My speedo is definately off that's why I asked about gears. I have a short geared (7:31?) '71 911 trans but I'm running 245/50/15's in back and the speedo's from and early '70s car. Unless the whole world just drives faster when I'm in the P-car my speedos probably 5-10 optimistic. Do chassis dynos read out MPH on there graphs? I'm gonna dyno test it anyway so that would be a great way to find out the variance.

Allow me to ask a stupid question. Where should the tach be just cruising down the road? For example, if I want to do the posted 55 mph (talking straight road, no curves), what gear should I be in? Where is the engine the "happiest" for each gear? 3K? 4K?

My rear tyres are 265/40ZR17. I know what the 17 means :p but I have no idea as to what the other stuff means :confused: ... stupid newbie, I know, I know...

edit: thought I'd add that when I'm cruising, I tend to be at about 4000 revs. Gives me room to slow down should I have to, and plenty of room to speed up.

Sheesh... Ask a question, get 30 replies in 5 minutes..

I love you guys ;)

265 is the tread width in mm, 40 is the aspect ratio it's the percentage of the width that the sidewall is high. For example your sidewall would be 106mm or 4.173 inches (mm/25.4=inches) in height. 4.173 x 2 (top and bottom sidewall) + 17 = 25.346 inches. That's your tire diameter. Diameter time Pi = Circumference so thats 79.63 inches. There are 63360 inches to the mile so your tire revolutions per mile would be 795.68 multiply that by MPH say 55 and you get 43762.4 tire revolutions per hour. Divide by 60 (minutes) and you get 729.37 tire RPMS. Mulitply this by final drive ratio (8:31 = 3.875:1) and you get 2826.31 transmission mainshaft rpms, then multiply that by the gear ratio of the gear you're in and you would get "theoretical" rpms to mph. I say theoretical because tires revolutions per mile are determine by the actual diameter as measured not from calculations. If you where in a 1:1 gear like 4th on many cars your engine would turn approximately 2826 rpms at 55 mph. ;)

Chris(tian) - 245/50R15 is 24.65" diameter theoretically. Factory was 25.20" (185/70R15). This impies 2% underreading.

Speed in gear? Depends on which year your tranny is, but a 901 has 7:31 r&p and the ratios moved around a bit IIRC.

Only ratios I can find right now (1967 911S) are:

1st 3.09
2nd 1.89
3rd 1.32
4th 1.04
5th 0.79

This would give about 3000rpm for 64mph in 5th if tyre size was correct. This should also be what the speedo reads. However, you will only be going 63mph (ie no big deal).

But this is all pretty theoretical!

(edit) and I see from your post I didn't need to explain any of the calcs for you :D. Hopefully the 901 gear ratios and factory tyre size helps!

eion - I have just come across this link which has (when you click "example" what are probably your gear ratios:

http://www.geocities.com/kharvatis/tspeed.html

Now this assumes a 225/50R16 tyre at 642mm diameter. You have 265mm (wide) x 40% (tall) x 2 + 17" * 25.4mm...

= 643mm or really freakin' close.

Wait, I dunno how he got 642mm for 225/50R16. It doesn't matter - if anything you are going faster than stock at any given RPM because your tyres have a greater diameter.

OK, based on what A Quiet Boom says (all makes sense now, thanks!), and with the help of my owners manual (and the solving function on my calculator), my top speeds should be as follows:
1st = 43mph
2nd = 72mph
3rd = 105mph (seems about right)
4th = 137mph
5th = 172mph
R = 52mph :p

That's based on a redline of 6800rpm (hmmm, the manual says maximum permissible revs of 6700, but I seem to have a close personal relationship with the rev limiter :rolleyes: )
Based on what I'm seeing at the moment (i.e. 105mph at the end of 3rd), my speedo seems to have been recalibrated). I was a bit concerned that my speedo was reading slow and that I was going faster than I thought - this doesn't seem to be the case :¬)

More likely you speedo was 2% optimistic and now you have the perfect size tyres for it.

Cam

Thanks Cam, good call on my name BTW. My tranny is a 911 (pull clutch version of the 901) and I don't know what year the speedo is. I do read approximately 3000rpms at 60 mph but that's irrelevant because the engine and speedo are mechanically linked through the tranny as you well know. I don't know for example if the speedo was set up for a 8:31 901/911 trans or even an early 915 since I don't know it's year, what I do know is that when I'm in the car driving everyone seems to be going approximately 5-10 mph faster than normal. There about a 13-14% difference between 8:31's and 7:31's so at 60 indicated I could be doing 51.6 actual mph, when I punch it up to 70 indicated to keep up with traffic (faster than most people drive in a 55 zone around here) I'd actually be going 60.2 which is more reasonable since in my other cars traffic tends to flow at about 60-65 in the 55 zones.

Thanks for making me think, I think I've got it now I just need to figure out what I need to do to fix it. Hopefully it's simple like a speedo gear change and not the whole speedo. This is one area about the Ford cars I own that I like, example I put a '68 toploader and 3:73 rear gears in my '81 Mustang, I went down to the local Ford dealer and picked up the correct speedo driven gear for that combo for like $15. I can get speedo gears for various tire sizes and gear ratio with about 15-20 to choose from and they're all listed with a formula in the Motorsport catalog. You'd think Porsche would make it that easy but I guess they didn't have to worry about a lot of combonations given the fact that these cars remained so similar for so many years.

-Christian ;)

For anyone that wants to check their actual speed, all you need is a highway with mile markers and a stopwatch. Any cheap digital watch will most likely have a stopwatch function. Just time how long it takes to complete a mile while holding a constant speed. I do this sometimes on my boring 4 hour drive to the beach house. I get very repeatable results, so those mile markers must be pretty accurate.

time mph
1:04 56
1:02 58
1:00 60
0:58 62
0:56 64
0:54 67
0:52 69
0:50 72
0:48 75

You folks that use those kilometer things will have to do your own math.

-Rob
1980SC

BTW, the only car I have that has a truly accurate speedo is my 1985 Plymouth minivan :)

Hehe, I saw your name on you Pelican page - better than AQB and easier to type than the full A Quiet Boom.

OK. What I understand is that virtually all 901 (unless specially changed) and 1972-1974 915 gearboxes had a 7:31 r&p. I think certain 1975 and 1976 cars also had the shorter r&p (the 2.7 Carreras with MFI).

What does your speedo look like? Does it have a silver dot in the middle of the needle (the '68 - '74 style)? Or no silver dot (74 on style)?

If no silver dot, you could possibly have a '75 speedo, which would be set up for an 8:31 r&p. Lets have a think about this. I think this would cause the speedo to underread not overread.

If you are going at 60mph and the speedo expects 2650rpm (and gets 3000rpm) it is going to read high. I think.

Something else is up - you need to do 1980SCs test, I think.

I would guess your chances of findng different gears as slim! You could talk to www.nhspeedometer.com or www.paspeedo.com.

I did that test on a 100 mile each way trip down the turnpike in my best friends 90,000 mile mustang, turns out the reason he got so many speeding tickets was his speedo was off by over 11% making his car a 100k plus Mustang LOL. At 55 he'd actually be traveling about 62 and he kept getting tickets for 67-69 in a 55 when he swore he was doing 60 ouch! You can test without a stop watch BTW, just set the trip odometer at a milemarker and drive a good 50 miles or so for accuracy's sake and compare the milemarker to your trip odometer reading. When I bought the car from my brother it came with 3 transmissions he said the one that's in it had "short" gears as well as one of the other ones, the third had "tall" gears, only later did I realize this meant 7:31 and 8:31. My speedo has the silver center FWIW. Was there a 8:31 901 or 911 trans? As far as over reading vs. under reading goes I'm very familiar with that having installed many sets of performance gears in drag cars. Lower (numerically higher) gears make the speedo overread while taller gears make it under read, much like tall tires on a lifted 4x4 cause the speedo to read much lower than it's supposed to. Lets do some easy math to explain this. Pretend your tires rotate 500 times per mile and you have a 1:1 high gear ratio with a 2:1 final drive, in this case the one mile would equal 1000 engine revolutions, let's also pretend that the speedo reads 1:1 of the tranny mainshaft so the speedo see's 1000 revolutions to the mile. Now substitute 3:1 for the final drive, now the engine and speedo both turn 1500 revolutions per mile but the speedo thinks 1000 equals one mile so it reads 1.5 miles instead since the odometer and the speedometer are driven the by the same mechanics the speedo mph would now be off by 50% higher with the lower 3:1 gears.

Here's the part where I have trouble, on most US made cars the speedo stays the same and they change the speedo driven gear at the end of the cable in the transmission to correct for different final drive ratios and tire sizes, I would think Porsche using VDO supplied gauges would do the same BUT that doesn't appear to be the case with my car unless the PO (before my brother who had it 9 years with only 3-4k miles) changed the driven gear in the trans for some reason, I know he rebuilt it since I have his receipts so he might have replaced it with the wrong one?

What do ypu think in my SC 79 wheels?
993 TT 17 rims with 215/45 in front and 235/45 in rear, what numbers will I get?
A too optimistic speedo?

25.33 diameter for 235/45/17 tires. pretty darn close to stock I think.

Thank for a fast response, I don´t have my 915 ratios in reach here at office, anyone have the numbers for a 79 915 gearbox?

Test this:
http://www.bmwpower.nu/tips/diverse.asp

Swedish though but easy to understand.
Hastighet = speed
Utväxling = ratio
Bakaxel = final drive
Bredd = width of tire
Fälgdiameter = rim size


Good luck!

I have taken my speedo past 150km/h in third. But my speedo indicates a bit more than I'm actually doing. (Most likely 8-10 km/h too much?)

I have an old road test here which states 149km/h at 6250 rpms. Seems realistic.

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

In my 79SC it´s about 150km/h, 93 mph for you

I was just playing around with this the other day, seeing how fast I could be going and still drop it down to third and second (of course, I did the testing on the way up). On my SC, third maxes out right around 90 mph.

Well Steve,

I can tell you from recent experience... It's NOT 100 mph. *sigh*