My old 325i (4-door) gave a consistant 25 combined mpg for the trips that I
make. I managed to slam the car into the side of somebody making an illegal
left turn, and replaced it with a convertable of the same year, 1994. The
convertable only gives about 22.5 mpg for the same trips that the other car
Can there be something that isn't set up right on the "new" car that can be
fixed, or does driving around with the top down cost 2.5 mpg? I wouldn't
have thought the top down would be so expensive, but if it is, then I'll
learn to adjust. I just wanted to know if something like plugs can use so
Also, is my convertable a 325iC or 325Ci. I'm thinking it's an iC because
the coupe is an iS, and the sedan is just an i.
First, are you basing mileage on the OBC (which is usually inaccurate) or on
measured consumption? If the latter, then 22.5 mpg does seem pretty low in
comparison. I don't notice any difference between top up and top down
mileage in my Z4 3.0, so maybe you need to drive a bit with the top up and
compare. If it is still 10% lower than your 4 dr., I'd have it checked out
by a good mechanic.
I believe the "S" referred to the sport package - available on both 2 & 4 dr
models. Currently, BMW uses 325Ci for both coupe and convertible - although
I think I once saw a reference to a 325CiC!
At any rate, enjoy your "new" ragtop.
It's not on my E39. I've checked it against full tank to full tank x 3,
and it was absolutely spot on. Of course if the mileage recorder is wrong,
then so would be the real results. But although the speedometer is poor,
the odometer seems pretty accurate - according to tests I've read.
*If one synchronized swimmer drowns, do the rest have to drown too?
Dave Plowman email@example.com London SW
No, at least not here in the US. The S in 325iS was used in the past to
denote a 2 door (coupe), with or without the Sport package, but this has
now been replaced with the leading C as in 325Ci. 4 doors are always
called 325i whether they have the sport package or not.
It's quite possible, IMO, as the coefficient of drag must increase by a good
deal. The easiest way to check would be to drive the same route with the
top up to compare.
Also, the convertible is heavier I believe, which, too, may account for some
of the mpg loss.
Are the tires the same size and inflated properly?
I'm assuming you've taken care of basic stuff like new air filter,
sparkplugs, clearing error codes, and the likes?
Inflated properly is open to debate, but they are in fact the same tires. I
took the tires from the wrecked car, 17" from an M3, and put them on the new
Before anybody has to ask, both cars have the same transmission. That is,
both cars have the 5 spd. The 4-door was built in Sep. 93 at the beginning
of the production run, the rag top was built in July 94, I assume one of the
last off the line for the '94 model year.
The error codes are cleared, and the filter was inspected. The oil was
changed the day before I bought the car - I know this happened because the
dolt put both of the flat washers on the long bolt that holds the filter can
closed. This resulted in an oil leak because the two washers are different
I am comparing the mileage of two cars with the same tires. Not the same
KIND of tires but the actual same tires. The original fitment isn't part of
I had a 4-door, now I have a rag top. The mileage in the rag top is lower
and I am wondering if the rag top itself is the problem, or if there is
something in the Engine Management Systems that I should be looking for.
The original spec. doesn't matter. The comparison is being made on two cars
with the same set of tires. The significant difference is one car is a
4-door and the other is a rag top.
Having said that, the tires that both cars used came off of an M3, and are
within a quarter inch of the same diameter as the tires that are included in
the Sports Package in 1994.
Here is the problem, again. I owned a car that was wrecked. I bought another
car and took the tires from the wrecked car and put them on the replacement
car. Then, I noticed that the replacement car doesn't give as good fuel
mileage as the wrecked car. The wrecked car was a 4 door, the new one is a
rag top. Other than the doors and top, all other parts of the car should be
the same, therefore the question is, what makes the mileage worse on the rag
FWIW, a perusal of the BMW NA website indicates shorter gearing and on the
current E46 convertible (compared to the coupe) - but only with the manual
gearboxes. Weight is 10% higher for the ragtop.
I still think that 22.5 mpg is low, unless it includes a high concentration
of stop & go traffic.
Well, It's not directly comparable, but I went from a 1997 M3 4dr to a 1999
M3 convertible - and they had different final drive ratios. Both were E36
automatics. Driving on I5 for long distances sorta burns the speedo and tach
readings into your brain.
And do they have identical gearing.? But I'd also be interested in what
the original wheel/tyre size was on each.
So both cars originally had the same wheel/tyre size? In which case,
disregard everything I've said. ;-)
Assuming the mechanical spec is identical, I'm not sure. Rag tops are
often heavier due to the extra 'chassis' stiffening needed and the motor
and pump for the power hood. etc. They also have worse CD. But I'd not
expect this to make a vast difference under the conditions where you're
getting low 20s. Might be so at the speeds where you'd be getting in the
30s with a 4 door.
Have you the handbooks for both cars? Is the mpg quoted the same? Is the
gearing (mph/1000) the same?
*It's a thankless job, but I've got a lot of Karma to burn off
Dave Plowman firstname.lastname@example.org London SW
Well, the 4-door had the Sports Package that came with 225/55x15s and the
rag top had 205/60x15s. The M3 rims that I have carry 225/45x17s. The
overall diameter of these is 24.74, 24.69, and 24.97, respectively.
Regardless of what the tires sizes were at one time, both cars ran with the
M3 wheels, and this is the mileage benchmark that I am using.
No, they both have the same size now, but different mileage numbers.
I was thinking the same thing, the CD difference ought not be the
explanation for a 10% variance in fuel mileage on two otherwise similar
They are within a quarter inch of being the same diameter, so that isn't the
Both cars had the same tires and presumably the same gearing, therefore they
should have the same speedo error, and the mileage should then be the same.
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