Fuel Mileage Sucks on '94 325 Convertable

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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 produced 25.
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 much gas.
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.
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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.
Tom
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Actual consumption. the OBC also reads much lower than the other car.
Drive with the top up? Can you do that? ;-)
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Yup, unless your 325 thinks it's a 50's British roadster!
Tom
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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.
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Tom K. wrote:

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.
-Fred W
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"J Strickland" wrote

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?
Cheers,
Pete
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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 convertable.
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 sizes.
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Have these the same overall diameter as the originals? If not, the odometer will be inaccurate.
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wrote:

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 the equation.
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.
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It is if the diameter is different to the original spec.
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wrote:

Dave, 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 top car?
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wrote:

Is it possible that the convertible has shorter gearing to compensate for the additional weight?
Mike P

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wrote:

I don't think so. I have a Bentley manual, and I'll check. But when I looked at the gearing tables in the past, I don't recall that the convertables have different ratios.
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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.
Tom
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wrote:

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.
Mike P
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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?
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wrote:

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 cars.

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I think he is talking about the original tires on your new car. If the tires which were installed previously were larger or smaller, the odometer will be inaccurate if not recalibrated.
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They are within a quarter inch of being the same diameter, so that isn't the trouble.
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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