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.

 

 

helpful reply mate

 

I wouldn't have thought a formula would work as things would vary from car to car. How about trying to use some kind of GPS navigator to calibrate your speedo?

 

You can work it out from your gearbox ratios and the circumference of your tyres. That should give you a value that is accurate to +/- 1 or 2km/h.

 

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.

 

Everybody knows they are way innacurate - that's all

 

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.

 

yes, I thought it is possible. Well assuming a bit of error from the rp meeter I think you are right, it could be quite accurate, which is good enough for me
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...

 

Your guesses might be right, thx for the help.

 

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.

 

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

 

wow that I call excellent computational skills, just what I was looking for.
Many thx mate!

 

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

 

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.

 

Sorry it's not as mathematical as Tims reply , but the 2005 brochure says " 24.5mph per 1000rpm" so 24.5 x 6.2=151.9mph "" IF "" your rev gauge is correct.

 

sounds good

 

odd my jdm sti with the converter fitted reads bang on gps speeds