how can I reduce speedo display after fitting 15" wheels?

[bugadifino]

G'day all,

I have 15" mags on the ute, I'm visiting Melbourne at the moment. Drove from Geelong to Melbourne & somewhere they have a speed display under a bridge. My speedo said I was going 90km/hr, but the display said 101km/hr. On the way to Melbourne I got done by a speed camera at what I thought was 110km/hr but now I know it will have been more like 120 or so.

I reckon this is due to having 15" on it, is there anyway of screwing with the manual g/box like in old holdens by fitting a different sized speedo gear? Or are they electronic & can't be changed?

[Joey_21]

those things that test ur speed are not very accurate but you mention how u got caught have u played with the dash display at all mayb ur speed dial is off

[Disperse_daredevil]

My initial thoughts were that the ecu send the speedo information, but dont hold me to that. I lean toward this because when you use the aldl plug from the ecu to a computer it displays your speed.

As for changing it, i have no idea. I was wondering the same thing

[immortality]

the speed is done electonically, best to pm dr bob to get an answer on how to fix it

[1991_Vn2nV]

G'day all,

I have 15" mags on the ute, I'm visiting Melbourne at the moment. Drove from Geelong to Melbourne & somewhere they have a speed display under a bridge. My speedo said I was going 90km/hr, but the display said 101km/hr. On the way to Melbourne I got done by a speed camera at what I thought was 110km/hr but now I know it will have been more like 120 or so.

I reckon this is due to having 15" on it, is there anyway of screwing with the manual g/box like in old holdens by fitting a different sized speedo gear? Or are they electronic & can't be changed?

Unless you are running truck tyres mate it wont make a real difference. Bigger rims = Smaller tyres which means the overall rolling diameter of the wheel remains the same, so the speedo stays reasonably accurate. If your tyres are the correct profile it should be fine.

It was most likely a faulty display, but even a brand new car will generally show 100 when in fact its doing 97km/h as the speedo's are never 100% correct (this is why taxi's are legally required to have the speedo changed from factory using a tech 2).

If there really is an issue, you can get a speedo corrector unit from Jaycar Electronics to fix the problem, its just a kit you make yourself using a series of resistors to alter the signal to the dash.

[bugadifino]

G'day all,

Thanks for your input, I have recently fitted a new carbon fibre look dash. The odometer reads now 3,127km so it is a brand new speedo head, I rfeplaced the whole dash assembly as one. The ute is a manual not auto, does this make a difference?

[Not_An_Abba_Fan]

You will need to get a certified sticker to display the original kms as what you have done is illegal by changing the dash and not either altering the odo to read the the same kms or displaying the original kms.

[pr0n]

my car has stock 15's but in the next week or 2 i will soon be rollin on 17's or 18's, will my speedo be out?

[pandaman]

my car has stock 15's but in the next week or 2 i will soon be rollin on 17's or 18's, will my speedo be out?

Depends what size tyres you use, if you use 235/45/R17 or 235/40/R18 or another equivalent, you're speedo will only be out by a tiny (unimportant) bit. If you go some weirdarsed size, then it might be out. Anyway, it's easy enough to work out in excel or if you're lazy, just google "tire size calculator".

[bugadifino]

I have 65 series tyres fitted. Therefore, the height of the wheel has increased by an inch as 65 series are standard fitment on a 14" rim. Other than fitting say a set of 55 series tyres is there no other way to reduce the read out?