[mattisere]

Hi all,

Weird one this time

So last week I put 20.18L of Diesel in my 330d (2011 e93) and got 175 miles out of it until it read 0 miles range.

I had reset the MPG gauge (and trip computer) once the fuel had been put in. Anyway, by my calculations that's 39.4 MPG (UK), but the MPG reading from the car was 32.3 MPG (UK).

Now I've heard of cars being optimistic with their reading before but never underrating them and to such a degree?!

Safe to say it was a huge relief to know that's what i'm actually getting rather than what the computer was reading.

Any ideas?? If it could be coded to adjust for a bad reading that would be ideal of course.

Cheers boys!

[MysticRob]

Originally Posted by mattisere

So last week I put 20.18L of Diesel in my 330d (2011 e93) and got 175 miles out of it until it read 0 miles range.

This part makes no sense to me. Why are you using, or even mentioning, the range to discuss calculated MPG?

Simply reset the trip computer and MPG average the next time you refill, then drive till next refill and use the actual miles driven and gallons pumped to calculate your driven MPG and compare that to what the car's MPG average says.

[jt335i]

Originally Posted by MysticRob

This part makes no sense to me. Why are you using, or even mentioning, the range to discuss calculated MPG?

Simply reset the trip computer and MPG average the next time you refill, then drive till next refill and use the actual miles driven and gallons pumped to calculate your driven MPG and compare that to what the car's MPG average says.

Exactly.
Also, the range will vary as the car's internal computer constantly recalculates it every time it is driven.

[mattisere]

Originally Posted by MysticRob

This part makes no sense to me. Why are you using, or even mentioning, the range to discuss calculated MPG?

Simply reset the trip computer and MPG average the next time you refill, then drive till next refill and use the actual miles driven and gallons pumped to calculate your driven MPG and compare that to what the car's MPG average says.

That's exactly what I did! Sorry if I over confused it, wanted to make sure ever basis for scruinty was covered. But yes, I did exactly as you just described.

Got 39.4mpg real world and the cars computer said over that time I got 32.3mpg?

[MysticRob]

Originally Posted by mattisere

That's exactly what I did! Sorry if I over confused it, wanted to make sure ever basis for scruinty was covered. But yes, I did exactly as you just described.

Got 39.4mpg real world and the cars computer said over that time I got 32.3mpg?

An ~20% improvement in real-world versus computed mileage is definitely odd.

My first thought is your wheels/tires may be a different combo than OEM sizes, or you have a different rear end gear ratio, different transmission, etc, than stock. Even worn versus new tires would only affect the calcs a couple percentage points, nowhere near enough to explain what you're seeing.

You sure you're mathing correctly? LOL

[Chappers 71]

There would have still been devils brew in the tank when you added your 20.18 litres. Even when the indicated range reads 0 there's still a fair amount of fuel left until it completely runs out. I once travelled 11 miles in my 335i with an indicated computer range of 0 miles
You'd have to drain the tank & fuel system & then add the 20 litres to get an accurate measurement.

[feuer]

Originally Posted by Chappers 71

There would have still been devils brew in the tank when you added your 20.18 litres. Even when the indicated range reads 0 there's still a fair amount of fuel left until it completely runs out. I once travelled 11 miles in my 335i with an indicated computer range of 0 miles
You'd have to drain the tank & fuel system & then add the 20 litres to get an accurate measurement.

Exactly! Let it ran out of fuel. Then test actual vs calculate MPG

[mattisere]

Originally Posted by MysticRob

An ~20% improvement in real-world versus computed mileage is definitely odd.

My first thought is your wheels/tires may be a different combo than OEM sizes, or you have a different rear end gear ratio, different transmission, etc, than stock. Even worn versus new tires would only affect the calcs a couple percentage points, nowhere near enough to explain what you're seeing.

You sure you're mathing correctly? LOL

Everything is standard aside from the wheels which are 1" bigger than standard at 19", but like you said it wouldn't account for anywhere near to that percentage!

Originally Posted by Chappers 71

There would have still been devils brew in the tank when you added your 20.18 litres. Even when the indicated range reads 0 there's still a fair amount of fuel left until it completely runs out. I once travelled 11 miles in my 335i with an indicated computer range of 0 miles
You'd have to drain the tank & fuel system & then add the 20 litres to get an accurate measurement.

When filling the first time it was also showing 0 miles and this would still not account for such a huge percentage difference!

So, any other ideas?

[MysticRob]

Originally Posted by mattisere

Everything is standard aside from the wheels which are 1" bigger than standard at 19", but like you said it wouldn't account for anywhere near to that percentage!

Woah now, I'm talking differences in remaining tread on OEM-spec tire differences, not entire wheel/tire size differences. What width and aspect ratio are you running on the "different" tire combo you have now? That might be the issue right there.

Originally Posted by mattisere

When filling the first time it was also showing 0 miles and this would still not account for such a huge percentage difference!

So, any other ideas?

Just kill that entire line and train of thought before anyone else gets confused.

[mattisere]

Originally Posted by MysticRob

Woah now, I'm talking differences in remaining tread on OEM-spec tire differences, not entire wheel/tire size differences. What aspect ratio are you running on the "different" combo you have now? That might be the issue right there.



Just kill that entire line and train of thought before anyone else gets confused.

Yes, lets just make clear it was like-for-like, both time measured on 0 miles

The rear tyres are 275/30/19, fronts 255/35/19.
Previously 255/30/18, 225/35/18.
(All on the same new tyres)

Using an online calculator it said it would make barely 1mph difference so it shouldn't affect the mpg reading at all right?

[MysticRob]

Originally Posted by mattisere

Yes, lets just make clear it was like-for-like, both time measured on 0 miles

The rear tyres are 275/30/19, fronts 255/35/19.
Previously 255/30/18, 225/35/18.
(All on the same new tyres)

Using an online calculator it said it would make barely 1mph difference so it shouldn't affect the mpg reading at all right?

I'm seeing something different.
https://www.discounttiredirect.com/l...ize-calculator
You're seeing a 1 MPH difference at what speed (not that it matters)?
What matters is your new setup shows a 4 MPH difference @ 65 MPH on the site I linked to. That's an addition of 4.65" distance traveled per tire revolution, or 49 less tire revolutions to cover a mile than the computer thinks it's taking. Over the 1000s of rotations, the 100s of miles you're driving on a tank, the computer thinks you're covering less ground than you actually are. Hence your real-world calcs are better than what the computer thinks you're getting.

I don't really want to do the calcs to determine how much extra distance you're actually travelling versus what the computer thinks you're travelling, but it's possible that 20% difference in readings can be found right there. Any other 'non-OEM' changes you haven't thought about?

[mattisere]

Originally Posted by MysticRob

I'm seeing something different.
https://www.discounttiredirect.com/l...ize-calculator
You're seeing a 1 MPH difference at what speed?
Your new setup shows a 4 MPH difference @ 65 MPH on the site I linked to, or an increase of 4.65" distance traveled per tire revolution. Over the 1000s of rotations, the 100s of miles you're driving on a tank, the computer thinks you're covering less ground than you actually are.

I don't really want to do the calcs to determine how much extra distance you're actually travelling versus what the computer thinks you're travelling, but it's possible that 20% difference in readings can be found right there. Any other 'non-OEM' changes you haven't thought about?

You're right, I've been mislead!

Everything else is stock, not long had the car. My query was answered a lot quicker than I expected! Thank you

Now I need to find a guide on coding the new wheel size to the car. Could you point me in the right direction? I've had a little search and couldn't find anything in depth enough. (I have NCS Expert, INPA etc)

[MysticRob]

Originally Posted by mattisere

You're right, I've been mislead!

Everything else is stock, not long had the car. My query was answered a lot quicker than I expected! Thank you

Now I need to find a guide on coding the new wheel size to the car. Could you point me in the right direction? I've had a little search and couldn't find anything in depth enough. (I have NCS Expert, INPA etc)

Way above my knowledge level, but hopefully someone chimes in.
And technically it's not the wheel size that matters, but the entire diameter of the wheel/tire together.
Example: Decrease the aspect ratio or size from those 275 rear tires (not that you can though) and you might get close to the stock diameter, which would mean the computer doesn't need tweaked.