Leaning out mixture is bad isn't it? - delboy100
Continuing my latest obscession with fuel consumption....

On a recent long trip with a number of friends driving the same cars (BMW E32 735i) we did a consumption comparison. We ran at exactly the same speed (in convoy) at 70 mph on cruise-control for over 100 miles. Two guys claimed 30 mpg on the run there whilst I was down at 25 mpg (on our on-board computers). On the run back (alone!) one of them claimed 36 mpg! This discrepancy (ie 11 mpg difference!) upset me somewhat and I've been trying to find explanations for it.

Firstly, a little more to put you in the picture...

His car has 163000 miles on the clock whereas mine has a mere 35K miles and has always been fastidiously Main-Dealer serviced (formerly belonged to Sir Frank Williams).

The car needed it's MOT the other day so I got them to check the mixture at the sametime...It's bang on correct. We did find a deficiency of ATF fluid due to a small leak from a cooling hose and I must admit I had noticed the gearchange was playing up a bit (now rectified).

One of the two other cars claiming 30mpg on the run out was carrying a completely full tank (like me)...I don't know about the chap who latered claimed 36mpg? The guy with the car claiming 30mpg both ways has no A/C fitted but the guy claiming 36 mpg does ...though neither he nor I had it switched on during the drive...Nevertheless, the system itself isn't exactly a light bit of kit!

Additionally, I have calculated that the rolling diameter of his tyres on his aftermarket 17" Alloys is LESS than mine so that would have put his gearing lower and worked against him in the 'consumption stakes'!

Nobody else that I'm in contact with with one of these cars claims 36 mpg (at an average of 75 mph...that's what he did on the way home apparently). I can explain some of my '25mpg at 70 mph) on the basis of the above...but not a difference of 11 mpg when he was going an average of 5 mph faster!

Hence, this brings me too my conclusion and question (you'll be relieved by now...LOL)

...There can only be 4 explanations:

1) There's a fault on the calibration of his computer

2) The mudflaps on my car (which he doesn't have) are costing me a huge amount in lost fuel consumption...LOL!

2)He's telling porkies

3)He's leaned his mixture out . (I do know that he has repeatedly sent secondhand AFM's back to his supplier saying they wouldn't 'adjust' properly.)

Leaning an engine's mixture out too far is a really bad thing isn't it?...Increased bore wear etc?

I'd be really grateful for some input on this and any other ideas.

Many thanks and sorry for the long post.


Leaning out mixture is bad isn't it? - J Bonington Jagworth
"formerly belonged to Sir Frank Williams"

Er, are you sure it's standard? How fast can it go...? :-)

If it really is the same as the others, I would check the fuel consumption properly before fiddling with the AFM system - I imagine it's pretty carefully set up, while I doubt that the fuel consumption meter is more than a toy.

Mudflaps may contribute more than you think. The car's aerodynamics were worked out without them.

35k is barely run-in for an engine like that. Your friends' ones are undoubtedly looser.
Leaning out mixture is bad isn't it? - delboy100
Yep...but the car dates from after his tragic crash so I don't think he'd have been behind the wheel himself.Also judging by the mileage and the V5 it was just a 'company car'...rarely did more than 2-3K miles per year...sometimes much less!

The car looks and drives like completely standard. The only thing non-standard as far as I can see was the fitting of later R132a A/C system so that it complies with present gas standards.

You're right though I'm not going to fiddle with the AFM as it seems properly set up as it is.

I'm just running through some of the obvious stuff now like tyre pressures etc Generally, I've always found BMW's recommendations too high leading me to wear out the central portion of tyres too fast as it's usually only me in the car. But maybe I've just been going a little too low.

Once I've got everything spot on I'm going to run a proper test and see how it compares with the 'book figures' for the car...I still maintain 36 mpg is going to be unrealistic...but we'll see.


Leaning out mixture is bad isn't it? - Cyd
With such a low annual mileage early in its life I'd be tempted to suspect it's more than just a bit coked up. That might then explain the, apparently, high fuel consumption.
Leaning out mixture is bad isn't it? - kithmo
There are other factors that you may want to consider. 1. Were you the leading car in the convoy ?. The trailing cars are helped along by the vortex created behind the lead car (what's known as slipstreaming in Nascar racing and caravaning circles). 2. The type of tyres can make a difference. There are tyres with less rolling resistance for economy and there are tyres with more grip (and more rolling resistance) for performance. 3. You say you didn't use the Air-con, on some vehicles (with Climate control) it works automatically in the background without you knowing it's on. 4. If you didn't use the Air-con did you have the windows or sunroof open, these contribute to the drag coefficient of the vehicle. 5. Do you have a roof rack (more drag). 6. Is your tracking OK. 7. Did you both have the same loadings. 8. were your tyre pressures correct for the loading. Finally regarding the claim of more mpg, by the other guy, on the return journey, the wind may have been in his favour. Each one of these factor mentioned only has a small effect on mpg but combine them and it could add up. Other things that could affect mpg to do with tuning are, air flow through the filter (different brands differ), Exhaust bore (smaller diameter exhausts give less performance but the increase in back pressure gives a little better economy) and even the type of fuel (Shell Optimax arguably gives better economy).
Leaning out mixture is bad isn't it? - delboy100
Thanks for such a detailed reply Kith. Taking the points you raised:

1. I was the middle car of three, then the third of 4 for the whole journey. Indeed, the guy with the '36mpg claim on his solo return' was the lead car on the whole journey there.

2. Tyre type...well I was running Michelin Pilots which were narrower than his Uniroyal Wets (would the 'Wets' be a stickier compound?)...I would have thought both these would have been in my favour?

3. The Climate Control thing crossed my mind too before we set off so I was careful to just have the 'AUTO CC' switched off.

4. I know he had his sunroof on 'Tilt' for the whole journey...I never opened mine but I did open the driver's window occasionally whilst having a fag!

5. None of us had roof racks

6. Tracking is bang on...had KDS Laser set up at BMW Maindealer (at some expense!) about 500 miles ago.

7. Loadings...Well, I had basically no luggage and weigh about 14 stone...at least 4 stone less than the other guy...and he had 'stuff' in his boot.

8. Tyre pressures...you may have something there as I've tended to run my cars about 4 psi below BMW's recommended...But after over 20 years of driving BMWs I've always found their recommendations too high for a single occupant without luggage and have worn the central portion of rear tyres excessively rapidly by following their recommendations...which are written to apply to either 4 occupants + small boot load (as a lower figure) and 5 occupants + large boot load (as the upper figure).
Nevertheless, I'm going to try increasing my pressures for a while back up to their recommendations.

On the return journey we both did essentially the same journey of approx 150 miles (bar the last 16 miles) at the same time of day. We were only 'alone' because we lost each other but could only have been minutes apart...so If the wind was in his favour it should have been for me too...yet I got the same approx 25mpg on the return that I got on the way down.

I have a new BMW air filter in place and we both have stock exhaust systems.

I don't know what fuel he was running but I have run a few tankfuls of Optimax through my car and I have to say I have not noticed any difference. The M30 six cylinder BMW engine isn't such that it would benefit from the increased octane level of Optimax but I have used to it as I feel the benefit may be in the quality of the detergents etc in the fuel.

Tomorrow, just for reassurance, I'm going to inspect my plugs, dizzy cap and rotor arm etc..but I've driven lots of cars with this particular engine and it sounds and feels better than any previous ones. I got the car a year ago with 26K on the clock and I've put 9K on it this year...all easy motorway/A Road miles so I doubt the 'coked up' issue raised above...the car never sits in city traffic.

I agree that a lot of these things can add up to a difference but we're talking an 11 mpg difference...that's nearly a 33% difference?

What I'm going to is to look up the quoted manufacturers mpg by BMW (even though their figures are usually 'optimistic' in my experience) and I'll post the answer. I just still find it difficult to believe a car this heavy with a 3.5 litre engine does 36 mpg at 75 mph...

I'll keep ya posted...

Leaning out mixture is bad isn't it? - JamesH
I agree with JBJ that it could just be that the fuel computer is out.

Have you tried measuring the mpg another method (e.g. HJs FAQ No 37)?

Leaning out mixture is bad isn't it? - delboy100
Thanks James

I think I should be able to do a 'Mini HJ Test' tomorrow. I'll be doing an approx 120 mile round trip tomorrow on what are usually empty motorways so I should be able to run on cruise control at a strict 70 mph as much as possible.

I'll check out the various things I mentioned above first and set the tyres to manufacturer's pressures, windows shut, no CC etc.

I'll fill with 'Optimax' and see what result I get (and then compare this with the car's OBC figure too...I've looked up how this can be recalibrated if needs be). I'm also aware of the fact that my tyre rolling diameter is -1.3% of the stock tyre diameter so I'll be able to calculate this into the 'distance part of the equation'.

Here are the figures BMW quote in their handbook (unfortnately, they never quote for 70 mph which is what I'm really interested in):

56 mph = 34.9 IMP mpg

90 mph = 29.4 IMP mpg

Urban = 17.7 IMP mpg

"* Due to the issue data fuel consumption figures may differ from the official ones"........What does that mean? Sounds like:..." This is what we're publishing but you ain't gonna get that!"...to me! LOL

I'll let you know


Leaning out mixture is bad isn't it? - delboy100
I think we may have resolved the difference in fuel consumption on my BMW 735i compared with my friend's. It transpires that the 1989 cars (like mine) carry a different final drive ratio to earlier cars!

We did a 'revs at 70 mph comparison'...whilst all the people with 1989 cars reported bang on 2500 mph at 70 mph...pre-89 cars were down near 2000 rpm!

To me this easily explains the 5 mpg difference at 70 mpg (ie. 25 mpg for the post '89 cars and 30 mpg for the earlier cars). It also confirms to me that the individual supposedly reporting 36 mpg at 75 mph is telling porkies or has their engine's mixture incorrectly set.

I can now sleep easy....LOL!
Leaning out mixture is bad isn't it? - J Bonington Jagworth
"..a car this heavy.."

Actually, weight makes little difference to fuel consumption at a steady speed. It has some bearing on rolling resistance, but that's about it. To keep a car at a fixed speed on a level road requires remarkably little power, as the main obstacle is just wind resistance, which increases with the square of your speed, so even weedy little cars can manage 70 or so, but even really big, hairy ones can rarely crack 200. 36 mpg at 75 sounds quite reasonable to me...
Leaning out mixture is bad isn't it? - Peter D
You all need to check you true distance travelled agains the computer distance at the MPG relies on the tyre diameter for calculating distance travelled. Use motorway km markers ( 10 of them ) or a GPS and a straight section on motorway some where.

Regards Peter
Leaning out mixture is bad isn't it? - Dwight Van Driver
Peter D

Have they changed the distances on Motorway marker Posts.
They used to be at half furlong i.e. 16 to the mile?

Leaning out mixture is bad isn't it? - Dave_TD
I believe so, the numbers on the marker posts indicate how many km from the start of the motorway - 208.6, 208.7 etc. Except on the M6, where they indicate the distance from the London end of the M1!
Leaning out mixture is bad isn't it? - Dan J
The only real way of checking fuel consumption is to brim the tank and record mileage/fuel used on next fill up and repeat several times.

36mpg out of a 3.5 V8 7 series? I think either your mate's trip computer got bored with giving the correct reading or he's yanking your plank. I'm lucky if I can get that out of my 2.2 Vectra.

The old Euromix fuel figures are highly inaccurate given when did you last drive at a constant 56 or 75 mph anywhere (no hills/change in wind resistance etc!). Even they don't claim the car would produce this level of consumption of this speed.

If his BM genuinely is pulling mid 30 mpgs I think BMW themselves would probably be interested in this miracle car - even the 728 averages well under this!

Dan J

Value my car