Long road trip startling #s
 

Took a road trip this past week to Montana so a good chance to check mileage and such. First thing I found out is my Speedo/odometer is 9% off. Only recording 9 tenths of every mile which I understand to some degree due to tire size but did not think I would be that far off.

After 800 miles in the truck we stopped and stayed the night checked everything fluid wise and under the hood the next morn and found that my dealer left the oil fill cap off oil everywhere. Level appeared ok so we pressed on.

I never went more than 300 miles between fill ups but checked them by hand each time I was runnin about 2400 rpms speddo said 75 so that was about 83mph got about 14 toned it down to 2200 rpms speedo read 70 actually 76 and was able squeeze 16.5 once hand calc countin adding the 9% overhead read 15.3 most of the time right around 15.2 15.6 trip totaled 2700 miles truck now has just shy of 13k on it

Not to impressed I really thought that on a trip like this the tires would help now I need some suggestions to help my mpg. ALso they supposedle did the reflash 18-003-06. REally noticed nothing compared to what I used to get.
Sometimes I wish I still had my gasser but oh the power. I do not expect to get 25 mpg IT is a 4x4 truck but this to me is pretty crappy.

What gears do you have? also are you adding 9% to the miles traveled on the ODO in your calculations? (You didn't mention it, I had to ask) 16.5 is not bad for so young a truck running @ high speeds. your mileage will improve as you get some more mileage.

That doesnt sound too bad to me. Got any pics of your truck? I want to see your tires.

Do keep in mind that the bigger tires are also more mass, which equates to more power required to turn them, and more friction on the road to boot.

I get that kind of mileage driving around town and roughly 20-21 when on the highway......but that is with OEM tires.

Speedo error when going from 265/70-17 (31.6") tires to 315/70-17 (34.4") tires is roughly 100%-31.6/34.4=8.1%. So your 9% number checks.

yes the 16.5 had the 9 % calc included

I was assuming thoughg with no hard pulls the larger tires yes need more power but once you get going the less times they gotta turn to cover some ground.

I left home in Mass and drove to Charleston S.C., 946mi oneway. Overhead
showed 16.5-16.0 all the way down, I used 48 gal of fuel, avg fuel economy is well into the 18 mpg range, cruise set to 70mph most of the trip, stock tires
and 4894 miles on the odo., 410 rear. I was expecting less, so needless to say I am pleasantly surprised. I'll post on the return trip for total overall fuel
economy. [coffee]

nitis,

I have gotten 20.08 mpg HWY (hand caluclated) on the last 2 short trips (about 800 miles & 1000 lbs total weight of people and stuff) and it now only has 8900 miles on it. The CTD gets it's best MPG at 55 to 60 MPH, 1500-1650 rpm's.

When I had only 2500 miles on it (with 800 lbs in the bed, plus two adults, plus one extra person on the return trip) I took a 4700 mile trip from IA to OR and back and got 18.35 mpg for the whole trip, but then I tried to keep the RPM's below 2100.

Have you used PSD & MMO to help with mpg? I use a mix of 8oz MMO and 16 oz PSD ( gray bottle) to 30 gal of fuel.

Maintaining 2200/2400 rpms is partly responsible for your fuel mileage.

Per the Executive Engineer @ Cummins, the "sweet spot" on these engines is in the 1700/1900 rpm range. That should be where you find your best fuel economy.

I know it's no fun to be driving that slowly but, that's what you will have to do if you want the good mileage. You just have to decide if you want to save money, or time.

FWIW, I have an '03,3500, QC, 6-sp., Campershell, & 110 gals. of fuel, on board. I did a "fuel economy" trip of over 400 miles with the cruise control at about the 62-65 mph range. Filled "to the top" before & after the trip & hand calculated the results. 21.47 mpg's. Half of the trip was into a strong headwind, also. On the trip back, the wind had calmed down by quite a bit.

Basically, most of these trucks are "capable" of pretty good mileage. You just have to have enough self-control to drive them economically. Normally, I don't!!!

Joe F.(Buffalo)

I had 35s on my ford and it was 10% off.

I agree with everyone else, you gotta slow down if you want the mpg.

I tried it but decied that my time was more valuable than my money.

ok so put a gear vendor overdrive in your truck and drop your rpm's by 500. your mileage will go up about 25%

Dont bet on that !!! Your pulling more load then, maintaining another trans, running another set of planitary gears, plus the cost of the thing, never get your money back. [dummy] [dummy] [coffee]

pulling more load? how do you figure ? [duhhh] well if he's running 2400 rpm's the way he said he was and he installs a gear vendor and drops 400 to 500 rpm's how is this more load? he will still be well within the engines optimum tourqe curve at 1900 or 2000 rpm. I knew a guy who installed a gear vendor in a 94 dodge ctd 5 speed and gained 4.5mpg. he actually got 24mpg hand calculated. now if your towing heavy I could see how there would be more stress on everything while the overdrive is on but you wouldnt have to run it while towing heavy. bobtail you will gain more mpg with an overdrive that with any other mod. as for getting your money back, they cost about $3000. and if you drive only 1000 miles per month you will save about $42 per month in fuel or $504 a year so it would take 6 years to become free of charge. however if you drive 3000 a month it would be a free unit in 2 years . not to mention the fact that there is way less wear put on the engine with lower rpms. in just one hour at 500rpm's more, his crank shaft would spin 30,000 more times without the overdrive unit . in just 12 hours at freeway speed thats 360,000 more crank rotations! some people use these trucks to drive 100,000 miles a year. it depends on your situation. big AZZ tires with fancy wheels can cost $3000. yet the tires will be used up and the wheels out of style in 2 years and they wont gain you any mileage at all. how long do you plan to keep your truck? there are a lot of guys on this forum who still drive gen 1 trucks. I bought a diesel because it will run for decades. I plan to keep and maintain mine till it quits and isnt worth the cost of repairing so an overdrive is worth it to me. it will see at least 10 years of use in my truck. oh , and you have to service the unit with I believe 2 quarts every 5000 miles , big deal. [laugh] and if you break one and are the original purchaser, gear vendor will sell you a completely rebuilt replacement for $600.

For starters your pulling more load when you shift into the vender, just as you pull more load when you shift from 3rd to 4th. Now you can get our trucks with a 4.10 rear or a stock 3.73 rear in the auto trans. Run them down the road side by side empty at a decent road speed and I dought you'd see a measurable difference in milage. Put 15k behind them and the 4.10 will do better in milage and performence. You gotta do a lot of freeway running empty in that high gear to make it pay. Any time you slow enough to downshift it 's not helping. So how many miles out of a 500 mile week are you going to be up in that gear. In town ,never, behind slow drivers never,. My 06 has a 3.73 rear. I have to get up to 50 mph for the TC to lock up in 4th. Then the engine is turning 1300. At 70 she turns 1900. 16-1900 is the sweet spot for these cummins. Just cant see a $ value to the vender. Or a $ value in bigger wheels and tires eather. The wheels and tires might be justified by ones taste in looks etc. But not in $. [duhhh]