Speedometer accuracy?

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Speedometer accuracy?

Post by Raggy on Sat Jun 25, 2016 1:44 pm

I did have 12" (155x70xR12) tyres on my van but have changes them to some nice 13" alloys (165x70xR13). Now didnt expect the speedo to read quite the same but its not showing quite what I expected. Due to the larger diameter it should be showing a slower speed than the actual speed i.e display 50 when I am actually doing 64 but been out and checked using a GPS speedometer on my phone and it tells me I am doing 55 when the speedo shows 60? Opposite to the expected. Checked using various web sites and it should be the reverse. Any ideas why and how accurate are GPS speedometers and should I just drive to the GPS one?
Also while on the subject does the speedo have a stop so that it never reads below 20mph?

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Re: Speedometer accuracy?

Post by dennispenis on Sat Jun 25, 2016 8:14 pm

They are usually show about 10% slower than you are traveling. Did you try the GPS speedo with the smaller wheels on ?

http://m.discounttire.com/dtcs/infoTireMath.do

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Re: Speedometer accuracy?

Post by Raggy on Sat Jun 25, 2016 8:25 pm

Think I did but cant remember how accurate it was and dont fancy swapping wheels again just to check.

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Re: Speedometer accuracy?

Post by bushwhacker on Sun Jun 26, 2016 5:19 pm

Your 12" tyre size is not correct anyway. The original equipment van tyres have a higher aspect ratio than 70 so comparisons are difficult.

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Re: Speedometer accuracy?

Post by dennispenis on Sun Jun 26, 2016 6:30 pm

What should they be?
It dosent state an aspect ratio on the door jar just 155R12-8PR

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Re: Speedometer accuracy?

Post by Logi on Sun Jun 26, 2016 6:34 pm

If no percentage ratio stated it's 80%

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Re: Speedometer accuracy?

Post by Raggy on Sun Jun 26, 2016 6:54 pm

Logi wrote:If no percentage ratio stated it's 80%
Thats very interesting, (and very correct as it seems just found details of the exact tyres Ovation V-02 155/80 R12) as my original (well they were on it when I bought it and they were recently new) didnt have it quoted I just presumed 70. If I use 80 in that website comparison its suggests that my current 13" wheels should show a speed near enough what is actualy being done, for 60 mph on the speedo I would actually be doing 60.8 unfortunately not whats happening. What does yours display Logi as you have the same size tyre dont you?

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Re: Speedometer accuracy?

Post by Raggy on Sun Jun 26, 2016 7:54 pm

Just had a thought, if my new tyres according to all the web sites should show virtually the same speed as being done, maybe its the speedo as last week it stuck then jumped from 50 to 60 (frightened the life out of me), Woody has advised me to lubricate it which will get done so I suppose I'll see what happens after thats been done and check again.

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Re: Speedometer accuracy?

Post by Logi on Sun Jun 26, 2016 9:01 pm

I have 165/70's on the front, 175/70's on the rear, mine's reads pretty close to the speed according to the "Your Speed" warning signs dotted about.

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Re: Speedometer accuracy?

Post by bushwhacker on Thu Jun 30, 2016 6:00 pm

I think you'll find they are 85%.

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Re: Speedometer accuracy?

Post by Woodie on Fri Jul 01, 2016 5:33 am

For once BW I have to disagree with you. My understanding is that tyre aspect ratio is 80% as Logi says. 
We have covered this subject several times now in the past 3+ years I have been on the Forum. I have 185/60/13's on mine and they give the same reading as the 155x12 's. The roadside speed check  signs here bare that out apart from one very tardy one. 

The easiest way to check the difference in your speed is to measure the Rolling Circumference of the two tyres and check the percentage difference of the two  tyre measurements.  Its easy enough.   One has to assume that with the standard 155x12 tyres the manufacturer has calculated the speedo correctly. 

Put a chalk Mark on your tyre and on the ground, then roll the wheel 1 complete revolution and measure the distance. Say for arguments sake the 155x12 is 40".
Do the same with the wider tyre you are using the chalk Mark and say that one covers 44" giving you an increase in distance covered of 4". That gives you a  10% increase in distance covered so your speedo is theoretically 10% slower. 
At   an indicated 27mph approximately, you will be doing 30mph, at an indicated  36mph you would be doing 40mph and so on. 

Using a Sat-Nav is the most accurate way to check out your speed obviously though.

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Re: Speedometer accuracy?

Post by Raggy on Fri Jul 01, 2016 3:44 pm

Checked out my original tyres and dont say anything on the tyre except 155x12C the web returns that they are 155x80x12 confirming the original thought of 80%. Got to agree with Woodie on how to test them but its wether I can be bothered! Using a spare phone for GPS speedo and the thing that started this whole thread off is that the difference accoring to the websites is 1.52% difference and should give a speed of 61mph when the speedo shows 60. Now thats acceptable but mine does not give that, it shows 60 but GPS displays 56! Opposite and bigger difference?

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Re: Speedometer accuracy?

Post by Woodie on Fri Jul 01, 2016 4:16 pm

So your low profile tyres have a smaller rolling circumference that is roughly 10% less than the original 12" tyres, giving you lower gearing and slightly better acceleration, but a bit more fuel consumption depending on how much hard throttle you give it.

At least you will be under all the speed limits if you go by the Speedo.

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Re: Speedometer accuracy?

Post by Raggy on Fri Jul 01, 2016 5:27 pm

Woodie wrote:At least you will be under all the speed limits if you go by the Speedo.
Yes thats the big advantage Very Happy although I'm only thinking about the lower speed limits, dont intend to ever get to the motorway limit in it! Went on the M62 the other day where it splits to go off to the M1 and you have to be in the middle lane... rather un nerving in the rain with all the lorries belting past either side...but she coped well with it. Did think there was an issue as could hear noises....untill I realised it was just the rain hitting the van! Live and learn!

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Re: Speedometer accuracy?

Post by Logi on Fri Jul 01, 2016 6:53 pm

Mine's will sit at 80 without too much aggro on the motorway, eats fuel like a fat bird in MacDonalds tho

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Re: Speedometer accuracy?

Post by Raggy on Fri Jul 01, 2016 7:04 pm

But mines just the 993!

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Re: Speedometer accuracy?

Post by HighlyJetted on Fri Jul 01, 2016 11:15 pm

Those 993's cruise at 80mph really nicely - On the back of a tow truck.

Giggles aside, I find the Hijets speedo to be pretty accurate, especially when the needle is wiggling about like a rattle snake.

When the needle is pointing way beyond the numbers for 90, and the little dashes, you know that your hijet is a good one! It's probably doing about 93mph on the GPS.

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Re: Speedometer accuracy?

Post by LloydAJN on Fri Oct 28, 2016 3:05 am

My speedo seems pretty accurate according to roadside speed limit signs.


Also while on the subject does the speedo have a stop so that it never reads below 20mph?

mine doesnt go lower than 5mph.

Mine goes past the speedo too, only done it once though, just had to try it, i assumed it was 95 or something.

and yeah, i can cruise at 70mpg anymore and the noise is a bit annoying and the steering wheels shakes. but yeah it drinks the fuel hard! i tend to drive it at about 64mph (according to the speedo) seems to be a relatively noticeable difference in revs between 64ish and 66ish so assume its about the most economical speed.

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