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How fast em i really going??? 

 

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 Post subject: How fast em i really going???
Posted: Mon Oct 02, 2006 1:17 pm 
Parts Gopher
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Location: Wollongong
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Hey, i'm worried thst my speedo is hellishly out. like when mines reading 100, i keep up with most of the traffic... and what concrened me over this weekend, driving at 100 on the freeway, i was passing people... now i'm on my green p's... so yeah its nice to know when your passing a cop car at 115, on a double dermits weekend when u think your doing 105.

I've know for a while the speedos a bit out, but yeh... not sure exactly how far. like yesterday i passed an eb with a digital dash, he was reading 97, i was read 92... and i was passing him.

i tried the calculations but the numbers seemed funny.

like i run 205/65/15, with a 1:2.77 and a T5 with a 0.7 over drive.

so the rims are 38.1 cm. the walls of the tyres are 205*0.65*2 = 26.65cm. total 64.15cm... now by pie. 2.015 m or 0.002015km.

so when i plug through those numbers at 100km/h... i get ~1600rpm.... which when it reads 100 its doing just under 2000rpm. and when i run 2000rpm through those numbers i get 124km/h.... now somethings not quite right... did i get a measurement worng??? does anyone have the same gears as me?


cheers,
jim
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Posted: Mon Oct 02, 2006 1:27 pm 
Getting Side Ways
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Jaycar has a speedo calibration kit.
Cant find the link at the moment. Browse through their website.

 

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Posted: Mon Oct 02, 2006 1:59 pm 
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bets idea find a mate with a new car ie 3-4 yrs old
or a car who u know's speedo id correct

and find a straight bit of rd and get him to do 60

then 70, and 80 etc up to 110 and then u can see how far your spedo is out

 

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Posted: Tue Oct 03, 2006 8:59 pm 
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new cars are nomally close but the best way is to try and get hold of a GPS some of them have actual rd speed, mine does anyway.

 

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Posted: Tue Oct 03, 2006 9:01 pm 
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or run it up on a dyno.. they can tell you kph.

 

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Posted: Tue Oct 03, 2006 9:19 pm 
Getting Side Ways
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I would assume (read below! :lol: ) that the calculations are innaccurate because they don't take into account rolling resistance etc... Best bet (without too much hassle) is line up a mates car as above. 8-)

 

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Posted: Tue Oct 03, 2006 10:05 pm 
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My dad drives past those police speed checker things. Those trailers with the large digital numbers on it.

 

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Posted: Tue Oct 03, 2006 10:29 pm 
Getting Side Ways
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lol steal a cop speed checker and put it in your street lol

 

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Posted: Wed Oct 04, 2006 9:28 am 
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now thats some thinking! but yeah get it on a dyno, get a tune a power run and get ur speedo checked out at the same time!

 

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Posted: Wed Oct 04, 2006 11:52 am 
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i checked my speedo while on the dyno (i swear my car lives there now :( ) as the dynos speed is dead on.

my car speedo reads 107 when the wheels are doing 100

 

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Posted: Wed Oct 04, 2006 1:35 pm 
Getting Side Ways
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try this - i got it off here, can't remember who made it :oops:

 

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Posted: Wed Oct 04, 2006 2:17 pm 
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xhute0 wrote:
I would assume (read below! :lol: ) that the calculations are innaccurate because they don't take into account rolling resistance etc... Best bet (without too much hassle) is line up a mates car as above. 8-)

LOL, your sig is right.
Rolling resistance really has nothing to do with measuring speed on a dyno. Your wheels drive the rollers and, so long as they are not "slipping" on the rollers which is very unlikely, its simply a matter of measuring the number of revolutions the roller does in a set time frame. Since the rollers circumferance is fixed and time is fixed... distance over a set time can be mesured. Convert that time to equal 1 hour and multiply the distance by the same factor and you have kph (or miles per hour, or even metres, yards, feet or inchs per hour)

 

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Posted: Thu Oct 05, 2006 3:46 pm 
Getting Side Ways
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huggiebear wrote:
lol steal a cop speed checker and put it in your street lol


i have been tempted to tow one of them somewhere else up a backstreet lol

 

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 Post subject: Re: How fast em i really going???
Posted: Thu Oct 26, 2006 2:18 am 
Fordmods Newbie
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Jim wrote:
Hey, i'm worried thst my speedo is hellishly out. like when mines reading 100, i keep up with most of the traffic... and what concrened me over this weekend, driving at 100 on the freeway, i was passing people... now i'm on my green p's... so yeah its nice to know when your passing a cop car at 115, on a double dermits weekend when u think your doing 105.

I've know for a while the speedos a bit out, but yeh... not sure exactly how far. like yesterday i passed an eb with a digital dash, he was reading 97, i was read 92... and i was passing him.

i tried the calculations but the numbers seemed funny.

like i run 205/65/15, with a 1:2.77 and a T5 with a 0.7 over drive.

so the rims are 38.1 cm. the walls of the tyres are 205*0.65*2 = 26.65cm. total 64.15cm... now by pie. 2.015 m or 0.002015km.

so when i plug through those numbers at 100km/h... i get ~1600rpm.... which when it reads 100 its doing just under 2000rpm. and when i run 2000rpm through those numbers i get 124km/h.... now somethings not quite right... did i get a measurement worng??? does anyone have the same gears as me?


cheers,
jim


You've got your numbers dead right, just checked em came up with the same. At 100km/h in fifth your engine is doing 1600ish rpm. So when your speedo says you're doing 100km/h @ around 2000rpm you are actually doing 120ish. Might wanna ease off the lead foot a bit until you get a corrected gear for your speedo drive.

You've gotta remember that T5 equipped E series, AU's etc. all do 100 in fifth gear at around 2000rpm, and they use 3.23 or 3.45 diff gears! Your 2.77 is way taller, designed to be used with the 1:1 top gear of a BW auto! So obviously at 2000rpm you're going to be doing more than 100!
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