Archives

I have been considering swapping out the differential for a better ratio but I was wondering how much of a gain it might make. Anybody better with a calculator than I am care to lend a hand?

Stock ratio = 3.25:1
alternate 1 = 2.93:1
alternate 2 = 2.79:1


I can get ahold of the 2.93 pretty easy, the 2.79 might take a bit of searching but they are out there. Any suggestions or thoughts on how much either might help my mpgs?

Thanks for the help.

Differential gear ratio calculator (http://www.csgnetwork.com/gearratcalc.html)

Let's get custom: http://www.currieenterprises.com/cestore/calculators/ringGearCalc.htm


This way when you figure what rpms you want to turn, at what speed, you can see what gear ratio you need. When you know the perfect ratio you can then work with the second link to find the exact ring and pinion gear.

Or you can do what you were asking and choose the best of the commonly available.

If I had as much money as I have time...or as much ability as I have dreams...

:D - Dale

2.93 will be 10% reduction in rpm's and the 2.79 will be about 14% change.

The scientific way to go about it would be to have the BSFC chart for the engine and use that to tailor the final drive ratio (not diff ratio - that is another component). It is probably unavailable as is true for most cars. I suspect that you are talking about the 533i. These cars have large high torque engines. The final drive was chosen by the manufacturer to be a compromise between acceleration and economy. Going to the lowest ratio commonly available will definitely improve your FE, but you will have to work out the numbers experimentally. You will be a little frustrated in making experimental measurements because that car does not have OBDII capability, meaning you can't install a ScanGauge, or SGII. You could use your fuel logs for a while and then, over time, you would get a rough idea of the improvement. JMO

MetroMPG tested rpm and mpg: http://metrompg.com/posts/rpm-mpg.htm

At the lower end of the rpm on the chart, the rpm / mpg change ratio is nearly 1:1, so a 10% rpm change would be about a 10% mpg change. YMMV, of course, but still a nice upgrade.

If changing the ratios was easy on my car, I'd have done it by now. It's not - I have to do a full transmission swap.

http://metrompg.com/posts/photos/chart-rpm-mpg-z.gif

If I was going to mod for MPG the first thing I would do would be to put a LRR set of tires along with the OEM spec size. Do you have access to take offs still? I would guess you could get 3+ mpg from doing that.

I did it on my old Volvo 740 wagon. Be careful that you don't go so low that your lugging the engine at 50-55 mph in top gear. Most manufactures drop a ratio for better acceleration with a minimal drop in fuel mileage.

For example my 740 which I swapped out the automatic for a manual would have had a 3.54 rear end had it come with a manual transmission. I dropped the rear gears to the next higher set which was 3.31. All in all I dropped my rpms at 65 mph about 125 with the change over including going from a 15" to a 16" wheel.

Remember the internal friction goes up just as wind resistance does as speed increases.

So going from 2000 rpms to 2200 rpms isn't a 10% increase it is more like a 30% increase in internal friction or drag. The same applies to when you drop the rpms at highway speed. 2200 to 2000 is a lot more than a 10% drop in internal engine resistance.

Plus that 200 rpm drop means that those cylinders are not filling with air and fuel an additional 200 times every minute your driving on the highway. Fuel savings to be sure.

I would love to drop the rear gears in my 960 from 3.73's to 3.54's. Problem is Volvo doesn't offer the gears. They only offer 3.90 and 4.10's as optional sets for my car.

All I have to do is find out who makes the rear end and find out if they offer a higher set.

One of these days I might spend some time finding out the information I need.

Awesome info, thanks everybody!


I'm still a bit torn on which ration to go with - but it will probably be whatever one I can get my hands on.

I wish it was as easy as calling up Currie or another shop. BMW doesn't let us have access to very much in the way of internal diff parts and no ring/pinion sets at all. There is only on shop that I know of that does rebuilds with custom ratios and they charge $1600:eek:

There is only 1 transmission choice for my car, so the only way I can change the final drive ratio is with the diff.

I still have access to some take off tires - but the original mix of 14" wheels and wretched TRX system wheels are off the car and scrapped. So I am currently running some 17x8 aftermarket wheels which are much lighter than the OEMs and running a 225/50/17 tire which is about 4.3% bigger than the stock size of 195/70/14 and 200/60/390 metric setup. It was the closest "modern size" that I could find available that still physically fit the car.

The tires are Nokian wr "super" all season tires and seem to roll about as nice as anything I have owned - even better than the Michelin MXV4s on the blazer. I can finally do some real p&g now!

The 2" extra width is extra friction. According to Miata.net (http://www.miata.net/garage/tirecalc.html), there is a difference of 4.5%. When the speedo reads 60mph you are actually going 62.7mph.

The 2" extra width is extra friction. According to Miata.net (http://www.miata.net/garage/tirecalc.html), there is a difference of 4.5%. When the speedo reads 60mph you are actually going 62.7mph.

Yea, I know the extra width isn't helping and my speedometer is going to be off - but a free set of Nokia tires with 8/32nds of tread and a sidewall max 51psi is awful hard to turn down.

As always I'm looking to do the best with what I have or can get my hands on - I scored a 2.93 diff today and for a bonus this unit has a limited slip! :woot: I am hoping to see 1 or 2 mpg gain with the new ratio, but the mileage log with hold the proof.

Thanks to everyone for helping out.

Yea, I know the extra width isn't helping and my speedometer is going to be off - but a free set of Nokia tires with 8/32nds of tread and a sidewall max 51psi is awful hard to turn down.

As always I'm looking to do the best with what I have or can get my hands on - I scored a 2.93 diff today and for a bonus this unit has a limited slip! :woot: I am hoping to see 1 or 2 mpg gain with the new ratio, but the mileage log with hold the proof.

Thanks to everyone for helping out.


A complete unit??? OK, I give! That may be worth it. I thought your were talking about changing out the ring and pinion.

Yup! a complete 2.93 limited slip diff from an 80,000 mile car with clean fluid for $50! Can't beat that.

I thought limited slip meant that a little torque limiter ties the two wheels together. So when the wheels need to turn different speeds, you're always wasting energy slipping this little clutch pack. Is that right? I'd think an open diff would be more efficient (if you don't count the tow truck's fuel if you get stuck in the snow).

LSD for $50.00 That's a real deal. Congrats! :woot: I payed $1650.00 for a new Integra Type R transmission with LSD back in my Honda racing days. Retailed for over $2400 if I remember correctly. - Dale

I thought limited slip meant that a little torque limiter ties the two wheels together. So when the wheels need to turn different speeds, you're always wasting energy slipping this little clutch pack. Is that right? I'd think an open diff would be more efficient (if you don't count the tow truck's fuel if you get stuck in the snow).

Yes, you are technically correct, since the most common type of limited slip is the friction clutch type. There are two other types which do not rely on friction and therefore avoid any theoretical losses. These are the Torsion, which has a complex gear setup, and the Detroit Locker, which has dog teeth and a "hard" sudden lockup.

But, I think the losses with the clutch type are mostly of a theoretical vice practical nature. A car spends most of its time traveling in mostly streight lines. So the frictional losses in the limited slip are very small.

Hey, $50 is my kind of deal.
I've always been a great believer in fewer RPMs means better mpg.
I would expect a 10% drop in RPMs-hy- to get a 5% improvement in FE(hy-city will be about the same,I think). Yes,I know frictional losses go up with the square of the speed(so 10% fewer RPMs means 21% lower frictional loss), but you never seem to get anywhere near as much are you calculate on such things.Most of the losses still go out the tailpipe as hot gas.

Let us know how it goes.
Luck
Charlie

Here you go - in the trunk of the car and ready to swap out next week.

http://www.cleanmpg.com/photos/data/500/medium/IMG00033-20100306-1541.jpg

Whatever losses I might see from the limited slip i figure will more than be overcome by the lower ratio. If I can see 5% that will get me at least +1 mpg that will make it worth the $50 spent.