Gas mileage question E30 vs E34

[StuntDrivin4U]

Hey guys and gals, I was the proud owner of a E30 that I sold a few years back and would really like to get a good commuter car in a manual transmission with inline 6. Now with a wife and kids I need a little more room than an E30 and would like to know what you're getting in your rides?

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[bblazen]

A M50 E34 would get better mileage than a M20 E30.

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[Siebenthal]

I have a 525it and I get 16 - 18 city

22-25 highway.

[Layne]

A M50 E34 would get better mileage than a M20 E30.

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Is the M20 that bad? I only ever had an M10 E30, and it got mid 30's. 25-26 Hwy is going to be the very best from an E34.

[bblazen]

Is the M20 that bad? I only ever had an M10 E30, and it got mid 30's. 25-26 Hwy is going to be the very best from an E34.It's not bad just SOHC vs DOHC one is definitely more efficient.

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[moroza]

No US-spec E34 will do much over 25mpg highway. An M50 manual with skinny and properly-inflated tires *might* touch 30mpg if you keep it below 60mph. My 544 gets 15-18 city, 22-25 highway, and most tanks end up right around 20mpg, running premium.

Of course, that depends on what you consider "highway". My OBC (seems accurate over 7 years' of use) shows ~24mpg at 70, ~20 at 100, ~26 at 50, and ~33 at 35.

[KVF]

/\
Just tallied up my first full month of driving a properly timed and tuned 544 and I’m getting the same.

My e30 325is did very similar in city driving 15-17 and was a bit less efficient on highway 19-23.


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[Mihnea"e34]

https://www.spritmonitor.de/en/overview/6-BMW/38-5er.html?fueltype=2&constyear_s=1988&constyear_e=1995&exactmodel=525&powerunit=2
this is a good start up point. A Europe spec 525i will do ~27highway 21 city

In US, LPG conversions are a thing? I have a 520i that will do double the distance with the same money, using LPG on a click of a button away. The only drawback is that it will take all the room in the spare tyre compartiment.

[moroza]

In US, LPG conversions are a thing? I have a 520i that will do double the distance with the same money, using LPG on a click of a button away. The only drawback is that it will take all the room in the spare tyre compartiment.

Not a big thing here, but it is done sometimes for older off-road trucks, because they have plenty of space for the tank and they see angles steep enough that carburetors might not work correctly. Sometimes they gain a little economy, sometimes they lose a little, but the fuel itself is much cheaper. There were a few ~2000 Honda Civics sold with a LPG kit, and I've seen some LPG government vehicles that never leave their city, but for normal cars it's quite rare. I wonder how the 518g did it?

[BleedsBlue]

If you're asking about an E34, I assume you aren't interested in the far more reliable, efficient, and boring commuter offerings from Japan? The word 'commuter' throws me off; I've been daily driving '90s BMWs for 10 years, but never had a true 5 days a week, 20+mile commute, or I would most likely have picked up something more practical :D

If you want a BMW for it, though, I get it, and more power to you. Just don't expect much efficiency from these heavy old cars.

I had a fun stint in an F30 320i with full Sport Pack, and had trouble getting below 34mpg combined (average speed of 35mpg).

[ross1]

Is the M20 that bad? I only ever had an M10 E30, and it got mid 30's. 25-26 Hwy is going to be the very best from an E34.
No. I've had M20 powered E30s and they all got better fuel economy than any e34 I've owned.

[StuntDrivin4U]

If you're asking about an E34, I assume you aren't interested in the far more reliable, efficient, and boring commuter offerings from Japan? The word 'commuter' throws me off; I've been daily driving '90s BMWs for 10 years, but never had a true 5 days a week, 20+mile commute, or I would most likely have picked up something more practical :D

If you want a BMW for it, though, I get it, and more power to you. Just don't expect much efficiency from these heavy old cars.

I had a fun stint in an F30 320i with full Sport Pack, and had trouble getting below 34mpg combined (average speed of 35mpg).I only say 'commuter' compared to both my V8 car and truck, but I catch your drift

[Dash01]

FWIW, my '89 535ia gets ~34 mpg cruising on mostly level to gently rolling I-5 freeway at ~75 mph, based on multiple tests in both directions. I don't know why, as my other E34 535ia only got about 23-25 mpg on the same road and conditions. I keep tire pressure at ~38 lbs. and installed Bosch/Mustang injectors with 4 hole pintles, which improves idle, throttle response, and engine smoothness.

[ross1]

FWIW, my '89 535ia gets ~34 mpg cruising on mostly level to gently rolling I-5 freeway at ~75 mph, based on multiple tests in both directions. I don't know why, as my other E34 535ia only got about 23-25 mpg on the same road and conditions. I keep tire pressure at ~38 lbs. and installed Bosch/Mustang injectors with 4 hole pintles, which improves idle, throttle response, and engine smoothness.
That is remarkable, to say the least. This has to be the highest reported MPG, by a wide margin, ever for this combination. I suspect some error.
I have never bothered to carefully calculate the MPG for any of my M30 cars as I knew it would be disappointing but mid teens in town and low/mid twenties with a steady foot on the highway is my experience.
Drafting trucks in the slow lane once got me 27mpg(obc indicated)in a 540/6.

[moroza]

34mpg? No it doesn't.

[Dash01]

I didn't believe it, either, having driven the exact same route under similar conditions for 25 years in BMW E24, E28, E34, and E36 cars, all but the E36 being M30 engines with automatic transmissions, except for the E28 535i 5 speed which regularly got ~28-29 mpg. My other E34 535ia got ~23-23 mpg on that stretch, for years. The I-5 section has posted 70 mph speed limit between the Snohomish County line and exit 206 at Marysville, WA, where the speed limit goes down to 60 mph. It's rolling to flat, but starts and ends at about the same altitude. A completely flat, straight section of highway would be best, in calm winds and low/no traffic, but that section of I-5 is commonly used by me, so that's the story here.

None of this was done drafting, but rather tested both north- and south-bound on the same day with similar moderate traffic (no stop and crawl, no tapping brakes--just reasonably smooth traffic flow), calm wind and weather. I-5 at the Stillaguamish River has a calibrated 5 mile section to check odometers, and mine have been quite accurate, i.e., after 5 miles always spot on. I've cross-checked my fuel consumption based on filling the gas tank right to the top each time, using the pump receipt gallonage figures, and noting the miles on the OBC trip odometer, which is corroborated by both the I-5 mile marker calibration and onboard GPS. GPS corroborates the speedo, which is fairly accurate but a tad high. Using an HP-12c financial calculator, I divide the odo mileage by the gallons pumped to full each time. My son (military pilot who owned the aforementioned E28) did not believe me, as he never got more than ~25 mpg in his E28 until he saw for himself: September 13 my wife drove the 3 of us to Seattle, she driving the 70 mph speed limit both ways: 34.6 mpg. The next day I drove him to Seattle with her aboard, driving 10% over the posted speed limit both ways: 33.2 mpg. Since then, she and I have driven other highways in moderate to light traffic (no drafting, no tailwind) with similar results.

My background includes flight test as a military aviator, long term member of Experimental Aircraft Association, member of Ecomodders.com fuel efficiency website (highly recommended, btw), and decades of experience with planes, cars, and motorcycles. I have nothing to sell here.

I've specifically used all these tricks in my 1995 E36 325is, which gets about 28 mpg on that route and speed. It has less frontal area and weight than the '89 535ia, but somehow the E34 gets a lot better economy. Same story with my Porsche 944 Turbo 5 speed, one of the slipperiest cars around with small frontal area and low drag coefficient, yet only gets ~25 mpg.

My best guess as to why I'm getting such good mileage is a combination of better fuel injectors; recent replacement of timing chain, guide, and tensioner; low rolling resistance tires due to 38 lbs. cold inflation; removing the AC belt; premium fuel which costs about 5% more but yields about 10% more mileage than regular; and careful but brisk driving with attention to traffic ahead. That way, I don't get stuck behind slowpokes or trucks, and simply coast when anticipating slow-downs rather than racing up and then slamming on the brakes. It all adds up.

[StuntDrivin4U]

Well done Dash01 if these are consistent results. I think a golf clap would be in order

[Neel]

I wonder what differential is in your car Dash01.

[ross1]

I wonder what differential is in your car Dash01.

5 spd 535s came with a 3.46:1 final drive which, with stock diameter tires, would mean the M30 is spinning~2800 RPM