Ok, first let me say I've searched/read thru a lot of other thread, but couldn't come up with answers to my problem of having poor gas milegae: I driving like an old lady with an egg under the gas pedal with overdrive on and only getting 10MPG (city only).

Background: I recently bought a 2005 F150, since changed the fuel filter, ran STP injector cleaner in gas, then changed oil to Mobile 1 synthetic w/proper Motocraft oil filter , cleaned throttle body/intake/filter, tires at 50 psi, only using 91 octane gas. The previous owner got a Airaid intake and custom tune (talk with who did the tune and it was done right, no worries there), the previous owner also hollowed out the cats and installed true duals. He claimed he got 15 MPG.

I drive the truck real easy 4 miles to work in somewhat stop and go traffic with overdrive always on and 1st gear seems kinda gutless when compared to when overdrive is off (I've tried it out a few times just to check). What I mean by that is I have to push the pedal further to the floor to get the truck rolling and at slower speeds/rpms are lower. But the few times I locked out overdrive the truck has lots of jump/torque with hardly touching the gas pedal/and rpms are higher.

Is this normal? A mechanic I know says this should not happen, that all overdrive is is a 4th gear that kicks in at higher speeds to conserve fuel/drop rpms, and that the tranny should behave the same way with overdrive on or off at lower speeds. Since I have to push the pedal down further, could this be whats killing my MPG? Any ideas?

Thanks

 

Unless you hit 40mph or above, you'll never see O/D. Overdrive is your 4th gear, and it doesn't really save you much money unless you can run in O/D for extended periods of time (i.e. hwy driving). You should still be getting mileage though.

 

Which company did the custom tune?
Have you cleaned the maf?

 

i think that a very realistic number. your driving 4 miles @ a time. the engine is probably not @ temp until the 2 mile mark. possibly even longer/farther when it was below zero outside. on top of that your doing the gas/brake/gas/brake which is not helping you any.

a true test for you would b to fill the tank, reset the trip, drive 200miles and then fill it up again and calculate your mpg. ill bet u jump to the 15/17 mark.\

as for the overdrive, mine is off unless im 50mph+

 

I've noticed the same thing with my 05... My mileage is 10.8-11.4 mpg (calculated). I have a shudder when in overdrive during city driving...over the last few days, I have turned the overdrive off during city driving. No more shudder, but the mpg's on the dash have dropped off to actually indicating 10.9. Btw, the dash mpg's and calculated are usually within 0.2mpg.

 

Actually, the 3-4 shift occurs at speeds as low as 25 mph under light engine loading. Even under moderate throttle, the 3-4 shift will occur at 35 mph. The "shift" you are feeling at 40 mph is the locking of the torque converter. However, the torque converter won't lock until the transmission fluid is warm. The lowest fluid temp at which I've observed my truck's torque converter lock is 85 degrees. (Usually, it locks for the first time at fluid temps between 87 and 95 degrees.) Regardless, I've noticed my truck gets its best fuel efficiency when being driven at speeds between 40 and 55 mph (with the torque converter locked).

This is spot on. The OP's truck isn't being driven far enough during his commute to reach optimal operating temperatures and, consequently, achieve the best fuel efficiency. On my truck, I've noticed it usually takes the engine oil, transmission fluid, and coolant at least 7-8 miles before they all approach their final operating temps (highly dependent on outside air temp, if course). Add idle time at stop lights/stopped traffic and acceleration to get the truck back up to speed, and you've got a perfect recipe for 10 mpg.

To the OP: Probably not much more you can do about it--however, even though your overall mileage isn't that great, you can feel good knowing your short commute is only costing you 0.8 gallons of gas per day. Folks like me who are getting better gas mileage are still going to pay more for our commutes because they're much further.

As far as the behavior of the truck with the O/D off, I don't have much to offer since I only turn off the O/D when I'm towing my trailer. However, turning off O/D does permit the torque converter to lock while in 3rd gear, and may adjust the shift points slightly. (It's tough for me to tell for certain because engine loading is much higher when towing, so the shift points are already being adjusted by the truck to compensate.)

 

Just in case you missed it in the OP's original post.....he's got a custom tune and that changes the 3-4 shift point and torque converter lock up speed engagement depending on how the tuner wrote the tune.....

You do bring up some good points .....if the pcm had the stock calibration tune loaded...

 

Just a thought but if you or the previous owner changed the tire size after the tunes were written that could be throwing your mileage calculations off? If the tire size has increased you are actually going farther than what the odometer is showing!

 

Thanks guys, I will have to check out the tire size. Will an OBD reader be able to show what size tire the tune is programmed for?

Also, does anyone know what % the odometer would be off by? The tires would have been stock size 275/65/R18 to 33" Toyo's.

 

Well, just from reading the tach, my truck won't lower the rpms (converter lock-up) in 4th until 40mph. I can sit at 38mph all day long and it won't drop, but the second I hit 40mph it will drop.

 

Try this....... http://www.discounttire.com/dtcs/infoTireMath.dos

 

Actually, I was responding to the information in SoonerTruck's post (hence, why I quoted it just prior to my response)...just in case you missed it.

However, the OP doesn't provide any information at all about whether or not the shift points or TC lockup speed have been altered. If the OP's truck is, as he puts it, "gutless" in first gear, then it seems unlikely the tuner made aggressive changes to shift points or shift firmness. Maybe these parameters have been adjusted, but the extent of such changes is not indicated by his post. Regardless, it was SoonerTruck's information that I was responding to with respect to the 3-4 shift point and TC lockup.

In any case, the OP's fuel economy is definitely lower because of his short commute, tune or no tune. The impact of O/D on or off will have little impact, if any, for a 4-mile drive. I also have a custom-tuned truck, but it doesn't get peak fuel efficiency until it reaches operating temp. That takes at least 7-8 miles of driving.

Yes, that's correct. The RPMs drop when the torque converter locks at 40 mph (under light engine loading). That's perfectly normal.

 

275/65R18 equates to a 32.1" diameter tire. The Toyos are 2.8% larger than your stock tires. If the tuner failed to compensate for the slightly larger tire diameter, your actual mileage would be 10.3 versus 10 mpg.

 

Again, thanks for the responses. Well, when I talked to the guy who tune the truck he did not mention if he changed the shift points, he just said the CAI/exhaust was set up right, as that's what I was concered with as I've heard F150 tend to run too lean with a CAI if it's not tune properly. I've tried to call him and ask some more questions, but I guess he's been busy. Maybe I'll go by and get him to give the truck a look just to make sure on the tire size and shift points. I'm thinking this might make sense and he may have changed them. The former owner was a farmer who lived out in the country (no city stop and go stuff) and worked the truck pretty hard towing stuff around the farm. So most likely the owner got a custom tune that provided for that kinda use. I guess he could have got a tune that when when O/D is not engaged had one type of shift pattern, and when O/D is engaged had a different pattern. Something like when towing O/D is always off. When getting up to highway speed O/D is off, but when at highway speed and not towing then O/D goes on and tuned to take advantage of the CAI?

 

What company wrote the custom tune?
Was it a laptop tune or did it use a programmer?