at the pump, etc.). I track all my vehicles' MPG this way, for the last 20 years, and I've never seen any modification swing it more than about 30% (and that was swapping out a small 6-cylinder in a full size K5 Blazer for a 590HP BBC which dropped it from about 9MPG to 6MPG). Even rather major changes (e.g. new high-flow exhaust system, intake or exhaust manifold changes, lifting a truck, changing to oversized tires, etc.) only end up being 5-10% incremental changes each. The energy required to push through the air at increasing speeds is also exponential, and is the single greatest source of the LMTV's poor fuel economy, in my opinion.
   

What are you seeing?

 

[spankybear]

GPS will fluctuate a tad depending on atmospheric conditions and other things. The speedo in the truck counts pluses. I would believe that the GPS is flutuation more than the speedo. The best thing to do is use the speedo check that you see along the highway to see how bad your speedo is. Another thing is if you have a known distance (like a mile) time how long it takes and do the math form there.

One thing you could take into account is they the tires are cooler at night and the pressure is lower a tad. This will change the wheel diameter. Since the who Pi thing this will change the number of pluses the speedo will see at a given distance.

 

[simp5782]

A0 truck 6.3 to 6.5 over the 278 miles to the shop. empty. 6.0 to 6.2 over the same distance back home with 1300 to 1500lbs in the bed. Pedal to the floor the whole way. Mileage varies there because coming back I am going more down hill as the countryside slopes. 298 sea level where i live and nearly 2,000ft where the shop is. You will always get better mileage running at night with the cooler temps on not just the tires but the entire truck, and lack of traffic. Regardless of stop and go city or cross country driving.

The 1078A0 on 4 trips recorded 4 to 4.5 mpg towing M931 tractors with a towbar the 278 miles back to memphis. They weigh around 20,000.

 

[coachgeo]

side note.... hwy ctis setting of 55lb per tire is perfect for Military usage , but is real low for ours. Others report that what they are doing with their LMTV's that either do lot more travellllll (overland/expedition use) or more generic civilian type use..... are going with round 80lb per tire if I recall right. Michelin that is. NOT they Goodyears

 

[1951M1078]

Just a question. If I fill the tires to 80 psi won't the ctis do a test and bring it back down to 55 psi ?

 

[coachgeo]

Just a question. If I fill the tires to 80 psi won't the ctis do a test and bring it back down to 55 psi ?

Unplug it..... you do not need ctis 99% of the time. You'll have to check your own air pressure when you get fuel though. If it works and your going off road.... plug it back in. Granted you should plug it in for maintenance/evaluation purposes too. Or go with a manual system as has been described in here while back and set your own desired pressure.

 

[1951M1078]

Thanks !