Evening all,

 

Since owning my Zed I've been skeptical about the reeding I've been getting from my speedo.

 

I went out for a spin earlier today with my laptop plugged into the OBD port. The laptop showed that my speed was consistently 10 kph lower than what the speedo was saying.

 

Just wondering if anyone has any suggestions, or advice on what to do to fix this (other than drive faster).

 

Thanks

How does it compare to a GPS signal such as a satnav? What wheels/tyres do you have?

Haven't tried it with a GPS/sat nav.

 

As for wheels, they're aftermarket running 235/45/17's all round. I know this would affect the accuracy of the speedo, but shouldn't the OBD port and speedo show the same thing?

How fast were you going when it was 10kph difference? 17" wheels are unlikely to make that sort of change.

 

I believe you can fit different resistors to change the the speedo needle rate.

I might be speculating, but, based on the ZX being RWD, I'd say it's just the rear wheel/tyre sizes that would make a difference. That; and whether the km/h to MPH conversion (if done) was done properly.

 

I have a UK spec car running 265/35/18 tyres on the rear. When comparing my speedo to the GPS, at speeds up to 90mph (not on the public roads - before anyone jumps on that), both read pretty much the same. Thereafter, at some speeds the speedo reads more, at others, they are even; but the difference is no more than 3 mph.

 

I'd also say GPS is more accurate; as surely a laptop plugged into the car would only pick up its reading from the car's own speed sensor anyway?

surely a laptop plugged into the car would only pick up its reading from the car's own speed sensor anyway?

 

This is the thing that has me puzzled... Surely the OBD and speedo should show the same speed, regardless of the tyre/wheel size. As you say, they are both reading from the same source.

 

The speedo has definitely been played around with as it lights up blue and has an 'mph' sticker in the middle of the dial. Despite the sticker, it definitely doesn't read in MPH!

 

When I had the laptop plugged in I managed to check at numerous intervals between 20 and 80 kmh (50 mph). Every time I checked it was reading exactly 10 kmh lower.

This is the thing that has me puzzled... Surely the OBD and speedo should show the same speed, regardless of the tyre/wheel size. As you say, they are both reading from the same source.

 

The speedo has definitely been played around with as it lights up blue and has an 'mph' sticker in the middle of the dial. Despite the sticker, it definitely doesn't read in MPH!

 

When I had the laptop plugged in I managed to check at numerous intervals between 20 and 80 kmh (50 mph). Every time I checked it was reading exactly 10 kmh lower.

In that case i would think that the needle isnt zeroed properly, if it is always a 10kmh interval. If it were wheel size it would get bigger the faster you went.

I think it was groover that done the blue led on the clocks looking back through old threads of this car mate, when 300zx-666 had it

Hi bud, there are two potentiometers (variable resistors) on the speedo board, you can drill a small hole in the top of the dash binnacle and adjust the resistor to recalibrate check here http://www.300zx.co.uk/forums/showthread.php?36601-Speedo-Calibration-Pots

 

NB i may have done the led conversion but dont touch any workings of the gauges or stick MPH stickers on :whistling:

Hi bud, there are two potentiometers (variable resistors) on the speedo board, you can drill a small hole in the top of the dash binnacle and adjust the resistor to recalibrate check here http://www.300zx.co.uk/forums/showthread.php?36601-Speedo-Calibration-Pots

 

NB i may have done the led conversion but dont touch any workings of the gauges or stick MPH stickers on :whistling:

 

I know you wouldnt mate, it just said in one of ians old post that you done the blue led conversion

All cars speedo's are out......don't know in UK but here in Belgium is stated by law for car manufacturers that speedo's must be out +/- 10 kph.

For the UK

The amended Road Vehicles (Construction and Use) Regulations 1986 permits the use of speedometers that meet either the requirements of EC Council Directive 75/443 (as amended by Directive 97/39) or UNECE Regulation 39.[14]

 

The Motor Vehicles (Approval) Regulations 2001[15] permits single vehicles to be approved. As with the UNECE regulation and the EC Directives, the speedometer must never show an indicated speed less than the actual speed. However it differs slightly from them in specifying that for all actual speeds between 25 mph and 70 mph (or the vehicles' maximum speed if it is lower than this), the indicated speed must not exceed 110% of the actual speed, plus 6.25 mph.

 

For example, if the vehicle is actually travelling at 50 mph, the speedometer must not show more than 61.25 mph or less than 50 mph.

 

I go by my satnav speed, this is why you can see so many vehicles travelling at different speeds when in roadworks with adverage speed cameras. You can set your speedo but most people know if their speedo is reading 30, 50 or 70 they will not be done for speeding.

I didn’t expect the speedo to be 100% accurate, didn’t think it would be out by as much as 10 kmh. Seems excessive (in my opinion…)

 

Anyway, turns out I have bigger problems than my speedo…

 

I was driving around yesterday when I noticed a knocking noise that was getting progressively louder. I quickly discovered that it was coming from my front nearside wheel. Went to jack up the car and take the wheel off when 3 of the wheel nuts came off in my hand. Needless to say, this didn’t put me in the best of moods.:cursing:

 

Whilst examining the wheels I noticed that the fronts have been fitted with spacers/shims so that the wheel will clear the callipers. Now I don’t know a lot about this style of spacer, but what I have heard isn’t good. What was clear to me was that with the addition of the spacers, there wasn’t much thread on the studs for the nuts to grip onto…

 

Given yesterday’s revelations I’ve come to the conclusion that wheels have got to go. I don't know if i'm being a little paranoid regarding the spacers and stud length, bug given that 3 wheel nuts were about to fall off, i'm not taking any chances.

That's not good, what a pain, and very lucky could have had a bad accident. I have just purchased extra long studs and spacers so I can put my new front wheels on, not had the time to do it yet.