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I have seen a lot of posts for performance parts but little on fuel efficiency. Does anyone know if there is any modification to change the gear ratios on the final drive or manual transmission to enhance fuel economy? I have seen some posts claiming that some of the M3's have done modifications that have taken them into the 40mpg range. I would think a 6th gear overdrive with a smaller final drive ratio would allow a high top end speed with less rpm's. Yet, still would have the lower gear speed for winding alpine driving. With the price of gas rising I would like our 87 325 to get a little better. When tuned and everything new (timing belt, coil, plugs, wires, oxygen sensor and canister) we see as high as 32 mpg with just highway. This is with cruise set at about 70 (found out that at 70 on speedometer we were only doing about 67). Around town and highway, we get about 23 to 25. If we push the car to 80 or faster, we get about 21 to 24 on the highway. Curious to hear other comparisons and thoughts.
 

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Cef5917 said:
I have seen a lot of posts for performance parts but little on fuel efficiency. Does anyone know if there is any modification to change the gear ratios on the final drive or manual transmission to enhance fuel economy? I have seen some posts claiming that some of the M3's have done modifications that have taken them into the 40mpg range. I would think a 6th gear overdrive with a smaller final drive ratio would allow a high top end speed with less rpm's. Yet, still would have the lower gear speed for winding alpine driving. With the price of gas rising I would like our 87 325 to get a little better. When tuned and everything new (timing belt, coil, plugs, wires, oxygen sensor and canister) we see as high as 32 mpg with just highway. This is with cruise set at about 70 (found out that at 70 on speedometer we were only doing about 67). Around town and highway, we get about 23 to 25. If we push the car to 80 or faster, we get about 21 to 24 on the highway. Curious to hear other comparisons and thoughts.
IF you could install a 6 spd for $2500 and IF it raised avg. mpg from 25 to 31.5 (25% increase) and you drove 15000 miles a year and fuel was $2.25 a gallon, it would take 8 years to recoup your "savings" :(
 

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The cheapest way to change your final drive is to fit larger wheels.
Going from 14" to 15" will give you about 8***37; better economy at cruising speed but you will also get a corresponding decrease in performance.
So, in the end you will give it more gas pedal and not save any money.
Another solution, if you haven't done it already is spend a few bucks and take it to a mechanic who knows his stuff and and have it sorted properly.
This will probably save you money in the long run and a good mechanic will also tell you the jobs you can do yourself before you get him to do the expert bits.
 

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Simply getting larger wheels won't do that, as the tires have a lower profile to account for it. Not to mention the spedo will be all messed up. To do this you need the final diameter of the wheel and tire together to raise.
 

· 1990 325i Convertible
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fuel economy in your e30

I had an '86 325es, and she got 32 MPG. Now I drive a 1990 325iC, and she gets 25 MPG. I'm wondering how I can get that kind of mileage in my "new" car...

Did you do anything special to your car?

I have seen a lot of posts for performance parts but little on fuel efficiency. Does anyone know if there is any modification to change the gear ratios on the final drive or manual transmission to enhance fuel economy? I have seen some posts claiming that some of the M3's have done modifications that have taken them into the 40mpg range. I would think a 6th gear overdrive with a smaller final drive ratio would allow a high top end speed with less rpm's. Yet, still would have the lower gear speed for winding alpine driving. With the price of gas rising I would like our 87 325 to get a little better. When tuned and everything new (timing belt, coil, plugs, wires, oxygen sensor and canister) we see as high as 32 mpg with just highway. This is with cruise set at about 70 (found out that at 70 on speedometer we were only doing about 67). Around town and highway, we get about 23 to 25. If we push the car to 80 or faster, we get about 21 to 24 on the highway. Curious to hear other comparisons and thoughts.
 

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my 316i is doing 3200rpm at 100km/h, yet is returning between 4 and 6L/100km (50-40mpg). final drive really does very little for changing the economy, because it has to work harder to get to the speed you want to get too. of course, if you are commuting 30km+ all highway driving where speed doesnt fluctuate, its worth looking at. much cheaper and easier to ensure your car has tyres around 36PSI if on stock 14" rims, and just accelerate gently away. ensure your boot/trunk is empty of stuff you dont need, get a wheel alignment, and if the majority of your driving is within 10kms from home, ditch the spare tyre as well.

then you can do little things like backing into carparks/driveways so the engine is up to speed and your driving in 1st gear instead of sitting idling. the car uses more petrol when cold than it does when its hot. i stop at this point because i like driving, but you could also plan trips if you dont already, eg going to the supermarket on the way home from work rather than coming home having a beer and going back out a few hours later. the car is cold again = more fuel, and you are doubling back on yourself = more fuel. mpg stays pretty much the same of course, but less miles covered in a week = less cost = less perceived mpg

i will say though, my fuel milage has gone down a LOT having a V12 in the driveway as well, if i want some fun i take that car, the 316i is an appliance to me now. oh no i probably jinxed the car!

all in all id say your milage is pretty good for a 20 year old 2.5L SOHC car.
 

· 1990 325i Convertible
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35 Posts
fuel efficiency

I think I am going to take your advice and empty out the car and make sure I have the right amount of air pressure in my tires.

I think the only modification I could ever do would be swap out the rear differential. But I would want to drive a car like mine that has already had the work done so I know what to expect.

The other thing is how much money would I actually save? If I drive 12,500 miles per year, and am able to do something to my car to increase from 25 mpg to 30 mpg, I will save $292 dollars annually.

The return of investment for swapping out the rear end would take two years if I paid 600 dollars to get the job done. It might not be worth it.

Not sure if there are "fuel efficient tires" out there, but if there were, I would buy them when the time comes to replace the existing ones.

I'm impressed that you were able to fit a V-12 inside your car. That car must FLY!!!
 

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yes you can buy fuel efficient tyres, Bridgestone have their Ecopia range, not sure about other brands as im pretty religious to Bridgestone tyres when i have a choice.

if your doing that sort of milage i would be looking at buying a year 2005+ car as they are more efficient. 12k miles a year is VERY high for just a civilian here, not sure about in the USA.

(If its for business reasons you do that sort of milage, Id imagine there are tax advantages of using a later model car too as well as being able to write off the depreciation etc, though im not familiar with USA tax law. im an accountant in training when im not playing with BMWs :thumbup:
 

· 1990 325i Convertible
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35 Posts
e30 fuel economy

Sadly, we Americans drive 25+ miles to work each way, and many people drive 50 miles each way.

I love how the e30 feels when zipping down the highway at 70 mph. And since I paid cash for the car, I want to get my money's worth out of her. And I just love the old Bimmer look. I would buy another just to have as a spare if I could fine one that I liked!

I take every tax advantage I can legitimately take, and mileage just isn't one of them. I figured that if I could increase my MPG to 30 from 25, I would save 300 dollars a year in fuel. Since you are an account, I don't need to say anything more about those numbers... buying a new car just won't give me a return on the investment.

Did I mention I love driving an e30? :)

My next set of tires will be the fuel efficient type.
 
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