Oversized tyres and trip computer inaccuracies

[Kev Stephenson]

at what speed Garth. You are not saying that when you are doing 5mph the speedo says 20 mph and the same 15mph applies when you are doing 70mph.

 

Steve, you usually find the speedo underreads by 10%ish on standard tyres, on oversized tyres (unless you go rediculous) the speedo usually reads dead accurate. The MPG on a Navara is calculated using the speedo reading so would change by about 10% (showing better economy) if no other factors influenced the MPG. If you are wanting to find out how much the oversized tyres is reducing your MPG then take your old MPG and add 10% (rolling radius adjustment), any reduction on this is due to the new tyres.

 

Note the 10% is an approximate figure. You can calculate or measure an accurate fiddle factor by comparing the rolling radius of the new and old tyres.

 

Hope that helps

 

kev

[garth mc garth]

my tyres are 37x12.50x15 so that quite an increase over standard size, i say the 15mph was at approx 45mph and increases as i get faster

[penfold]

if you're caught speeding,you can ask for a ten percent leeway apparently.

[Dean Lofthouse]

if you're caught speeding,you can ask for a ten percent leeway apparently.

 

10% + 3

 

ie 70mph + 10% + 3 + 80mph

 

dunno about the legality

 

What you can ask for is the calibration cert

[Jonesie]

10% + 3

 

ie 70mph + 10% + 3 + 80mph

 

dunno about the legality

 

What you can ask for is the calibration cert

 

I got done by a camera, 3 points for doing 36 in a 30 zone.

[Dean Lofthouse]

I got done by a camera, 3 points for doing 36 in a 30 zone.

 

Should have asked when the camera was last calibrated

 

I was told by a traffic cop the hand held are supposed to be calibrated before each use.

[educated arborist]

I got done by a camera, 3 points for doing 36 in a 30 zone.

 

a good friend of mine works at speed procesing centre bristol (ex-friend once he took the job)

 

It depends on the method by which you are caught, if the method relies on a calibrated speedo eg a cop following you with a camera or car mounted laser then it can be argued in court that his speedo calibration is as much as 10% out hence you often find, as i have, that a coper will do you for 90/99 on a motorway unless he manages to clock you at 110+. (different charge over 100) where as certian cameras are far more acurate, best friend just done for 43 in a 40.

 

HOWEVER

in a thirty zone you cannot be prosecuted for any less than 36 dont ask me y but u cant. also if your between 36 & 40 you'll usually get the option (if its 1st speeding offence) to go on a speed choice workshop but over 40 u don't get the option.

 

as for over sized tyres, it depends on the OD of your tyre (overall diamiter)

the best system of tyre sizing is american: 29/9.25x15 meaning 29"overall diamiter 9.25" wide on a 15" wheel

those pr**ks in europe decided that 235/75 r15 was better though but this means a tyre that is 235mm wide with a sidewall height that is 75% of the width on a 15" wheel

they really do over complicate things:mad1:

the above sizes are for the exact same tyre so with a european sizing you have to multiply 235mm by 75% then by 2 then add the wheel size to get the OD.

 

now work out the od of the standard/previous tyres with wich the speedo was corect then the new ones then find the %increase in OD then you know what % your speedo will overread by.

 

most speedos (not all) over read by 3-10% so you could actually correct an under reading speedo if not you can add a specially designed resistor of the correct rating between the reader & dial to correct it or on certian veicles you can simply change the cog which the sensor reads off for one with the relevent number of teeth.

 

i really should start a pib quiz team but know sweet fa about sport and tv:laugh1:

[treedave]

If you put your old & new sizes in this, it'll give you various measures inc. % changes on your speedo and revs per mile, which may help tweek your mpg figures.

 

http://www.1010tires.com/TireSizeCalculator.asp

 

Steve,

I'm shopping for a newer 4x4 - what made you go for the navara?

[Stephen Blair]

a traffic cop told me years ago most cars are 10%out, not cop cars because the speedometer is off the flywheel??

[educated arborist]

a traffic cop told me years ago most cars are 10%out, not cop cars because the speedometer is off the flywheel??

 

if u see him again give him some **** it dont matter where your speedo reads from & if it read from the fly wheel it would be the most inacurate ever, every time u dipped or dipresed the clutch the speedo would read 200mph or whatever + when the clutch is engaged it would need a second sensor on the gear shift to know what gear youre in, what a nob

 

If he pulls u again & your sober, when he asks you " have u had a drink 2nite sir?) say yes but i'm fine to drive i've only had about 2 pints and a couple of shots in the last couple of hours & anyway those breath detectors don't work on me

 

he'll be there for ages getting more & more pissed off as you blow numerous readings of 0 saying "see i told u they don't work on me"

 

This is my fav party tric lol:w00t: