JTD MPG Computer… is it lying to me??? - Alfa Romeo Forum
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(Post Link) post #1 of 6 Old 22-02-07 Thread Starter
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JTD MPG Computer… is it lying to me???

Hi all,

I have a 2004 1.9 JTD 16V 147, on 37,000 miles.

Had it a couple of months now, and I just can’t seem to get any more than 39 mpg.

I don’t drive particularly heavy footed, and most of my commute is dual carriageway and M1. I know the stated figure for my engine is 47 mpg, but I was expecting to be able to achieve at least in the region of 42-43 mpg but so far nothing more than 39.7.

Is this possibly a sign of something at fault in the engine, or is my computer just lying to me?

Also can the mpg figure on the computer be reset? And over how many miles is the mpg calculated?

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Re: JTD MPG Computer… is it lying to me???

I find that my trip computer isnt very accurate when it comes to MPG, the best way is to fill up your car, then drive till its empty, record how many miles you did and how many litres you put in when you fill it up, then go to http://www.geocities.com/mpgcalc/ to work out your mpg!
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Re: JTD MPG Computer… is it lying to me???

I haven't worked out whether my computer is bang on accurate, but I used to work out my mpg manually with big hopes when I used to drive a Peugeot 306 diesel regularly.

Judging by the average mpg I get by the computer and number of miles I get out of a tank(not alot!), I think the computer is about right. However, the only entry which is clearly way off is the average velocity.

I'm sure this concern over computer accuracy has been discussed before - do a search.

The way to reset it is to hold down the 'change mode'/RES(?) button on the end of the stoke for a few seconds until the display clears.
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Re: JTD MPG Computer… is it lying to me???

Just filled up the GT, 555 miles on 48.23 litres, simple arithmatic makes it 52.3 mpg. The computer shows 54.6 on 1300 miles, which is about 3 too much. I am still trying to verify the mileage, I think it is about 3% too high on my 17's.
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Re: JTD MPG Computer… is it lying to me???

Currently averaging about 50mpg out of a re-mapped 2.4 156 driven quite gently (honestly, 90% of the time anwyay), no city or motorway driving but a mixture of short runs and a few 20-30 mile runs) so if the 1.9 is really only making 39mpg you either need to take off the lead boots or get it checked imo.

My trip computer says i average 52.3 MPG but fuel measurements (fill tank, drive, re-fill at same pump) give me the 50mpg figure.

At a total guess from just looking at the clock and remembering how it was driven the MPG seems to be averaged over about the last hundred miles. A spell of hard driving can make my car drink diesel and it takes about 100 miles more of easier driving for the mpg figure to rise back up.

Average velocity seems to me to be quite accurate on the 156, you would be surprised how low it can be at times even when you think you have been driving quite fast. Mine sits at about 28 normally.

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Last edited by okenora; 08-03-07 at 00:29.
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(Post Link) post #6 of 6 Old 08-03-07 Thread Starter
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Re: JTD MPG Computer… is it lying to me???

Forgot about this thread.

Anyway, appears that I wasn’t resetting my TripB the correct way.

Finally sorted it, and now getting 48-49 MPG on a decent run.

Last 250 miles averaged 46.5, and that was with a fair bit of city/rush hour driving.
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