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| Jim |
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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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| Timmeh |
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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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| Disco Frank |
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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
_________________ RIP SCOTT |
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| provked |
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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.
_________________ 05 BA futura wagon, dumped, 19s, limo tint ( now the wifes car! |
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| Casper |
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or run it up on a dyno.. they can tell you kph.
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| The Dog |
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I would assume (read below!
 ) 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. 
_________________ You'll ride a black tornado across the western sky
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| Sandstone_EB_Fairmont |
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My dad drives past those police speed checker things. Those trailers with the large digital numbers on it.
_________________ -1994 EF XR6 + Factory 5speed = Fun |
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| huggiebear |
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lol steal a cop speed checker and put it in your street lol
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| MikGan |
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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!
_________________ I couldnt fix your brakes so I made your horn louder
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| rushed |
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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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| mr_sperm1 |
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try this - i got it off here, can't remember who made it
_________________ Maloo - Holden's word for ugly. |
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| Casper |
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xhute0 wrote: I would assume (read below!
) 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. 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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| wicksy |
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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
_________________ Now i have a EF XR8 5spd.........mmmmmm 5L |
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| pandaman |
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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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