Trip Computer Miscalculations?

[dogbiscuit]

Having made some long motorway journeys last week I noticed that after resetting Trip B Computer at the start of one journey I managed to cover 124 miles in 2 hours (as shown by the trip computer) but the average vehicle speed was only showing 46 mph. Surely it should have been closer to 62 mph or am I missing something. Anyone else noticed inaccuracies in the trip computer?

[guest598]

Yes I agree I think the whole trip computor system is very hit and miss

[bucksfizz]

On a USA website for the 2009 Fit[Jazz] I found mention of a Honda Service Bulletin 09-086,titled 'Information Display shows higher MPG than actual' which includes the following under the heading of' Probable Cause'-'The PGM-F1 software bases its fuel calculations on an incorrect data set'
 The  Operation number for the corrective action is 125517  and the time allocated is 0.3 hour.
   Has anyone in the Uk with a GE jazz had their software updated in accordance with this bulletin?

[dogbiscuit]

So it looks like Honda recognizes there is a problem with their computer data. Why haven't Honda done anything about this as the bulletin is dated November 2009 and my car was manufactured after that. Perhaps Honda is happy to have you believe you are getting better MPG than you actually are.

[guest2213]

I bought my Jazz last September from Ruislip Honda. It was a demo model and generally it's OK but it does have a problem with the trip computer/MPG readouts and now its seems that these are generic to the design. The MPG calculations are hopelessly wrong and the trip computer is downright misleading and almost caused me to run out of fuel last March.

I have come to consider this gismo as totally unreliable and it causes me to wonder whether I would have bought the car in the first place if I'd have known that it's fuel consumption is no better than the 306 that preceeded it. When I did the test drive I was getting what appeared to be 53mpg from the machine and this was a major factor in the decision to buy the car. Although the car is OK in many respects I can't help but feel that another car from a different manufacturer would have been a better bet.

Regards

[Geoffers]

My Jazz always reads 3 or 4 mpg greater than the actual. Last 8 or 9 tankfuls have all been 51+mpg with the highest being 53.7!
There are three options with the trip computer :
1) miles this trip
2) miles since last reset (multiple trips)
3) total mileage
I forget which is a, b, or c, but it should be obvious when looking at them!

Bog standard 1.4 ES.

[guest2213]

Yes I can get 53-54mpg on the gismo but this tends to work out as 48 or so when actually doing the arithmetic ie (miles completed/litres used)/.22

Personally I have come to the conclusion that the gearbox ratios are too close together, either that or it needs another gear for the higher speeds.

The range calculator is horrendous. Last March I started a journey at Minehead (190 miles from home) with the gismo telling me that I had a range of 300 miles or so; 180 miles later I was being told that I had one mile left but the average mpg screen was saying that the journey had be done at 55mpg! Now I just fill up every 300 miles or so (measured by the odometer) and ignore the damned thing altogether.

As good as the car is, and it is very good indeed, the fuel economy is mediocre for a modern 1.4l car. I'm sure other manufacturers do better.

Regards

[John Ratsey]

I have a suspicion that the computer MPG reads high by design so that users feeling that they are getting closer to the advertised MPG. If the error were caused by data inaccuracies then some users would be complaining that the computer MPG was lower than actual.

My other gripe with the computer is that the reported miles remaining before the tank is empty is credible until the gauge gets towards empty. Then, I think, the computer switches over from calculating based on the fuel consumption from a fulll tank to calculating the miles remaing until the gauge reaches zero. At my last refill the computer told me that the fuel would run out in 13 miles. Yet the fuel I could squeeze into the tank was 6.5 litres less than the claimed tank capacity. That suggests there was at least a futher 50 miles to go to empty.

Why the computer is so inaccurate given Honda's normal attention to detail is one of those mysteries.

John

[Geoffers]

Yes I can get 53-54mpg on the gismo but this tends to work out as 48 or so when actually doing the arithmetic ie (miles completed/litres used)/.22

Those figures I quoted are actuals, not from the trip computer!

[culzean]

I seem to have the opposite problem on my GD 1.4 SE (85K) - my trip normally reads around 54 - 55 mpg but over the last 4 tankfuls I have actually calculated an average of 60 to 62 mpg.  I have to add that this is using V Power, and has been since the weather has warmed up. But the truth is I am not really trying to get this kind of mpg figure, ie I don't go everywhere at 40mph and accelerate reallllly slowly!!  The last tankful included a motorway trip of about 40 miles doing 70mph+

Maybe my driving style (ie looking ahead, coasting downhill, coasting up to islands and 'slotting in' on islands without stopping if possible) is so ingrained that I don't realise I'm doing it any more. I have Michelin energy saver tyres fitted, but they are only inflated to standard pressure.

I have to say that I commute about 60 miles round trip every day, mainly country roads and little or no town driving.   I do keep up to 50 or 60 mph speed limits.

I am going to drive really carefully on the next tank-full and see if I can break 65mpg!

[Geoffers]

What's the numbers you are using to convert litres to mpg culzean? Perhaps that's where the problem lies!

[guest2040]

I have used the brim to brim method several times and i find the trip computer to
 give readings 10% above what is correct.

[eljuero]

Mine is too optimistic for about 5%.

[culzean]

What's the numbers you are using to convert litres to mpg culzean? Perhaps that's where the problem lies!

4.546 litres to a UK gallon - no mistake there methinks! I did say I am using V power and mpg is not so good in cooler weather. I also know when my tank is full to the brim. Driving style, fuel octane rating and weather have an influence on consumption.  I am lucky that on my journey to work I can go many miles without stopping and starting - these are the thief of fuel. I was roundly criticised on this forum not so long ago for coasting down slopes - I do this naturally without even thinking about it any more - from trials have found it at least as economical (and easier on the manual transmission) than leaving it in gear.  Most auto cars 'coast' down hills anyway, which is why brake pads on auto cars wear out so quickly.

[Geoffers]

Thought it was worth checking the obvious first culzean. It's often something very basic that is the cause, but not in this case!