the flyin' scotsman

Its a matter of personal preference as to what diff gives the best overal ratios for your style of motoring.

For me the 2:20 in my '88 S4 is wrong for my style of driving............engine is at the wrong revs and the auto changes at the wrong time when I get on the go pedal to pass the line of slow pokes.

The 2:73 has the revs up but then they're always up..........a little harder on gas and vibes.

The 2:64 is the happy medium IMHO.

dr bob

Rich in Phoenix made an interesting observation a few years ago. He has to choose between the GTS and the 2.20 S4. They choose the S4 for trips because of the bettr/quieter/calmer final drive ratio. The '89 in the garage here is a good compromise at 2.54, but I still yearn for an overdrive gear when out on the open road. Recovering 15% of engine speed with the 2.20 seems like a great idea. I need to spend some time in a 2.20 car and make sure that the gear 2/3 performance is suitable for the local driving though.

Seems a shame... The car will launch to 60 nicely, rolls past 120 with ease, cruised at 100 effortlessly across west Texas, yet the 'average speed' value from the dash computer usually says '26 MPH' in my from-home normal driving pattern. Maybe with 2.20 I'd average 15% higher speeds. That's the ticket...

pcar928fan

I thought all the final drive gears in the box plus the final Diff ratio's pretty much left all the cars at about the same place... ie. 2100ish rpm at 60 mph... Is that incorrect? I have not driven my '81 in a LONG time, so that 3 spd box may be off a bit, but I know my GTS's and S4 all spin right above 2000 rpm at 60mph on the highway in top gear...

the flyin' scotsman

I had my S4 out this morning; 2k rpm at just above 100kph top gear with the 2:20 rear end.

Couldn't handle that for long so dropped it to 3rd and moved up to 160

Vilhuer

Yes, its incorrect. All 928 gearboxes have 1:1 top gear no matter which version they are. This means ring&pinion is only thing which determine how fast engine revs in 60mph or any other road speed. There is big differerence between 2:20 and 2.73 diffs. Even 2.54 used in late automatics is too short for me and 2.20 diff gears are going into automatic in few weeks. Will loose maybe .1 or .2 in 0-60 time and few mph in top speed but gain several 100 rpm lower revs in motorway cruizing speed and mpg or two better fuel consumption.

Vilhuer

Diff is only driveline component which determine how fast engine revs on highest gear other than tires of course. Using nominal 1984mm diameter of225/50-16 as reference:

60 mph is
2.20:1 = 1780 rpm
2.27:1 = 1840 rpm
2.36:1 = 1910 rpm
2.54:1 = 2060 rpm
2.64:1 = 2140 rpm
2.73:1 = 2210 rpm
2.75:1 = 2230 rpm

80 mph is
2.20:1 = 2380 rpm
2.27:1 = 2450 rpm
2.36:1 = 2550 rpm
2.54:1 = 2750 rpm
2.64:1 = 2850 rpm
2.73:1 = 2950 rpm
2.75:1 = 2970 rpm

100 mph is
2.20:1 = 2970 rpm
2.27:1 = 3070 rpm
2.36:1 = 3190 rpm
2.54:1 = 3430 rpm
2.64:1 = 3570 rpm
2.73:1 = 3690 rpm
2.75:1 = 3720 rpm

'87-88 US S4 5sp with 2.20:1 and 40% LSD is best 928 gearbox. 1st to 4th aren't that far higher than in other gearboxes but 5th acts like 6th.

James Bailey

Oh Damn now Kibort will be posting soon....... note that in gears 1-4 the layshaft gear ratio is often different changing the actual gear ratios so as stated in fifth you get the above numbers

pcar928fan

Erkka,

Sorry to burst your bubble, but your numbers are wrong dude...

I have an '88 S4 with 2.20 rear end and at 60mph I am turning 2060ish RPM...Malcolm is seeing the same thing. I am telling you rear end ratio is not changing engine speed because the gears in the box are different. My GTS's (BOTH OF THEM) turn 2060ish RPM at 60 mph and they have the 2.54 rear end I think...I KNOW they don't have a 2.20 rear end...

My car is an automatic...I guess I should say that too and all my cars are and all the cars I have driven are and all of the 4spd gear boxes I have driven have 60mph coming up at engine speeds just above 2000...

Maybe there is a big diff. in the manual boxes, but not in an '84, '88, '92 or '94 auto gear box...they all turn the same engine speeds at the same road speeds within probably 150 rpm...

Randy Carter

Easy, unless it's modded, it's the car behind the GTS. Way behind.

Just kidding.

Vilhuer

'84 and '88 US automatics should have 2.20:1 and both GTS 2.54:1 diff from factory. All 4sp automatics have 1:1 4th meaning exact incoming rpm come out at other end to pinion gear. There aren't any other gears between engine and tires other than possibly slipping torque converter which can have any effect on ratio. So either you have slipping converter, tach shows wrong rpms or have wrong diameter tires in '84 and '88. Maybe even combination of all three. But those numbers I listed are correct within small rounding error unless I did some individual errors in them. In general they are correct for properly working transmission no matter which version it is. 2.20 vs 2.54 on top gear has to be clearly noticeable when everything is ok. This is simple mechanics.

pcar928fan

Erkka,

I am telling you they all turn roughly the same RPM at the same speed in top gear even w/ different diff numbers. Maybe 60 is the magic number or something, but my cars (2 GTS's and the '84 and '88) all are right at 2100 rpm at 60 mph. Come take a ride with me...the cars are in exceptional working order too and the numbers have not changed over the years that I have owned the cars (considering your comment on PSD's here).

Are you SURE that the 1:1 is not the final to the wheels...that would make sense considering I am seeing the same numbers...

BTW, your numbers have a bit more than a small rounding error since all of us with 2.20 rear ends in auto trans cars are running right at 2100 rpm not less than 1800 rpm at 60 mph... I am not working the numbers with math, I am looking at my speedo and I am looking at my tach. Most of those cars have had recent emissions testing too where they use their gear to look at your RPM and they have matched what I am looking at on my Tach, so it is not a tach reading error. I guess all of my speedo's could be off... doubtful though as they tend to match up withing a mile an hour or two of my GPS...

AO

James I think you're wrong. My experience jives with Erkka's numbers.

pcar928fan

Ok...I'll go out and drive the cars again... that said, NONE of my cars are even CLOSE to 1800 rpm at 60 mph! NOT EVEN CLOSE! They MAY BE 2050 on the S4 and 2150 on the GTS, but MAN ARE THEY CLOSE and nowhere near 1780. Oh, stock wheel/tire sizes too guys.

the flyin' scotsman

Missing from my sig is the S4 tyre sizes which are somewhat larger than stock.............275/35x17 rear.

There must be an effect over stock but my SOTP comparo to the 90 GT I'm working is substanial.

Snow pic:

Jim Chambers

I'm too lazy to go down a flight into a cold garage to find my owners manual, but doesn't Porsche still include the graphs of rpm/speed in each gear as with the old air cooled cars? If so, maybe the answer lies in comparing owners manuals. I'll show you mine if you show me yours.