is my speedo out by 7mph?

S

starletsportif

Member +
hello. i need to know can my speedo be out? by 7mph. my mate in his 106 vtr had a digital reader what plugs into your lighter hole. and when i was doing 105 he was doing 98? well thats what my mate thinks when i looked and i was doing 100 so can it be out by 7mph?

can it even be out 7mph. only way im going to find out is to get my car dynoed.
 
AdDaMan

AdDaMan

Member +
LOL the sat navs arent out! there you actual speed. nearly all car speedos are out and yes 7mph at 100 is defo likely. GPS speed that stanavs show is your 'actual' speed.
Pretty much every car ive had has been out between 3mph-10mph at 100. My bimmer however is bang on which is weird.
 
sanchez

sanchez

Member +
so what if your going through a camera at what you think is 30mph and its actually 37mph? flash flash
 
addz

addz

Member +
My perception is yes when you change wheels, age etc cars generally read wrong on speedo. Also I always believed GPS devices gave very good readings, if not nigh on exact - could be wrong
 
W

Wills21

Member +
Is it possible they could read 100mph when actually doing 80mph?
Reason I'm asking is because even doing 60mph I'd swear (and other people that are in the car) that I am doing around 50mph
 
ianmcintoshi

ianmcintoshi

Member +
i always thot that most speedos are out by around 10% so the faster u go the more u will be out., ie if ur speedo say 30mph u could be doing anything between 27 and 33 but if ur doing 100mph you could be doing anything inbetween 90mph and 110mph. so it get worse the faster u go.
 
AdDaMan

AdDaMan

Member +
thats right. there always out more the faster you go.
And starletsportif if you want to know your exact speed get a sat nav in the car and stay at a steady speed and it will tell you your exact speed.
 
Dan3SGTE

Dan3SGTE

Member +
Adam is 100% right. Your speedo will always show a higher speed.

Pretty much all cars ive tested have been around 5-8MPH out at 100MPH. My EK9 on the other hand is pretty much bang on right...... God knows why but at 100MPH im doing 99-100MPH Sat Nav reading. So when im doing 150MPH i must really be doing near/if not that :haha:
 
P

Paul_JJ

Member +
Dan3SGTE said:
Adam is 100% right. Your speedo will always show a higher speed.

Pretty much all cars ive tested have been around 5-8MPH out at 100MPH. My EK9 on the other hand is pretty much bang on right...... God knows why but at 100MPH im doing 99-100MPH Sat Nav reading. So when im doing 150MPH i must really be doing near/if not that :haha:
Click to expand...

The Gps would be the best way to check your speed! It is in fact very accurate!

If you have different to standard alloy wheels - you will have different speed then it should be.

When I've bought my car I had 4 mph less on the speedo than it should've been. So I was doing only 30 but in real it was 34. However, after cleaning the rear calipers, the speedo is accurate - I can't really explain that, but it's true and the car is exactly the same as it was and the gps is the same!
 
Dan3SGTE

Dan3SGTE

Member +
Paul_JJ said:
The Gps would be the best way to check your speed! It is in fact very accurate!

If you have different to standard alloy wheels - you will have different speed then it should be.

When I've bought my car I had 4 mph less on the speedo than it should've been. So I was doing only 30 but in real it was 34. However, after cleaning the rear calipers, the speedo is accurate - I can't really explain that, but it's true and the car is exactly the same as it was and the gps is the same!
Click to expand...


I know, that's what I'm saying. GPS is the correct speed not your speedo, but on my ek, the speedo is bang on, I'm using identical wheel/tire size specification as factory. Most factory cars I've been in are out too.

I know overal rolling radius effects it. You'll be surprised how much you can actually extend your gears through wheel size too. That's another discussion though.

Strange how your speedo read lower, pretty dangerous if u ask me.
 
P

Paul_JJ

Member +
Dan3SGTE said:
I know, that's what I'm saying. GPS is the correct speed not your speedo, but on my ek, the speedo is bang on, I'm using identical wheel/tire size specification as factory. Most factory cars I've been in are out too.

I know overal rolling radius effects it. You'll be surprised how much you can actually extend your gears through wheel size too. That's another discussion though.

Strange how your speedo read lower, pretty dangerous if u ask me.
Click to expand...

Well, strangely enough I have a spot on readings now and 195/50R15 wheels!

So how does the wheel changes the gear ratio???? Bigger wheel - bigger ratio?
 
S

shizerproject

Member +
not all sat navs are accurate which is why it says in the manuals that it may not be your exact speed as they have to cover themselfs from lawsuits from people getting speeding fines and saying ohhh my sat nav told me i was doing this speed. especially on the older ones and you can test this if you have two diff sat navs as i did and got diff readings only by a 1-2 mph but it shows that there not all accurate as if they were they would have both been bang on the same... propper sat speed mapping units are but they cost a fortune. but that is why the police have to give you 10% as most speedo's are inaccurate to no more than this or atleast they shouldnt be.
 
P

Paul_JJ

Member +
shizerproject said:
not all sat navs are accurate which is why it says in the manuals that it may not be your exact speed as they have to cover themselfs from lawsuits from people getting speeding fines and saying ohhh my sat nav told me i was doing this speed. especially on the older ones and you can test this if you have two diff sat navs as i did and got diff readings only by a 1-2 mph but it shows that there not all accurate as if they were they would have both been bang on the same... propper sat speed mapping units are but they cost a fortune. but that is why the police have to give you 10% as most speedo's are inaccurate to no more than this or atleast they shouldnt be.
Click to expand...

Well, I am pretty certain my sat nav is accurate - may be it has 1-2 mph infelicity, but according to the speed cameras which shows your speed - I do have very accurate readings as the speedo, gps and the camera - show me the same speed!
 
L

Lawrence

Member +
This is interesting thread as i brought a sat nav and my speedo is miles out even at 30 it reckons im doing 25 on the sat nav.
 
Dan3SGTE

Dan3SGTE

Member +
Paul_JJ said:
Well, strangely enough I have a spot on readings now and 195/50R15 wheels!

So how does the wheel changes the gear ratio???? Bigger wheel - bigger ratio?
Click to expand...

Thats not strange, every car is different.

Well like i said your changing the rolling radius so your your shaft is now doing a larger cycle per rev so a larger cycle will/does effectively give you a bigger gear. I proved and tested it with the amount of wheels ive gone through.

Ive gone from 14'/15'/16' with my starlet and from 14/60 to 16/50 i got a MASSIVE difference in in gearing like a true 7-9MPH more in 3rd. it went from 83-85MPH 3rd to around 92MPH. Verified by Satellite. You could feel that it had lost lower down power/response due to the longer gear.

shizerproject said:
not all sat navs are accurate which is why it says in the manuals that it may not be your exact speed as they have to cover themselfs from lawsuits from people getting speeding fines and saying ohhh my sat nav told me i was doing this speed. especially on the older ones and you can test this if you have two diff sat navs as i did and got diff readings only by a 1-2 mph but it shows that there not all accurate as if they were they would have both been bang on the same... propper sat speed mapping units are but they cost a fortune. but that is why the police have to give you 10% as most speedo's are inaccurate to no more than this or atleast they shouldnt be.
Click to expand...

Interesting what you say. But 1-2MPH isnt really worth saying its inaccurate. Such a small difference your still doing pretty much the same speed so accurate enough.

Lawrence said:
This is interesting thread as i brought a sat nav and my speedo is miles out even at 30 it reckons im doing 25 on the sat nav.
Click to expand...

Thats normal, My van is about 4MPH out at only 30MPH. Most cars ive driven are about 3MPH out at 3MPH. There are exceptions though.
 
You must log in or register to reply here.
Top