real speed ?
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real speed ?
I was just wondering, is there a formula to calculate the real speed I was doing, assuming that my rpm meter is accurate ?
For the record: the speedo showed 250 kph and rpm was at 6200
I know that the Impreza speedo is quite accurate, but I'm also pretty sure that at that speed it showed a bit more than what I was actually doing.
For the record: the speedo showed 250 kph and rpm was at 6200
I know that the Impreza speedo is quite accurate, but I'm also pretty sure that at that speed it showed a bit more than what I was actually doing.
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I wouldn't assume your rev counter is accurate, any more than I'd assume that "Impreza speedos are quite accurate". I know my rev-counter over-reads by ~5% compared to a diagnostics PC plugged into the OBDII port. I also know my speedo, compared to a GPS device, reads about 7mph high at a genuine 80mph. You'll be hard pushed these days to find a production car that isn't over-reading by at least 5mph at 3 figure speeds, I'g guess 250kph (~155mph) indicated is probably near to 145mph actual.
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They are inaccurate. I have a Road Angel or i use my Racelogic Box, and my speedo overreads. ALL production cars over read a little, or in our cases a lot.
Mine get worse the faster i go, so what starts out as like 2-3 mph at say 40, becomes easily 5-6 mph out at like 70 or 80 mph.
Mine get worse the faster i go, so what starts out as like 2-3 mph at say 40, becomes easily 5-6 mph out at like 70 or 80 mph.
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So any help on putting it together ? My car is my05 sti, so 6 speed gearbox and tyres are 225. No idea on gearbox ratio, or how to relate that to the 6200 rpm, though...
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I wouldn't assume your rev counter is accurate, any more than I'd assume that "Impreza speedos are quite accurate". I know my rev-counter over-reads by ~5% compared to a diagnostics PC plugged into the OBDII port. I also know my speedo, compared to a GPS device, reads about 7mph high at a genuine 80mph. You'll be hard pushed these days to find a production car that isn't over-reading by at least 5mph at 3 figure speeds, I'g guess 250kph (~155mph) indicated is probably near to 145mph actual.
#11
Id say they would still be someway out...at an indicated 150MPH by UK Turbo was travelling at a genuine 138MPH and when my bike speedo is registering a rather exciting figure of 190MPH im actually travelling at a more disappointing true 176MPH.
Get yourself and your car along to a straightliners day and find out for sure, it doesnt even cost that much money to enter and its great fun.
Get yourself and your car along to a straightliners day and find out for sure, it doesnt even cost that much money to enter and its great fun.
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I make it 225 kph assuming 6th gear and 225/45x17 tyres.
revolutions at wheel per hour =
6200rpm x 60
(6th gear ratio x final drive)
which is
6200 x 60 = 113283.3912
0.842 x 3.9
tyre rolling circumference, when new, is
((2 x 225 x 0.45) + (17 x 25.4) ) x pi = 1.99271m
1000
km/hr = revs per wheel per hour x rolling diameter
1000
which is
113283.3912 x 1.99271
1000
= 225.74 km/hr
revolutions at wheel per hour =
6200rpm x 60
(6th gear ratio x final drive)
which is
6200 x 60 = 113283.3912
0.842 x 3.9
tyre rolling circumference, when new, is
((2 x 225 x 0.45) + (17 x 25.4) ) x pi = 1.99271m
1000
km/hr = revs per wheel per hour x rolling diameter
1000
which is
113283.3912 x 1.99271
1000
= 225.74 km/hr
Last edited by TimH; 14 April 2007 at 04:23 PM.
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I make it 225 kph assuming 6th gear and 225/45x17 tyres.
revolutions at wheel per hour =
6200rpm x 60
(6th gear ratio x final drive)
which is
6200 x 60 = 113283.3912
0.842 x 3.9
tyre rolling diameter, when new, is
((2 x 225 x 0.45) + (17 x 25.4) ) x pi = 1.99271m
1000
km/hr = revs per wheel per hour x rolling diameter
1000
which is
113283.3912 x 1.99271
1000
= 225.74 km/hr
revolutions at wheel per hour =
6200rpm x 60
(6th gear ratio x final drive)
which is
6200 x 60 = 113283.3912
0.842 x 3.9
tyre rolling diameter, when new, is
((2 x 225 x 0.45) + (17 x 25.4) ) x pi = 1.99271m
1000
km/hr = revs per wheel per hour x rolling diameter
1000
which is
113283.3912 x 1.99271
1000
= 225.74 km/hr
Many thx mate!
#14
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Speedos are designed to over read a little. It is illegal for a manufacturer to deliberately make a speedo under read, as you would get caught for speeding when you did not think you were. Expect 5% over read for a good speedo, but that percentage can change. Things can go wildly inaccurate at higher speeds.
eg, mate got pulled in his GTE Nova, he was bricking it as he was "doing 110mph", copper's gun said 92mph
eg, mate got pulled in his GTE Nova, he was bricking it as he was "doing 110mph", copper's gun said 92mph
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My Fiat uno (see worst cars thread) said 92 mph on the clock with my one and only ticket. Copper said 73mph (A rd). This was 8 years ago though.
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