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Old 04-12-2008, 08:30 PM   #1
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RPM/Gear & actual road speed

I love my Boxster -- but I never know how fast I'm going. I don't trust the digital speed read-out, other threads have indicated that this can be off by a lot. The speedometer guage is just worthless, you can't tell if your going 35 MPH or 50 by a glance.

This makes me wonder:

Does anyone regulary use the tachometer to know your speed? I drive a 6 speed (its an '02 S) and use the stock 18" wheels. I know that there are some variables, but I think this method should be as accurate as any.

In my old BMW, the tachometer was great for telling speed because 4k RPM in 2nd gear was 40 MPH. 4000 RPM in 4th gear was double (80 MPH). I am very aware of my engine RPM & I am also able to "read" the tachometer quickly and accurately at a glance. I just haven't been able to correlate RPM, gear & speed yet in the Boxster. Is anyone doing this? Any hints or tricks?

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Last edited by fatmike; 04-12-2008 at 08:32 PM.
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Old 04-12-2008, 08:32 PM   #2
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I suggest you buy a $200 GPS and use it as a speedo.
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Old 04-12-2008, 08:33 PM   #3
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Quote:
Originally Posted by Paul
I suggest you buy a $200 GPS and use it as a speedo.

Ha! Good advice, but I'm too cheap to ever buy a GPS.

I'm looking for more of a low tech "Zen" solution -- being more aware/connected with the car.
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Old 04-12-2008, 08:50 PM   #4
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OK then, just subtract 4 mph like the rest of us. 69 is really 65, etc.
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Old 04-12-2008, 09:06 PM   #5
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Quote:
Originally Posted by Paul
OK then, just subtract 4 mph like the rest of us. 69 is really 65, etc.

the digital speedometer is incorrect?
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Old 04-12-2008, 09:10 PM   #6
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The odometer is correct, both the digital and analog speedos read about 4 mph high anywhere above 20 mph.

Borrow a GPS to verify.
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Old 04-13-2008, 05:31 AM   #7
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The 3-4 mph "safety factor" has been discussed about 986s, but is it also built into 987s? I haven't had my new one long enough to discover on my own. I live in Omaha and who needs GPS in Omaha? I'd assume Porsche built the "safety factor" into 987s but varification here would be better than finding out via law enforcement that it's not.
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Old 04-13-2008, 12:26 PM   #8
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I'm more precise than subtracting 4 MPH (or so). Drives me nuts. I want to set my cruise control at certain places on the highway at very precise speeds. For example in certain spots -- 73 is OK, 76 is not.

Maybe someone with a Garmin can post their speed at 3K RPMs in 3rd gear? (We'd need to know "S" or not & also your tire size). How fast is 3K RPM in 6th? Anyone?
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Old 04-13-2008, 02:34 PM   #9
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If you are good at converting kph to mph...

To access the AC diagnostics:
Hold down the Recirculating & Air Up buttons for 5 seconds. The + - buttons go up and down through the list of "c" codes. The center vent button switches the display between the "c" code and its actual value. Press the Auto button to exit.

Go to 20c.
20c - Vehicle speed in kph, updating only once per second. (real speed, not speed+safety margin as in the speedometer)

I haven't verified the speed with a GPS yet but it does read lower than the speedometer.
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Old 04-13-2008, 02:43 PM   #10
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If you really want accurate you will need to beg, borrow or steal a GPS. Anything else is loose guesswork.
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Old 04-13-2008, 05:31 PM   #11
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Most highway mileage markers are pretty darn accurate. In our state they are marked every 0.1 miles. Start by setting your cruise at 60 mph. And time 1.0 miles using a sweep second hand wristwatch, a handheld stop watch or sport-chrono if you have got it.

If the mile does not take exactly 60 seconds, change the cruise control setting until it does. Now you know what indicated speed is 60 mph. Simply put the car in 3rd, 4th, 5th and 6th at that speed and you will have the RPM at 60 in each of those gears. You can do the math easily to figure out any other RPM per MPH in gear calculation you want.

PS don't get to engrossed in looking at the watch that you lose track of your driving.
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Old 04-13-2008, 06:05 PM   #12
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as others have said, the speedomoter typically reads high. I'd wager the 4mph high when traveling moderately fast is about right.

I would trust the digital and knock off a few. The analog is too hard to read! I also use my GPS as my primary speedometer in my other car since it too is off-cal.
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Old 04-13-2008, 06:55 PM   #13
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We have radar guns all over the place and in front of them is a read out of the speed. I have tested these many times when I am alone on the road and my car always reads 4 mph higher than the radar is indicating.

BTW,
I am glad my radar detector shows me multiple signals.
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Old 04-14-2008, 07:06 AM   #14
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Quote:
Originally Posted by derb
If you are good at converting kph to mph...

To access the AC diagnostics:
Hold down the Recirculating & Air Up buttons for 5 seconds. The + - buttons go up and down through the list of "c" codes. The center vent button switches the display between the "c" code and its actual value. Press the Auto button to exit.

Go to 20c.
20c - Vehicle speed in kph, updating only once per second. (real speed, not speed+safety margin as in the speedometer)

I haven't verified the speed with a GPS yet but it does read lower than the speedometer.
Yes, this works. It is actual speed in KPH, not MPH.

To convert KPH to MPH, you use the formula:

KPH / 1.609344 = MPH

But, this only works on cars through the 2000 model year only. These diagnostics cannot be accessed on later cars.
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Last edited by Lil bastard; 04-14-2008 at 07:51 AM.
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Old 04-14-2008, 07:39 AM   #15
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Quote:
Originally Posted by fatmike
I love my Boxster -- but I never know how fast I'm going...
This makes me wonder: Does anyone regulary use the tachometer to know your speed? I drive a 6 speed (its an '02 S) and use the stock 18" wheels. I know that there are some variables, but I think this method should be as accurate as any
...Any hints or tricks?

Yes, it's fairly easy to do:

First, you need to calculate the circumference of the rear tires
(important if you've changed tires sizes).

Assuming OEM Continental ContiSportContact 2

Tire data on the sidewall gives you:
Rim dia = 18 inches
Width = 265 mm
Tire aspect ratio = 35

Calculate the tire height - tire aspect ratio x tire width/100
= 35 x 265 /100 = 92.75 mm

Calculate tire OD - Rim dia x 25.4 + 2 x tire height
= 18 x 25.4 + 2 x 92.75
= 642.7 mm

Calculate tire circumference - π x tire OD /1000
= 3.14159 x 642.7 mm /1000
= 2.019 m

Now you need to know the Gear ratios and Final Drive ratio. For the Boxster 6-speed, these are:

1st = 3.82
2nd = 2.20
3rd = 1.52
4th = 1.22
5th = 1.02
6th = 0.89
Final Drive = 3.44

Then, calculate wheel revolutions for a given RPM. For example, at 3700 RPM, in 5th gear:

wheel revs - RPM / (Final Drive ratio x Gear ratio)
= 3700 / (3.44 x 1.02)
= 1054.49

Then calculate the speed (at the given RPM of 3700) - tire circumference x wheel revs x 60/1000
= 2.019 x 1054.49 x 60/1000
= 127.74 KPH

Convert to MPH, from KPH - KPH / 1.609344= MPH
= 127.74 / 1.609344
= 79.37 MPH.

Using these formulae, you can calculate the car's speed for any combination of Gear, RPM, Tire.
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Last edited by Lil bastard; 04-14-2008 at 04:39 PM.
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Old 04-14-2008, 09:03 AM   #16
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Lil Bastard, as my daddy (being into the use of the occasional spoonerism), used to say to me years and years ago, "Yer a pretty fart smeller."
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Old 04-14-2008, 11:53 AM   #17
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When auto magazines/shows are testing 0-60 times what do they use for the speed (radar, speedometer, other)? If speedometer, then 0-60 would actually be slower than they report?? Guessing they use something more precise but is anybody sure?? Sorry, a little off topic.
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Old 04-14-2008, 12:04 PM   #18
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Quote:
Originally Posted by Frodo
Lil Bastard, as my daddy (being into the use of the occasional spoonerism), used to say to me years and years ago, "Yer a pretty fart smeller."
Thanks... but not really.

I didn't invent any of this stuff... just passing it along. Hope some find it useful...
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Old 04-14-2008, 12:11 PM   #19
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Quote:
Originally Posted by lguard
When auto magazines/shows are testing 0-60 times what do they use for the speed (radar, speedometer, other)? If speedometer, then 0-60 would actually be slower than they report?? Guessing they use something more precise but is anybody sure?? Sorry, a little off topic.
I believe they use several methods, each Mag does it differently.

Some use the Speedo.

Some use a Track Wheel - a 5th wheel attached to the side of the car and fed into a PC with calculating software.

Some use a Radar Gun.

Some use a Drag Strip and timer.

Probably other methods as well. That explains alot of the variation you read from Mag to Mag.

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