#1: Speedometer Accuracy Author: Mr-Tigger, Location: EssexPosted: Fri May 18, 2012 12:13 pm ---- Not sure if its due to the change of alloy/wheel size but im pretty sure it is,my speed reads around 20% out (too high) which i THINK is also illegal/mot failure currently. Is the only way to correct this by paying an extortionate rate for a garage to plug into the ecu and reset the wheel size parameters?
Just wanted to check this before i go in and do it incase the guys taking me for my pay-cheque
Oh,its a 206 quicksilver as the sig says lol
#2: Re: Speedometer Accuracy Author: marc206, Location: Bourne, LincsPosted: Fri May 18, 2012 12:16 pm ---- well i got 17inch on mine and i do the speed limit past speed cameras and never get flashed
#3: Re: Speedometer Accuracy Author: -Dave-, Location: North ManchesterPosted: Fri May 18, 2012 12:26 pm ---- Mines 5mph out. No issues with MOT and I just adjust my speed accordingly. Never bothered me so wouldn't pay out the money to get it sorted.
#4: Re: Speedometer Accuracy Author: Andrew, Location: Black Country Ay IPosted: Fri May 18, 2012 12:30 pm ---- Very useful tool: www.alloywheels.com/Tyre_Calculator
Use 175/65/14 as the base size, and whatever your alloys are in the new
#5: Re: Speedometer Accuracy Author: Mr-Tigger, Location: EssexPosted: Fri May 18, 2012 12:30 pm ---- 5mph aint too bad,but 20% out at 70 is a big difference. i can compensate easilly enough,but in turn its also bumpin up the mileage on an already high mileage pug. just wondered if there's another way
#6: Re: Speedometer Accuracy Author: spike_202, Location: West midsPosted: Fri May 18, 2012 12:32 pm ---- Do they even check your speedo on an MOT as they don't drive the car do they. I heard that a sat nav speed is just as accurate as the one on the car?
#7: Re: Speedometer Accuracy Author: Lee, Location: EnglandPosted: Fri May 18, 2012 12:43 pm ---- GPS is a lot more accurate than the speedo on constant speed. Speedo is more accurate on harsh acceleration.
Them calculators are inaccurate and shouldn't be trusted.
#8: Re: Speedometer Accuracy Author: Andrew, Location: Black Country Ay IPosted: Fri May 18, 2012 12:48 pm ---- Really? Because when I changed my alloys and comapared the speedo change to my GPS it was on song with the calculator I posted.
But of course I'm an idiot and that calculator should be ignored...
#9: Re: Speedometer Accuracy Author: Lee, Location: EnglandPosted: Fri May 18, 2012 12:49 pm ---- Depends on your speed sensor
#10: Re: Speedometer Accuracy Author: spike_202, Location: West midsPosted: Fri May 18, 2012 12:54 pm ---- Do you have a sat nav that has a speedo and compare it to that
#11: Re: Speedometer Accuracy Author: Ash, Location: Running from Ant and Lee and Adam...........Posted: Sat May 19, 2012 5:24 am ---- MOT doesn't check speedo because they don't drive the car.
#12: Re: Speedometer Accuracy Author: Jaeger, Location: UKPosted: Sun May 20, 2012 9:50 am ---- Well atm I have 195/55 15'' and when I do 70MPH on the speedo the GPS shows I'm doing 66-67MPH.
#13: Re: Speedometer Accuracy Author: Spiky, Location: CardiffPosted: Mon May 21, 2012 12:47 am ---- thats normal for the speedo to over read
#14: Re: Speedometer Accuracy Author: spike_202, Location: West midsPosted: Mon May 21, 2012 8:19 am ---- That's sounds about what I get.
#15: Re: Speedometer Accuracy Author: Mr-Tigger, Location: EssexPosted: Sun Jul 08, 2012 10:27 am ---- yeah,that caluclator doesnt work for me,it says speed on 30mph then real speed is 29.59mph,when actually it closer to speedo on 30 then real speed is 24-25 lol,and it gets bigger as you go
#16: Re: Speedometer Accuracy Author: kandlbarrett, Location: SwindonPosted: Sun Jul 08, 2012 5:16 pm ---- 20% suggests you have a much bigger problem. While the calculators are not spot on thta is due to manufacturers slight differences NOT the calculator as the maths is simple.
196 / 65 / 14 tyre has the following diameter:-
Convert 14 inches to mm = 25.4 x 14 = 355.6mm
195 / 65 tyre has a wall hieght of 195 x 0.65 = 126.75mm
there are two lots of wall hieght in the overall diameter so the overall tyre / wheel combo diameter is:-
The calculator results may not be spot on but they are within a few %. If you are 20% out then either you are using the calculators incorrectly or you have another problem.
NOTE: most speedos overread by between 5 and 10% (indicated 30 = real 27) so any change the calculator gives needs to factored onto the original innacuracy. 20% still suggests another problem though!
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