Fixing MPG

[RodneyBur]

Can you make changes to the way that this is calculated using SCT software? I've noticed that my MPG seems to be off a little and just want to see if its possible to get an accurate reading. Also interested in the mileage till empty reading.

I thought that the ECU measures the amount of gas in the tank along with the mileage travelled to formulate the MPG. Does it look at anything else?

[FastCougar]

I believe it's a seperate board that calculates that, but get's inputs from the ECU or the same wires that feed into the ECU harness ... someone correct me if I'm wrong. I browsed through all the adjustment tables in the PRP last night ... nowhere is there such a table.

I wouldn't worry about it ... you should be calculating MPG manually anyways. I personally keep all my gas receipts and jot down the trip mileage and vechicle mileage with the Corolla. When I get home, I transfer the data to a spreadsheet and then toss the receipt in the trash. I have been doing this since Day 1 with the Corolla :)

[RodneyBur]

Blah... Im lazy. I usually keep my gas receipts too but they usually either end up on my floorboard or on my dresser when I empty my pockets for the day.

I usually reset my tripometer each fillup and calculate the mileage based on that. Its not a hundred % accurate since I do alot of rounding. 9 gallons= 10 gallons. 11 gallons=10 gallons. My beater only holds 10 gallons so that made things easy.

I was just wanting to get the car to have reliable, accurate instrumentation.

Can you tell me how the speedometer changes work if you move to a different tire size. Say from a 16 to 18. How do people change that?

[GrandMasterKhan]

WHEEL size changes do not affect your speedo. However if you get the properly sized tire then you should have a minimal effect on your speedo.

http://www.miata.net/garage/tirecalc.html

For example.

215/50/R16 16" tire
215/35/R19 big pimpin 19"

your speedo would read 1.9% too slow. Marginal difference. 1mph

its when you get oddball tire sized when you throw off the speedo. Like say getting a low profile 215/35/17 series 17" tire. lol not only would it look stupid but your speedo reading 5mph too high @ 60mph.

[RodneyBur]

Ooops. I wasn't looking for info on tire size changes or anything like that. I was just wondering how they correct for the differences. Do you go to ford to get it changed though their software? Change the speedo gear on the differential? Change the sensor? Just wondering since there maybe another way for the mpg correction.

[scrupul0us]

Hes saying the difference is so minute that they dont bother... period

[Blackcoog]

The MPG calculation is in done in the dash cluster not in the ECU. You won't be able to change it unless you figure out how it is calculating it and modify the signal to the cluster. Mine always reads a bout 3-4mpg lower than it should with 19lb injectors. I'd assume larger injectors would really throw it off. It must be reading something to do with the fuel deleivered through the injectors (injector pulse witdth?) to calculate the mpg.

[jaged]

when i had my car tuned i think there was an option on the superchip software that would accound for a different wheel height. but dont quote me on that one

[ilovemycar]

In the SCT software there is a value you can change to "fix" an off speedo. It is the Tire Revolutions Per Mile. When you buy a tire from tirerack or someplace like that...look closely at the description and it will tell you how many revs per mile that specific tire does.

[TrackCat]

Most ECU's that I've worked on ask the tire size for accuracy. AEM, Autronic, Megasquirt, EFI Technology,SDS, and Motec.

[warmonger]

yes, you can adjust the revolutions/mile setting or you can change the final drive ratio if you want