Tree DR

Why is there some OH MPG almost correct and others so far off? Could it be the float on the fuel tank sender? I've installed fuel tanks and I noticed differences in fuel levels and what the gauge said.
Do the ones that are more acurate on the OH MPG actually have a half tank of fuel when the gauge says so? My truck is about 7-8 gallons off at half tank on the gauge.
I other words the fuel tank sender/float is what make so much variance from one truck to another?

t-boe

You might be right. My fuel guage varies as well as my overhead. Sometimes it's dead on with my hand calculations and sometimes it's been as much as 4 mpg off!

timcasbolt

Mine varies a lot, and I run almost the same routine every week. The truck stop where I refuel is on a slight grade, so it matters if I fill up uphill one week and downhill the next. Since I pull the nozzle out when the auto shutoff stops, it matters if the pumps at the island are not pumping with the same pressure because higher pressure will create more foam which kicks the shutoff quicker.
The thing I've always wondered about is, what information does the computer use to calculate MPG?

LFD2037

Mine OH is exactly 2.5 MPG's high EVERY time.

Tree DR

That's a good question. Distance to empty has to be based on fuel level in the tank. I've watched my DTE as I fueled and it increased as the tank filled.

Mark03Ram

I know on gas motors the computers monitors the injector pulse width vs the speed o ticks. I did a test on my Hayabusa, I used a speed o healer to calibrate my odometer to 1/10th at 50 miles, best I could get lol. With the odometer dead on my speed o was about 6% off, my fuel mileage was 5% off when I hand calculated it. I reset my speedometer to be right on at 70mph and drove it another tank the mpg gauge was less then 1% off.


I have never seen a speed o be exactly on at 10 mph and 110 mph always seems to be some variance in there. So in my bikes case the slight difference in hand calculating and on board gauge could be the % of error at low speeds (or really high speeds :-) ) or simpley if I put in a few drops of fuel more.

I think that is a big issue with allot of guys, 1. speedo is not calibrated to be right on at high way speed. A few mph will make a big difference over 30 gallons. 2. not filling up to the same spot in the tank, getting a few gallons more in this time then last would change it. I seen in Diesel power challenge one of the trucks pulled around after filling up and got 3 more gallons in.

My truck is pretty much dead on at 65 mph, my on board gauge has been within 1/2 a mpg on all but 1 fill up and I idled it allot that tank, it was only 1mpg off. So not sure that is much good info, just my experience.

Tree DR

I don't think the speed MPH would be part of the equasion. The OD would but what else? Pulse width? Why would a moded truck that is increasing the pulse width show better MPG? What is the performance mods doing to affect the MPG computer? Some have said their MPG comp was closer to hand calc after moding the truck.
As to the difference in MPG because of different tank levels at fill up. I could see that as a major factor. I'm sure that others not just me have seen a consistancy of 4mpg above hand calc. I've checked my speed O as well as my odomiter, each were so close you couldn't ajust to get closser.

own1bigtruck

mine was almost dead nutz on stock (within .2). I added the smarty, and it still almost dead on, even went down (.1). I think it has to do with the truck, also if you change tire size, you have to reprogram to the tire measurement not what the sidewall says. My 305's are supposed to be 33's, however in reality they are 32.6's

Mark03Ram

Well from my research the mpg gauge has nothing to do with the on odometer, its about the ticks or pules sent out by the speedo sensor. The computer is reading those and not the odo on the dash.

Most ppl that have problems reading high are running pressure box's. The programmers hold the pulse width open longer so the computer knows this. But the extra pressure is allowing more fuel in during that same time period. SO the computer would have no way of knowing the extra pressure is allowing more fuel.

I wonder how much more fuel is injected with a few k more psi in the same pulse width as stock??

Either way that is the variance with pressure box's. I use GPS to calibrate my speedo's very accurate if u hold the speed for 15-30 seconds. MY truck speedo is dead on mph wise, but off on the odo. That is how I got the truck has stock tire size on it, but not oem tires. This is odd to me as most vehicles the odo is dead on and the speedo is off as much as 8% depending on the manufacture. I am chalking it up to the type of tire the previous owner put on, it might be a little taller then stock tires and that would explain why the speedo is right on.


I have never ever ever seen a speedo and odo be dead on in any vehical ever. There is always some degree of variance, useually 4-8% range. My bike for example is about 6% off, it also has a variance of 2% over the range. What I mean is at 100mph is perfect at 20 mph its about 1 mph off and at 180mph its about 1 mph off. My math is a little rounded but you get the idea.

**edited due to typing before I wake up and had to clear the fog **

GMScott

I noticed that my OH would read a little high on the avg. m.p.g. during the first couple of hundred miles but would even out over the long haul and be almost dead on by the time I'm finished a 1,300 mile trip.

Tree DR

I still don't get the reason for the wide difference from one truck to the next. Is there not one thing that it can be contributed to?

rockdonon

My OH always is off by 2.5, It reads 13.5 MPG and hand calculated is 16 to 16.5.

dale29

Mine is just the opposite it reads 16-17 and I'm really getting 12.5 - 13.5

Tree DR

What you last couple of guys said. Some high some low. There's got to be something in common that could be ajusted. That's why I thought the float in the fuel tank might be the problem. Do you guys that are almost dead on find that when the tank said half full it actually is?

Foxborough

I have been keeping track of fuel since new. Just this year it has started tracking more off, up to 2 mpg higher than hand calculation. Some fillups have shown the hand calculatons to be better than OHD. The only pattern has been city driving they are closer then highway for me. I drive mostly combined miles. Drive into Chicago & back or up to La Crosse WI for the weekend. Hauling my #6500 boat up to La Crosse back and forth 3 of 4 times. I have 33k miles on truck and 20k miles on motor. I mostly fuel here at home, at the same place (Mobil). I have assumed the OVH is like my wife, unpredictable and a little off sometimes