Ram95

In May of this year I put a after market air cleaner (cone stile on the end of the sock intake tube) Then I cut out the cat. My MPG went from average of 13.20 to 15.56. I then put on a 4" exaust from the turbo back and stayed at 15.56. In Sept I moved the stock plate and housing all the way forward and put a new set of 295 17 Toyo OC Mt on. Now my MPG are averaging 13.33. Any idea in the drop? I had 33 x 12.5 on before so I did not really go up in size on ther tires. The only thing I can think of is the air temp. During the summer it is in th 80's-90's now it has been in the 60's-50's. I would appricate any help, advice. It is hunting season and I'm gone every weekend and it's getting expensive at $3.79 a gal.

William12Valve

Is it an auto or manual? When you are figureing your milage are you correcting the miles you went to make up for the larger tires making the speedo and odometer/trip meter off.

wcbcruzer

You said you moved the fuel plate forward. That could do it.

Ram95

No I havent been correcting my miles. How would I do that?

Ram95

Would putting the plate back at stoke postion help> What about going to a different plate?

Ram95

almost forgot, auto trans

Dartmouth 12V

Tires have a lot to do with it. Diameter is important(both for rotational inertia and gearing) but also width(larger contact area equals worse mileage). Also, tires with more aggressive tread will get worse mileage.

If you haven't corrected your mileage for the new tire size, you can search on google or here and find a gear calculator.

As far as the plate goes, moving the plate won't necessarily have an effect on mileage but it can. The plate only limits the maximum throttle allowed so if you never use full throttle, it doesn't do anything. However, most people will use more throttle once they have moved the plate.

Ram95

What should my engine temp be?

Buckshotram

The needle on my truck stays straight up at the (190 ?) mark.

Ram95

Mine never gets to 190 just a little below. I wondering if I have to cold a thermistate in it?

Big Blue24

Thermostat should run 190* or higher for best mileage. You correct tire size by dividing current tire size by stock tire size. 33/31 =1.065 you need to multiply miles driven by 1.065 or 6.5% to get true figure. Make sure your tire pressure is up around 50-60 psi if you can run that high without wearing the tires out too bad.

Hercules

You could probably get some milage back out of the afc. I noticed you said you moved it forward. Did you move it full forward then screw the star wheel all the way forward too? Did you screw with the smoke screw on the back? If so, that's probably contributing to your milage. The afc controls fuel at no or low boost conditions until the afc arm moves out of the way at +/- 6 psi boost. So, since the majority of the time your driving, your probably under 6 psi boost(I know mine usually is), you should be able to tune the afc to get some milage back, control smoke, etc. There's a real good right-up on here about tuning the afc. Probably just search "afc" or "tuning afc". It goes into the trial and error process of tuning it with the afc position, the "smoke screw" on the back of the afc, and the star wheel. It gives real good info on what the afc is actually doing and the "what" and "why" of how the tuning "parts" work. If I knew how to do a link I'd just find it and send you there...hopefully someone else knows of the right-up I'm talking about and knows how to link to it.

Your right about the cost of diesel...it's getting rediculous! IMO, there is no reason why diesel should cost more than the highest octane gas(heck, it should be less than gas period). I understand heating season and with the holidays around the corner, shipping is placing a large demand on diesel and heating oils, but, if I'm correct(and I could be wrong), the refining process for diesel is not nearly as involved as gasoline...therefore, it should cost less as a result. Sorry for the rant, I had to get it out. Maybe someone will correct me if I'm way off base on this and yes, I understand there are other factors contributing to it, but, to justify a cost of 30 cents or more over premium octane gas is a stretch for me.

Ram95

Thanks for the info guys. Gives me a good place to start.

Big Blue24

Easiest way to find out your correction factor is to drive down the Highway for ten miles using the road side mile markers as a reference. Reset the trip o meter and watch to see what it reads at exactly ten miles of roadside mile markers. If your truck says 9.6, your correction factor is 4%. 1% for every tenth.

If you wanna get a real good figure, do this for 100 miles, every 10th is 1/10 of one percent, every mile off is 1 percent. Pretty easy to get a good correction factor.

Then when you fill up, divide miles driven by gallons pumped. Then multiply answer by correction factor: if 4% then multiply by 1.04, if 6% then multiply by 1.06 and so on. Just FYI, if your truck still has factory speed o meter tuning/ correction, 285's need a 6% correction, 315's need an 11% correction, 265's need a 3.3% correction factor, 245's need no correction. Every tire varies a little so these numbers will vary somewhat between different tire manufacturers. But we're talking like 0.1% or 1.001, or not enough to matter.

In my experience, the two factors that have made the most difference on highway trips are air pressure/tiresize and good thermostat that lets the cummins run 190* or more. I had a broke/ stuck open thermostat for a few months in the winter and highway miles dropped 1.5-2 mpg.

I hope the tire size correction makes sense. *If you have bigger tires, it should make sense that they turn around fewer times to go the same distance.* That is why the speedo is off when you buy bigger tires. The speedo runs off of the odometer so if you ever want to know actual speed, you can just multiply your speed by correction. With my 315's at 11% it was easy since 10% is an easy one to do in your head, 60 mph on speedo was actually 66+ mph on the road.

One more thing, when doing the correction factor, you can apply it to the miles before you divide by the gallons or after, it makes no difference.

The next best advice for good mileage is to keep your foot out of it. Rolling up/coasting down instead of brakes and throttle all the time.

If your truck has poor alignment, and is toed in a ton, that will add road friction/ driving resistance and lower mileage as well.