Computer Gas Mileage Lies if you've changed Gears/Tire Size!

[MikeVistaBlue06]

I've made 2 or 3 trips to GA in the s197 and each time it says I'm getting 25mpg. That's with the Saleen SC and 3.73 gears. The very first time I drove her to GA with 3.31 gears and no Saleen SC, she got 22-23mpg.

Well I finally got off my ass and measured the gas milage the proper way...fill it to the brim, drive x number of miles, refill to the brim, and do the math. The math sez about 21mpg on a good day with Saleen SC and 3.73 gears!!!

Yes, 3.73 has been entered into the tune, and I am running the stock size tires/wheels that came with my car. Speedometer is right on the money (60sec between mile markers at 60mph) Odometer is ok as well.

This tells me the fuel mileage is calculated based on the computer still thinking the car has a 3.31 gear!

Math sez so too: 25mpg X 3.31/3.73 = 22.18mpg!

Not that it matters. I didn't buy the car for gas mileage.

There has to be another parameter in the tune that needs to be overwritten OR there is a separate computer calculating the fuel mileage and that needs the 3.31 replaced with a 3.73!

HTH

Mike

 

[bcb06gt]

I been down this road before. It has nothing to do with the gears, directly. If your mileage is correct and the speedo is right then you are good. It is the tune. Did you check before the gears when you only had the blower? Mine has been off since the blower and JDM tune. There is another parameter that can be adjusted to make the fuel consumption correct. The car calculates gas mileage by fuel consumed and miles driven. Check your fuel consumption - this is the parameter that is wrong.

 

[bcb06gt]

Here you go. My take on the subject is this: Basically applying older methods of programming to "force" the system to work and not really learning the new system.

From Eric Brooks:

Before the 2005 Mustang came out with its newer processor, there was a limit of airflow that the PCM would recognize.

63.5 #/min of airflow was the limit. The MAF transfer function could not have any number higher than this value. In fact the software is setup so that you can't even enter a number higher than this. If you had a car hit this number, then you could not add more fuel in.

This number is roughly 480 RWHP give or take, so it is fairly easy to hit this on older cars, especially 2003-04 Cobras.

The fix was to scale down the injector values, along with the MAF values, along with the engine displacement scalar so that the calculation could still be correct, but the actual number would be under the limit.

GM cars have the same issue on certain year models as well.

Scaling at first seems like a hack job, but in fact, Ford used this technique on the Ford GT Supercar, since it used an EEC-V processor.

With the 05+ Mustangs, there is no limit to the airflow calculation (technically there is, but you would need a top fuel engine to hit it).

If you have ever read the instructions that come with a Diablo MAFia you will find that they just tell you to scale. This will work on an 05+, but there is no reason to do so.

If things are scaled, there is parameter called "Pulses Per Gallon Conversion Factor" located in Misc > Scalars in SCT's software. I don't think this parameter is in Diablo CMR. The default value is 48000. I have no idea what unit this is in, but it needs to be scaled if the tune is scaled.

Me personally, I have never found a reason to scale a newer processor in order to tune it properly. There are some tuners (tooners, hehe) that don't realize you don't have to scale on the newer cars.

If you tune the car properly, use the correct injector values, and tune the MAF properly, the fuel calculator works perfectly.

Hope this helps.


Entire thread:
http://www.s197forum.com/forum/showthread.php?t=30101&page=2

 

[iDance]

That concept seems obvious to me...

 

[bcb06gt]

That concept seems obvious to me...

not too much...look and see the tunes that have a problem with mpg after injector size change

 

[trill gear head]

I don't recall where but I heard that the mpg computer usses the assumption that you have stock fuel injectors. If your speedo is correct, then that would explain why it is wrong.

 

[Vapour Trails]

My computer calculates pretty close to the truth. I've verified it enough times I feel no need to do it by hand.

 

[Steps]

Mine is always .5 - 1 MPG under. This was calculated manually on two trips to TX, one this year and one last year. Close enough for me. I average 23.5 - 24.5 MPG with the Blower, 4.10's, and other mods.

 

[bcb06gt]

Check fuel consumption and compare that to the actual amount pumped when you refill. This way tire inaccuracies and gears are not involved.

 

[phatpony]

My consumption is way off. I fill with 15 gallons, and the computer says it's 11.2 or so. My mileage numbers are awesome on the computer, consistantly 29.9 mpg, but the reality is I'm only getting 23 mpg. When I get a tune done, I'll see if we can fix that.

 

[NastyStang113]

My computer calculates pretty close to the truth. I've verified it enough times I feel no need to do it by hand.

Same.

 

[Shotokan1509]

My tune na 3v with 4.10's is pretty close, if anything the computer calcs lower mpg's than actual, but like .2 mpg's on average per tank