Easy question. How fast is 3rd gear in an SC?

[cstreit]

Just curious... If you are maxed out in 3rd gear in an SC (8:31 rear end) at about 6500 RPM, what kind of MPH is that?

(I have the speedo covered in the racecar)

THanks!

Chris

[autobonrun]

1st,2nd,3rd,4th,5th

30,60,90,120,150mph

I misplaced my owners manual but I recall a speed vs rpm graph in it that reflects this.

79SC

[GSpreeman]

Would the same hold true for the Carrera?

[CamB]

Depends on your tyre circumference. You should measure it to be certain.

MPH = revs * 60 / (rear end * gear ratio * wheel revs/mile)

Wheel revs per mile = about 63346/tyre circumference in inches (I usually do this metric). With 25" diameter tyre, this is about 807.

So... 6500 * 60 / (31/8 * 29/23 * 807) = 99 mph.

Unless I screwed the calc up .

I made a spreadsheet (anal sob that I am). I go to Tire Rack to find out the wheel rev per mile (eg my size S03 is 864 wrpm). If anyone want's a copy, I'll clean out all the BMW 2002 crap in it and share. Steve Timmins (Instant-G) has a spreadsheet at his site too. (edit) This is what mine looks like - Chris' would have a different circumference/wheel rev per mile and redline.



The same will, more or less, apply to a Carrera - depends on the ratios (yours is a G50 box so might be different).

[A Quiet Boom]

Cam are you saying Carreras are actually capable of 90MPH or is this just theory? muhahahahaha!

[CamB]

Well, theoretically yes. But they are so heavy it may never get there. Ho ho ho!

[eion]

Well, on my car (also a Carrera )
1st = 35mph
2nd = 75+
3rd = 110
4th = no idea, I don't use 4th very often, and I've never been on the track. (interpolating 3rd and 5th, probably 140)
5th = at a guess, 170 or so.

[makaio]

eion,

Since you are up in Siskiyou County, do you do any high speed runs in Oregon, and then slip back across the boarder ?

[GSpreeman]

EION!!!!!!! Finally someone chimes in for the Carrera folks.

[eion]

High speed runs in Oregon? Not really, it only gets up to 65 there as far as I've found, seems to be mostly 55, and apparently you get plainclothes police cars there too. I-5 around here is 70mph (I'm near Yreka). Next week I'm going to be driving down the coast, though, via Eureka - I'm really looking forward to that!

[emcon5]

This is what my owners manual says:



Top of third for me is ~90mph.

Tom

[autobonrun]

Cam, I'm coming up with a different tire diameter which could explain our difference. I base mine on the equation from Korman below. Then I'm multiplying this by 25.4 to match your mm's. I tested my SC all the way up when I first got it and the rpm's vs speed very closely matched my owners manual. Having 225/50's in the rear and 205/55 up front (where the speedometer reads) may make a difference also.

http://www.kormanfastbmw.com/ttiredia.htm

[CamB]

Yeah, the problem (as I have so anally calculated and checked) is that theoretical (ie the Korman calc) and actual are usually different.

The best plan I figure is to use either the manufacturers spec (if you can find it - note the S03s are 2.7% smaller than theoretical (actually quite a bit) - or do as I did on the BMW, which was make a mark on the ground and tyre and roll the car a couple of wheel revs and check the diameter (1775mm). Told you I was anal!

I wouldn't trust the speedo. As far as I am aware it reads from the transmission and (obviously) makes an assumption about tyre size. Even if you stick to the factory size, you can get variation - 215/60R15 doesn't have the same diameter as 225/50R16 despite these being the two factory options at the back of an SC. Whats more, virtually every road test I have ever seen shows manufacturers have optimistic speedo (usually a few %).

The best way is to assume your rev counter is correct and figure out your tyre diameter and circumference, then apply the formula (the one I have above might not have the right number of inches in a mile).

You also need to track down your gear ratios - it seems Porsche messed around with 4th and 5th over the life of the later 915 gear boxes.

[dean]

92 MPH exactly. Ha

[A Quiet Boom]

Speedos read of the tranny not the front tires! While where on that subject does Porsche or anyone else sell different speedo gears for different final drive ratios, tire sizes, etc. ?

[eion]

Originally posted by A Quiet Boom
Speedos read of the tranny not the front tires! While where on that subject does Porsche or anyone else sell different speedo gears for different final drive ratios, tire sizes, etc. ?

So, if the wheels on my car have been upgraded to 17s, wouldn't that make my numbers lowballs? (except for the 4th and 5th gear numbers, obviously).

[CamB]

Depends on what size tyre you have eion - if it is the same diameter then the speedo is as accurate as it was before you changed.

I assume Porsche offered different speedos with different final drives (eg 8:31 vs the turbo 9:whatever it is). I think they (electronic ones) can be recalibrated.

Early (up to '75 I think) speedos were mechanical and would need to be matched to the tranny (7:31 r&p or 8:31 r&p in '75 only (I think)).

Guess who is interested in gear ratios?

The BMW should do 180kph at 6600rpm in 4th (top) gear. I have had problems with it revving past 6150 so I haven't checked...

[eion]

I doubt the tyre diameter is the same diameter as it originally was, my car would have come with 16" wheels from the factory. I have no idea how my speedo works, I'd guess it's electronic, but I don't know if it's been recalibrated.

[autobonrun]

Being equally anal, I should have probably not have used the theoritical but taken actual measurements. I used a similar spreadsheet to decide what new differential (changed from 2.93 to 3.73) to put in my BMW 740 but I used the theoritical numbers for the wheel diameter. I played around with the final ratio until I got a good balance between a higher low-end speed and not to high of a rpm figure at cruising speed.

You are absolutely correct about the different aspect ratios making no difference. Excluding rim diameter, the 225/50 on the rear gives a 112.5 mm figure and while the 205/55 gives a 112.75 mm figure; close enough that the wheels spin at virtually the same speed so the calculated speed from the rear should equal the measured speed from the front; plus or minus speedometer error of course.

It's too late here in Kansas for me to be doing these type calcs. This is 9am type of thinking, not appropriate for 11:30pm.

[CamB]

Your speedo is definitely electronic. Porsche phased mechanical out in time for the CIS age.

It probably hasn't changed the diameter.

If you has 225/50R16 (my guess at a C2s factory rear tyre size) and you ended up with 245/40R17 (my guess at your rear tyres) you have a virtually identical theoretical diameter of a little under 25".

Hence, no change in speedo accuracy. If it was inaccurate from the factory though, it is the same now!

(edit) And autobonrun - despite me making a song and dance about it, I think it is safe to say that <3% calculation error is ok!