MG-Rover.org Forums banner
1 - 11 of 11 Posts

·
Registered
Joined
·
34 Posts
Discussion Starter · #1 ·
A question for some of you technical folks out there. How does the in car computer calculate the MPG ?
I am guessing it looks at the throttle opening and the speed or distance traveled and makes a calculation from there.

I do have a reason for asking other that just curosity but I'll come to that later.

Steve.
aka - schwantzz
 

·
Registered
Joined
·
1,872 Posts
I reckon it uses distance as measured by counting the pulses from the wheel and fuel level as sensed from fuel sender. MPG changes relatively slowly so I am assuming it counst in chunks - the same way the odometer counts up ... all assumptions though.
 
G

·
Fuel used is often calculated by trip computers on the basis of how long the injectors open up and how often.

Therefore, if you know the flow rate in litrs per minute, the amount of time the injectors have been open in the last minute and the number of injectors you have you can work out how much fuel has been ejected.

Speedo and odometer (usually operated electronically) give you distance.

Fuel used, time and distance give you all you need.
 

·
Registered
Joined
·
251 Posts
I know that the figures are corrupted if you have a Powerbox attached to diesel engines! Does this help with your assumptions?
 

·
Registered
Joined
·
38 Posts
On my 75 V6 Conn SE the computer mpg has always shown between 4 and 5 mpg more than my very accurate checks of several fillings, tank full to tank full, usually covering about a thousand miles.
I use 4.546 litres to the gallon for conversion.
My speedo reads 8% fast when compared to the satnav reading and I feel that this difference may contribute to the apparent error, although the odometer reading seems to be fairly accurate.
The original tyres are 225/45 ZR17.

How accurate is the computer intended to be? As a toy it is great fun to use!
 

·
Registered
Joined
·
364 Posts
On my 75 V6 Conn SE the computer mpg has always shown between 4 and 5 mpg more than my very accurate checks of several fillings, tank full to tank full, usually covering about a thousand miles.
I use 4.546 litres to the gallon for conversion.
My speedo reads 8% fast when compared to the satnav reading and I feel that this difference may contribute to the apparent error, although the odometer reading seems to be fairly accurate.
Explain a bit more?
 

·
Registered
Joined
·
34 Posts
Discussion Starter · #7 ·
aga said:
I know that the figures are corrupted if you have a Powerbox attached to diesel engines! Does this help with your assumptions?
This is kind of where I am going with this one. I've read lots of posts about how the car was transformed by a powerbox and yet returned the same or better mpg.
 

·
Registered
Joined
·
38 Posts
JohnA

I was trying to state a few facts and hoped that someone could offer some explanations to me!
Basically my computer always shows 4-5 mpg more than I actually achieve.
 

·
Registered
Joined
·
2,322 Posts
schwantzz said:
This is kind of where I am going with this one. I've read lots of posts about how the car was transformed by a powerbox and yet returned the same or better mpg.
Well my MPG with Ron's power box was always 10MPG optimistic but the actual fuel consumption (calculated brim to brim) stayed within 1 or 2 MPG of that without the box.
 
G

·
CFS75 said:
JohnA

I was trying to state a few facts and hoped that someone could offer some explanations to me!
Basically my computer always shows 4-5 mpg more than I actually achieve.
If the speedo over reads by 8% then there is a chance that the odometer (mileometer) will over read by the same amount.

An 8% over read on 40mpg would cause the mpg to be about 3.2 mpg optimistic.
 

·
Registered
mg_zt
Joined
·
307 Posts
You can work out your true MPG.

Fill your car up with petrol, divide the figure by 4.54 (to convert to gallons) and then divide your miles on one tank by the figure you come up wiht.

i.e. i get 370 miles per tank (about 60 litres).

60/4.54 = 13.21

370/13.21= 28MPG

You need to watch your comparrisions as my computer says i get 30MPG when it is nearer 28!
 
1 - 11 of 11 Posts
Top