MPG: convertible vs coupe/sedan

[CorvallisBMW]

So after doing a full tune-up on both my cars (plugs, wires, cap, rotor, all filters, all fluids, rebuilt injectors, valve adjustment, O2 sensors, cleaned ICV/TB/AFM) my vert is still getting consistently poor mileage, 20-21 MPG at best. The coupe on the other hand is getting it's normal 25-26 MPG. Now I know the verts are heavier by 250lbs or so, but that can't explain all of difference in fuel consumption. Both are running G260s with 3.73 LSDs too. What gives? I'm consistently getting 5-6 MPG worse, no matter the driving style, roads, fuel, etc. I've put the vert up on stands and can't detect any signs of a sticky caliper either. Do any other people out there who own both types of car notice such a big difference? Anything else I should be checking?

[kronus]

Check your fuel system. I'm not sure if the e30 one is pressurized, but my friend's e36 was getting like 22 mpg until he replaced the fuel cap, since the old one was leaking and decompressing the gas tank. He started getting around 28 mpg after the fuel cap replacement.

[twothrees]

FWIW, I am getting 24 mpg average(3.73 diff). That is mostly around town with intermitant highway driving. I can imagine that driving with the top down will decrease your mpg.

Are your wheels the same size from coupe to cabrio? Would that screw up the odometer when your checking your mpg.

[Aerton]

Crap when I drive my convertible around town I get 15-17 mpg as per the computer? I guess I have some work to do or lighten up my foot.

[smonkbmw]

Check your fuel system. I'm not sure if the e30 one is pressurized, but my friend's e36 was getting like 22 mpg until he replaced the fuel cap, since the old one was leaking and decompressing the gas tank. He started getting around 28 mpg after the fuel cap replacement.

damn thats worth a try. sorry to thread jack darin, but i just checked the MPG on the ETA and i just got 23mpg on this last tank!! yeesh.

[twothrees]

Forgot to mention that my head is practically brand new: cam, rockers, etc. I am sure that is helping with my mpg.

[SamE30e]

damn thats worth a try. sorry to thread jack darin, but i just checked the MPG on the ETA and i just got 23mpg on this last tank!! yeesh.

I get shit on my ETA.. like under 20.

[Turf1600]

According to my OBC I got 35 mpg on the way home last night. I can get 400+ miles to the tank on my 90 325is if I drive with a light foot.

[RedStar240]

The first big difference between the vert and the coupe is its a vert, it has way more wind drag. Especially if your cruzin around with the top down and all the windows down. Wind drag is going to kill MPG, although e30s are not the most aerodynamic cars.

[RedStar240]

According to my OBC I got 35 mpg on the way home last night. I can get 400+ miles to the tank on my 90 325is if I drive with a light foot.

when i drove an average of 55mph (max) i got 29.8 mpg and driving like a granny.

[nando]

I get 16-20 on my stroker and my tune is NOT fuel efficient - it runs 13-14:1 on overrun (stock ECU cuts the injectors, I haven't tuned for that yet), 13.5:1 at idle, 12.5-13:1 at WOT. I also have 4.10s, more drivetrain loss, and a stuck front caliper that I need to rebuild. so if you're getting that with a stock M20 and a 3.73, yeah, something is wrong.

The first place I would start is the tests in the bentley (I get the feeling most E30 owners don't have one..) There are tests for most of the sensors, the only ones you can't test are the AFM and O2. Coolant temp sensor will make it run rich if it's gone bad.

also, a clogged cat can make you get poor mileage too - might want to check to see if there's any loose honeycomb inside. I don't think E30 cats go bad very often though (at least they're worth a mint in scrap metals).

[CorvallisBMW]

Check your fuel system. I'm not sure if the e30 one is pressurized, but my friend's e36 was getting like 22 mpg until he replaced the fuel cap, since the old one was leaking and decompressing the gas tank. He started getting around 28 mpg after the fuel cap replacement.

Good to know, I will check on that.

Are your wheels the same size from coupe to cabrio? Would that screw up the odometer when your checking your mpg.

Nearly identical according to the tire size calculator. The 'vert actually has a slightly larger OD, so it theory that should help mileage.

According to my OBC I got 35 mpg on the way home last night. I can get 400+ miles to the tank on my 90 325is if I drive with a light foot.

Your OBC must by lying, I’ve never heard of an m20 getting over 30mpg. Even M42s have to work hard to get 35. Is your commute entirely downhill? :)

The first big difference between the vert and the coupe is its a vert, it has way more wind drag. Especially if your cruzin around with the top down and all the windows down. Wind drag is going to kill MPG, although e30s are not the most aerodynamic cars.

Yeah I know, which is why I waited like 4 months before posting this. I wanted to drive it with the top up, top down, hot and cool weather, etc. It doesn’t seem to make much of a difference up or down unless I’m going 65+.

The first place I would start is the tests in the bentley (I get the feeling most E30 owners don't have one..) There are tests for most of the sensors, the only ones you can't test are the AFM and O2. Coolant temp sensor will make it run rich if it's gone bad.

also, a clogged cat can make you get poor mileage too - might want to check to see if there's any loose honeycomb inside. I don't think E30 cats go bad very often though (at least they're worth a mint in scrap metals).

Good to know, I look through the ol’ Bentley. What’s the best way to check for loose honeycomb? Tap the cat and see if stuff comes out the tailpipe? Maybe I’ll take it to an exhaust shop and have the backpressure checked.

[Miasma]

You prolly already know about this but...

http://autos.groups.yahoo.com/group/bmwe30ic/

Nothing but vert owners talking about verts.

[Kruzen]

I average 20 mpg, I get 300 miles to the tank with normal driving, and about 270 if i am really revving the shit out of it all week.

[smonkbmw]

According to my OBC I got 35 mpg on the way home last night. I can get 400+ miles to the tank on my 90 325is if I drive with a light foot.

yeah i dont have any faith in my eta's obc as it says i get 28mpg cruising at 80 on the freeway.

do verts have the same size tanks?