Cadillac Owners Forum banner

1 - 19 of 19 Posts

·
Registered
1995 Sedan De Ville (4.9 L)
Joined
·
581 Posts
Discussion Starter #1
I have noticed that the MPG indicated on the dash board
is somewhat higher than the actual MPG value calculated
from the amont of fuel added and the trip meter.

When I had a 83 Eldorado, I had the same problem.
Why is that? Is this normal? Or does anybody have a car
indicating always lower MPG than actual??

Also, how does the car calculate the MPG? I assume that
she knows for how long injectors are open... But she
doesn't know actual fuel pressure (there is no sensor to
measure it, right?) and thus, she is not possible to accurately
guess the actual amount of fuel going thru an injector...

Just curious.

Thanks.
 

·
Banned
Cadillac
Joined
·
16,105 Posts
Maybe you have something there? My 4.9 says 18-22 or so in city and 24-35 avg. highway. Whereas my 5.0 Ford got 16-18 city and 26 highway. I would think these engines would be closer, and according to my Caddy computer it is higher. I've never worked it out manually for the Caddy assuming it was accurate. Maybe it is optimistic?
 

·
Registered
1995 Sedan De Ville (4.9 L)
Joined
·
581 Posts
Discussion Starter #3
Your MPG is really good! I am impressed. While I am on highway,
the dash shows pretty good numbers like 25-30 MPG, but as an
average, it is 16-19 MPG. I have never driven on highway all the
time until next refueling, so I don't know what kind of number I would
get.

Do you think it is because Canadian gasoline is higher in octane value?
It is very low in the US. I am from Japan and when you buy premium
(we call it HIGH-OC), most of them are 100 and just regular is 90-91
in Japan.
 

·
Banned
Cadillac
Joined
·
16,105 Posts
I've heard on the CBC National news that Canadian fuel is a higher octane, but I cannot believe it is that much of a difference because my engine knocked while using regular. I am happy with my car's mileage because we actually pay more here I believe. I would estimate that there are more small cars here than large ones, and I see more Toyota Echos every day! :crying: I do think (in Canada) the government is slowly trying to force people out of larger vehicles with initiatives such as Kyoto, etc. Fortunately Russia has doubts they will adopt it because they are a poor country and cannot pay the increase in fuel costs for cars, home heating, etc. In a colder climate like ours, they have no business messing with forcing environmental policies down our throats. I am glad Cretian left office today as Prime Minister, he really pushed for Kyoto.
 

·
Registered
Unicycle
Joined
·
11,009 Posts
100 said:
Your MPG is really good! I am impressed. While I am on highway,
the dash shows pretty good numbers like 25-30 MPG, but as an
average, it is 16-19 MPG. I have never driven on highway all the
time until next refueling, so I don't know what kind of number I would
get.

Do you think it is because Canadian gasoline is higher in octane value?
It is very low in the US. I am from Japan and when you buy premium
(we call it HIGH-OC), most of them are 100 and just regular is 90-91
in Japan.
Why does Japan have 100octane gas, if all their cars need regular gas here (87)?

USuALLY I get about 18mpg town and 28 on the highway....but with the winter, alot omre short cotwn/city driving, and shoorter trips, it is around 14-14.5..... all according by the dash computer, I'll figure it out sometime....
 

·
Registered
1995 Sedan De Ville (4.9 L)
Joined
·
581 Posts
Discussion Starter #6
Many Japanese cars sold in Japan REQUIRE high octane gasoline.
But most of them come with a knock sensor, so they run OK
with regular. But if you want power and torque indicated in the
catalogue, you must put high octane.

I have never bought a Japanese car here, so I am not sure how
it is here.

Well, but gasoline in Japan costs about 3 times more expensive
than that in the US!!! So it better be hyper super octane gasoline!
 

·
Registered
01 frontier , 89 Shelby CSX vnt
Joined
·
14,607 Posts
"Also, how does the car calculate the MPG?"

best i can figure is it knows how many gpm (lpm in canada) the fuel pump is running at , bounces that across figures on throttle position (would give injector duty cycle) and miles per hour

something like this , im pumping .5 liters per minute , injecotrs are at 25% capacity ,which means "x" ammount is being returned to the tank and im going 100 kmh (62 mph i think), so im useing what is left over which equates out at this moment and condition to 27 mpg , but 2 miles ago i was useing at a rate of 9 mpg when leadfoot was mergeing , so ill avaerge it out to "X" over the last 10 miles i was able to measure

the is the best i can figure , but i have no real info on this , just thought id give my idea
 

·
Registered
Joined
·
146 Posts
I was curious so I figured mine out by myself. I calculated 14 (mostly city driving), while my computer said 17.2. Also, it said 8.2 gallons used, but I was able to put 10.2 in (just stopping after the first click). I really wish someone would test theirs and compare it to the computer to see if all of them are off
 

·
Registered
01 frontier , 89 Shelby CSX vnt
Joined
·
14,607 Posts
mine is usuly spot on , but i reset the mpg at every fill up , otherwise its avaerageing total miles the car was driven on multiple tankloads ever since the previous reset . a tankload used only at highway speed will have a higer avaerage than a tankload used at city driving , but the monitior doesnt know any diffrent and avaegares all of them together

if you never reset the mpg montitor thing it will still be accurate , but you would have to figure your gas milage over that span of time and tankloads , i use microsofts excell with its graphing
 

·
Registered
1995 Sedan De Ville (4.9 L)
Joined
·
581 Posts
Discussion Starter #10
I do reset every time I put gasoline. But MPG is ALWAYS optimistic,
whereas the sum of amount of fuel left in the tank and amount of fuel
I add comes close to 20 gal (usually difference stays within 1 gal)...

Because my previous Cadillac did the same thing, I thought being
optimistic is one of the Cadillac's characteristics. I am surprised that
Stoneage_Caddiy's car doesn't do that!
 

·
Registered
2005 CTS-V, 1994 Infiniti Q45
Joined
·
7,658 Posts
Stoneage was right about how it figures it out..... Execpt its based on injector open time (super accurate)..... You can use the FUEL USED setting and that should be the best way to do it.... Just divide the mileage by the indication on the fuel used thing, assuming you reset it....!

They have to make these things mostly accurate, because if they dont most people would start complaining... However, I think a 1mpg variation is acceptable for the electronics.....
 

·
Registered
Joined
·
146 Posts
ok, just did another tankful. It said I used 9.5 gallons, but I was able to put in 12.3 gallons. It said I got 16.3 mpg, but I calculated it to just over 13. Mine is WAAAY off and Id like to know why!!!
 

·
Registered
2005 CTS-V, 1994 Infiniti Q45
Joined
·
7,658 Posts
Do you reset it every time??? Its possible some of your injectors are leaking, meaning they dont close all the way....
 

·
Registered
Joined
·
146 Posts
yep, I reset it every time I get gas
 

·
Registered
1995 Sedan De Ville (4.9 L)
Joined
·
581 Posts
Discussion Starter #16
So, as Stoneage_Caddy said the computer knows fuel pump's GPM
(or liter/km) as it is new and ASSUMES it stays the same (there is no
sensor to monitor actual pump's GPM, right?). Also it ASSUMES
injectors follow what the computer says to do (close and open).

Does it mean that if the actual MPG is off from the value that the
computer says, either the fuel pump's performance is getting lower or
the injectors are leaky???
 

·
Registered
2005 CTS-V, 1994 Infiniti Q45
Joined
·
7,658 Posts
The car has no way of knowing GPM on the fuel pump, nor would that be of any major importance......

When the car comes off the assembly line, and probably for the first 50k or so, the display is probably right on.... The computer has to ASSUME from the data that comes in, as it cannot detect a leaking injector (unless its super bad causing one bank to register more rich than the other.... o2 sensors).... I would assume that if the computers reading is off then the fuel pump, filter, or injectors are faulty.... However, unless the mileage is really bad, I wouldnt worry about it.... Thats normal wear, and I think unless its really bad to go and replace all the injectors would be a futile effort....
 

·
Registered
01 frontier , 89 Shelby CSX vnt
Joined
·
14,607 Posts
yah hes right , it just takes a reading on how long the no 1 injector is open , bounces that across speed

boy if that injecotr was messed up it would really thorw it off since it just multipys that injector times 8
 

·
Registered
1995 Sedan De Ville (4.9 L)
Joined
·
581 Posts
Discussion Starter #19
Yeah, you guys are probably right. Actually, I don't really worry about.
Only thing I worry about is the fuel pressure. It is 40 psi at idle that
is the lowest edge, I think. However, I don't experience any trouble
so far, so I think it is OK for now, unless it progressively goes down
(which is not happening, I think).
 
1 - 19 of 19 Posts
Top