I found out in a German S2000 article that the S2000's airodynamics coefficient (CD) is 38 with the roof up and 42 withthe roof down. I found the numbers in the S2000 vs the yellow BoxsterS at the Nurburgring and Hokenheim circuits. It's on this site:
http://www.honda-s2000.de/home.html

Anyway, if the CD is 38 for the roof up, then what is the possible top steep for the car? I mean real possible top speed on a flat piece of road where there's miniman wind ... not what the speedo shows running downhill. Can someone work this out (maybe Sev)? According to all the articles it's somewhere between 235 - 241km/h (147 - 151mph) ... 241km/h would be showing about 256km/h (160mph). Does that match the .38 CD?

Also, what about with the roof down where the CD is 42?

ps. The BoxsterS has a CD of 32.

[Edited by DavidM on 03-27-2001 at 12:06 AM]

 

The best I have managed on a flat road was 240.1 km/h, measured with a GPS. At that speed the car was showing 248. ( See my post max speed ).

I think it should be very very difucult to make more than 250.

 

Hi DavidM

I can confirm that the Boxster has excelent aerodynamics..my friend

 

David, interesting to see your comment that 256kmh on the speedo equals 241kmh actual or approx 6% variation.

I don't really know what the top speed of the Stook is - I guess I don't have any ambition to drive any faster than the longest straight on a track will allow (and I think we agree that in Oz that is at Calder).

However, the issue of speedo variance is one that piques my interest.

I have just installed a bike computer on my Suzuki motorcycle. I have calibrated it precisely by measuring wheel rotation and inputing this.

Comparing the odometers on both the computer and bike speedo I find they are accurate to within 0.05% of each other (50 metres in 100 kilometres or as close to perfect as I think one could reasonably expect). However, the bike speedo reads 13% high compared to the computer for actual speed.

Now given that Suzuki can get the odo to read 99,95% accurately why should there be such a huge differential in the speedo reading. This leads me to believe that they must have deliberately calibrated the speedo to read high. Of course the 6% high reading on the S2000 is less than half the 13% of the bike but in my opinion is still unacceptably inaccurate.

I wonder if we have a case for false advertising or misrepresentation against the vehicle manufacturers?

 

I remember somebody in Germany who did 268KMPH going slightly downhill. There was also a photo from it.

 

I had a 1981 Saab Turbo 4 door sedan. If I recall correctly, it had a cd of 38.

 

Hey 2kturkey, The speedo innacuracy is common with all cars. Also, it is not linear. Usually at 100km/h there should is no more than 1 - 2 km/h error. The error gets greater with speed and by 200km/h it is about 10km/h ... by 250km/h it is about 15km/h. I worked out the error for S2000 by seeing a few shots of the speedo at high speeds - you can read the revs from it and revs tell you what speed you're doing much better than the speedo. Afterall, if you know exacly what speed each gear will do at 9k then you can work out what speed it is doing at other revs. There's even a screenshot of a speedo with 252km/h (or something like that) on that German site with S2000 articles. See what the speedo is doing there (I already made a reference to it in detail in the 'S2000 TOPGEAR review...' thread. I worked things out exactly in there).

Anyway, this kind of 'error' is pretty common and all cars (to my knowledge) have it. From my experience the sportier the car, the smaller the error ... though, there's always a parabolic error curve.

What would be good is if people who reach these high speeds actually took note of the revs and then posted the revs as well as the speed shown on the speedo. That would make it eazy to work out the 'true' speed.

ps. There's more to the speed/error than just revs. The amount of tyre wear will make a difference. Think about it, a difference in a tyre that is brand new and worn out must be at least 1cm in diameter. The circumference for 225/50/16 tyre is 2 * 3.142 * (11.25 + 40.55) = 6.286 * 51.1 = 325.6cm. That in turn means you need 307.1 revolutions for each km. Now if you take 1cm of wear into account then the cirumference of 225/50/16 tyre is 6.286 * 50.1 = 314.93cm ... that revolved 307.1 times will give us 967.15m (0.967km). So even if you have 'true' speed with brand new tyres (which is highly unlikely) then after having them worn (0.5cm wear) you will be getting 3.3% error ...that is 6.6km/h (4.1mph) at 200km/h (125mph).

As you can see, it's impossible to have a 100% accurate speedo while reading revolutions of the tyre to calculate the speed. On top of that there is some 'on purpose' error dialed in.

 

A little bit off the topic of this post but the accuracy of speedometers and odometers has me thinking.

There are odometer checks along NSW highways with a post at each km for 5km. Now last time I checked one of these wasn't in the S2000 but keeping a constant speed of 120kmph using cruise control, these posts were going by at 30 sec intervals. I know it isn't the most scientific method of measurement but it indicates that miscalibration of speedometers is perhaps not consistent and widespread. Unless odometer checks (or digital watches) are accordingly miscalibrated.

I do recall that in the S2000, these km checks seemed to be out of whack with the trip meter. I can't remember if it was under or over but by the time 5km had gone by, there was a difference of a few hundred meters.

Sorry for being so vague and ambiguous. I will endeavour to be more informative next time.

 

[QUOTE]Originally posted by DavidM
[B]I found out in a German S2000 article that the S2000's airodynamics coefficient (CD) is 38 with the roof up and 42 withthe roof down. I found the numbers in the S2000 vs the yellow BoxsterS at the Nurburgring and Hokenheim circuits. It's on this site:
http://www.honda-s2000.de/home.html

Anyway, if the CD is 38 for the roof up, then what is the possible top steep for the car? I mean real possible top speed

 

Nothing fishy Sev. The real world is just not a good place for measuring flat ground top speed.

Every magazine test that radar measures top speed for the S2K has come up with a speed in the 150 mph range, +/- 1 to 3 mph. Magazines testing top down have come up with top speeds in the 135 mph +/- range. That jives almost perfectly with the calculated speeds, especially if you remember that the S2K gets a mild cool air ram effect on the intake for a couple more hp at high speed.

The front lip spoiler may help, and I'll bet the rear spoiler really helps (helps smooth air coming off the roof). But the 260-270 kph readings are the result of speedo error, tail winds, downhills, etc, not actual speeds.

UL