my last gas mileage thread for a while (m20 vs m50) I promise.

[Kozworth]

Ok guys, heres the deal.

(325is with bav chip 3.73 lsd)

I was always working on some gas mileage issues on my e30 since I got it. First, it was the tank leaking. Then valve adjustment, reconnected purge valve, new o2 sensor after torching that baby multiple times to remove it, etc. etc.

I am getting a pretty solid reading of anywhere between 20-22mpg. From multiple posts ive read, the m20 is generally capable of about 18 city and 23-25 highway. Going off my obc, and having some fun with it doing mostly highway driving, I feel that my car is pretty much running as it should.

Anyways, Its becomming difficult for me to offord 4$ a gallon, or a tank of gas every 4-5 days (commute to school and work).

I would really not want to have to sell my car, but I am honestly considering a m50 vanos swap on behalf of your input.

Ive read the vanos can get upper 20s, but I am wondering what you e30m50 swapped guys get, if its much better then the m20. And i know a lot of you guys have this swap. Better yet, maybe a m50 with m3 cams and mgmt.

Id really rather do that then go through the trouble of selling my e30, bumming cars from people until I found a local yet to my liking 318.. So please help me out. Figured this could help others out in a time of increasing gas prices.

Thank you

[SlamedIAm]

Before considering a m50 swap to improve your fuel efficiency I would suggest doing the math to see how much you will be actually saving from a few additional mpg vs. the cost and time of an engine swap.

If you just want an m50 and this is your excuse then have at it. If it's about saving money over the long haul I don't think this is the answer.

[nrubenstein]

1). Never, ever go off your OBC. Calculate mileage based on the odometer and fill up.
2). You will be lucky to come out even on fuel economy.
3). If you want better economy, try fitting a taller rear gear.

[squidmaster]

#3 ^^
I get 25-30 everywhere with a b27 and 2.93.

[Kozworth]

Im getting about 300-330 miles for 14.5 gallons. Or more or less onboard with obc. Ive had a post and obc goes off pulse width, duty cycle of injectors, and speed, so technically it would be a good guideline if injectors arent leaking.

From what i read the m50gets high 20s and i assume that would be regular gas. Im looking to keep my car for a while so it would help me out. If i could get a solid 25mpg driving moderately i would be really content, and it seems like this option would give me that.

[Farbin Kaiber]

Sell it and buy an m42 car.

[dothadance]

From what i read the m50gets high 20s and i assume that would be regular gas. Im looking to keep my car for a while so it would help me out. If i could get a solid 25mpg driving moderately i would be really content, and it seems like this option would give me that.

you're willing to swap for an extra 3-5 mpg? not really worth it imo

[jlevie]

The OBC and econometer will be off if there is a chip. Both are driven by injector pulse width, which changes when a chip has been installed. To find out what the true mileage is, fill the tank & record the mileage each time. After several tanks of gas you will have a good number for average mileage. If your commute is 90%, or better, highway driving the number will close to best possible mileage. But if a good bit of the time the car is driven in an urban setting the number will be closer to the "city mileage".

If everything in the engine & management system is working as it should ~25mpg is realistic when cruising at 55-60mph. Intake leaks, a bad O2 sensor, compression loss, worn head, aged ignition wires, wrong plugs, leaking injectors, cold running engine, etc., will reduce mileage. An M50B25TU might gain a few mpg, but it would take many years to offset the cost of the transplant.

[Kozworth]

I did the calculations and its about 1k more a year to drive a m20w/ premium then an m50 on reg.

So basically i would get a slight increase in torque/hp, reliability, and the swap could pay for itself within the year. I am more or less looking for what people get mileage wise then whethor the obc is correct/how to track my mileage. I know what i am getting, and half of what you guys posted is 100% contradicting what others have posted in my other thread about the obc. But i still track my miles.

[jlevie]

If you revert to the stock chip, the engine will be totally happy with 87 octane. There will be a minor loss of performance, but for commuting purposes that won't matter.

[nrubenstein]

If you want to swap for economy and you have a decent shell, just put in an M42. Keep the 3.73 for better highway economy, or even swap it out for a 3.46.

You are NOT going to see the gains that you think you will with an M50.

[scottinAZ]

having gone down this road, the M50 is NOT more economical than the M20, given a decent state of tune with both.


The M20 I replaced was giving 22-25 mostly highway, and the M50 is giving 24-26 over the same exact route.

The slight difference in fuel economy will be hardly noticeable even over the course of a year, in my case, its about a $300 difference. Keep in mind that I do about 20K a year in mileage commuting. The difference in the price of gas makes more of a difference than the engine swap ever would.

[Kozworth]

I looked up changing my overdrive gear, and of all e30s the 325i has the lowest already (.81:1).

If I were to change my 5th, what would be lower then that and would it be pretty straight foward or would it have to be custom

[dnguyen1963]

This is the same argument I have with Chevy Volt owners! You will never recoup your money doing the swap. Switch back to stock chip then use regular 87 grade gas. My 2.5 L engine average 25 mpg city and 30 mpg hwy...not bad for a 23yrs old six cylinders.

[tinkwithanr]

Why would your want to change your overdrive gear? I think your confusing that with the final drive ratio, which is in your differential. Swapping diffs (you probably have a 3.73, could go down to a 3.46, 3.25 or even a 2.92) is easy enough, and you would come close to breaking even after selling your current one. As far as I know it is impossible to change out the gears in the transmission short of having custom ones made, and if you have to ask you definitely can't afford that.