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wille

Last night, after washing my filter I went to find out how fast my 33 really goes. Before washing it revved about 5500rpm and didn't go much more. Washed, dryed and oiled, I managed to get sth like 5800 to counter, before I had to slow down. The reason I'm asking this is that my speedo don't want to show numbers above 120km/h, and sometimes when driving at 100km/h (~60mph) it suddenly jumps to 140 km/h. Is there any easy way to calculate how fast I was doing?
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-Ville-

bobbber · Posted: Fri Apr 02, 2004 9:43 am Post subject:

Hi,

I have the opposite problem - my speedo works, but the rev counter is broken Crying or Very sad

I think the thing to do is fix it!!!!

0.854 is the ratio for 5th gear however, and with 185/65R14 tyres

185mm x .60=120.25mm x 2=240.5mm + 355.6mm(14")= 596mm or 23.4" outside diameter

Therefore, for each revolution of the wheel you cover 73.5"...

When the car has Rev's of 5800rpm - the wheels are turning at 5800/0.854... 6791rpm.

Therefore, every hour the car covers 6791*73.5*60 inches!

12 inches in a foot, and 5280 feet in a mile.

SO... (6791*73.5*60) / (12*5280)... phew! = 472 mph!!!!

Thought that was the end for a second but forgot the DiffRatio of 9/35 Exclamation

Therefore, final speed @ 5800rpm with 185/65R14 tyres = 121mph (or 195kph).

Seems pretty fast... I think I've got that right?!?!?>

This ofcourse assumes you have 100% traction at that speed on your clutch, and I'm sure other things I can't think of!!!

Is this correct?!?!?! Anyone else wanna shed some light on this?!?!

Willie, did it seem this fast?!
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Fiat Bravo 165 Sport

Rolle · Posted: Fri Apr 02, 2004 10:04 am Post subject:

Did you mean 185/60 or 185/65? You talked about both there and I don't bother to count Razz

Anyway isn't 185/60x14" the (standard)/right size?

If Wille was running on summer tires they where 195/50x15" and that would make a final speed of 184,4km/h, m'kay?

bobbber · Posted: Fri Apr 02, 2004 10:17 am Post subject:

Yes! I put 60 in the text, but calculated it for 65!!! Laughing

If you say so!!! I'm no expert Exclamation

I make it 189kph with
195/50x15" though...
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Fiat Bravo 165 Sport

bobbber · Posted: Fri Apr 02, 2004 10:23 am Post subject:

Then again... it's been a long time since I worked out any math!!!

My head hurts now! Shocked

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Fiat Bravo 165 Sport

Rolle · Posted: Fri Apr 02, 2004 11:31 am Post subject:

wille · Posted: Fri Apr 02, 2004 11:53 am Post subject:

Well..I think that was fast enough for student anyway Smile

It seems that my made-it-myself-ram-air-whatever works, because the straight was a little too short to catch the "ultimate top" Cool

Got to be satisfied for that, as long as I have a permission to drive.... Rolling Eyes

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-Ville-

bobbber · Posted: Fri Apr 02, 2004 11:54 am Post subject:

This is getting silly!!! Laughing

Surely:

195/50R15 = 195 x .5 = 97.5
97.5 x 2 = 195 (so far!)
15" = 15 x 25.4mm = 381mm

195 + 381mm = 576mm outside diameter... which is 1.809792m per wheel revolution!??????

5800rpm/0.854=6791rpm

6791x1.809792=12290.297472m/min -> 737.41784832 km/h

737.41784832x9/35 = 189.62kph
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Fiat Bravo 165 Sport

bobbber · Posted: Fri Apr 02, 2004 11:56 am Post subject:

Fast enough for me too.... I think my dodgey gearbox would explode at that speed at the moment!!! Laughing

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Fiat Bravo 165 Sport

Rolle1 · Guest

Guest · Posted: Fri Apr 02, 2004 4:15 pm Post subject:

I posted a list of theroetical tops speeds a while ago in the racing thread - I found a mistake in my calculations last week! Some division/multiplication error which resulted coincidentally in some possible values!

Paddy - lazy to log in....

Rolle · Posted: Sat Apr 03, 2004 9:45 am Post subject:

Guest · Posted: Tue Apr 13, 2004 6:00 pm Post subject:

Do you think that speedo is THAT precise in calculating the speed!!! - No it won't know if you put 65 or 60 or whatever may cause mistake. I even think it is pretty incorrect (unless you put exactly the same tires it was made for of course)
The truth is somewhere there Wink

vassil

Rolle · Posted: Tue Apr 13, 2004 7:18 pm Post subject:

Ken McCarthy · Posted: Wed Apr 14, 2004 12:10 am Post subject:

Because of a failed speedo, I worked out speeds Vs rpm from the gear ratios, differential ratio and tyre diameter (for 185/60x14). The gearbox is from a 16V which has slightly different ratios from earlier gearboxes but for this set up 100 kph is 4850 rpm in 3rd, 3700 rpm in 4th and 3150 rpm in 5th.
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Ken McCarthy - 16V Sprint

Guest · Posted: Tue Apr 27, 2004 4:32 pm Post subject:

changed air filter (i am using now BMC air filter instead of standard) and car is going much better..
100mph at 4500rpm, so i am suspecting that 125 is possible now..

Peter D · Alfasud Joined: 15 Aug 2003 Posts: 60 Location: Denmark

Hi

I think that you bother too much with the calculations.

First you cannot calculate the rolling circumference of a tyre by using the size and profile etc. Deformations change the result.

The official circumference for a 185/60-14 from the ETRTO (European Tyre and Rim Technical Organisation) is 1763 mm.

Second: Why do you not use Alfa Romeo's official data, which says 31,8 km/h by 1.000 rpm in 5th gear?

5.800 rpm then makes 5,8 x 31,8 = 184 km/h.

Peter (DK)