Had to travel to Bath the other day to visit my bro and decided to borrow a tom tom gps from work.. Had it set to show my speed and to my amazement when my speedo was reading 60, the tom tom showed I was doing 76Mph. When I was doing 70 the tom tom showed 89Mph!!

 

Even worse we're I would normaly cruise at 80 on a duel carrage way or motor way the tom tom showed 106Mph!! I know i've got 18" rims on the back so can appreciate the rolling radius will afect the in car speedo to a certain extent but this is way out!!

 

No wonder when I was sticking at 70 and the police pulled me recently when it was wet to inform we I was border line to weather they would have me or not, now it all makes sense! Just wondering if it under reading till it gets to a certain speed then corrects it's self but the needle seems to be loosing a few MPH the faster I go!

 

Just makes me think now the times I have done 120, 130 or even 140 what speed the Z was actually doing... Scary....

At a constant speed with say 3 or more sats locked on they are very accurate!

Thats odd John, normally the speedos are 10% too high as standard. ie 100mph on the clock is really 90mph and so on. I've never seen a speedo reading that much lower than the actual speed, even with bigger rims.

did you reset it for your car, apperently you cant just swop from car to car withouit doing a run

the tom tom?

 

Its doesnt make any difference wether your running with it, falling down the stairs, driving, it will be right as long as the speed is constant and the signal is good!

sorry i may have been thinking road angel

I tested a RoadPilot MicroGo(camera warning unit). I tested in my Scenic which has digital dash, found that approx 30mph it was 1 mph different and at 60mph about 2mph. Cant remember which was faster/slower, the speedo or RoadPilot. Was told the GPS should be more accurate

 

Barry

Increased rolling radius of the driven wheels will cause the car to travel further for every wheel revoloution - resulting in the speedo under-reading the vehicle road speed.

 

 

 

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road angel is the same, i have both lol

Increased rolling radius of the driven wheels will cause the car to travel further for every wheel revoloution - resulting in the speedo under-reading the vehicle road speed.

 

That's the outside radius of the tyre, not just the wheel, so should make little or no difference in Jon's case.

my tom tom is spot on to my speedo .

btw have you had the dials converted if so the calibration may be out and may need to be adjusted via the pots on the back of the speedo.

its a gps, it isnt affected by wheel radius

Tyre profiles are lower on 18's, so most likely the speedo is out of cal.

Thats odd John, normally the speedos are 10% too high as standard. ie 100mph on the clock is really 90mph and so on. I've never seen a speedo reading that much lower than the actual speed, even with bigger rims.

 

Yeah that's what i always thought, I drive my boss's Mondeo a lot and when your sat at 70 cars are still flying past. I know my speed sensor plays up everytime it rains or it's wet, the power steering goes heavy and the speedo just sits at 0mph.. Maybe it's all realated somewhere and the dash isn't getting enough signal???

btw have you had the dials converted if so the calibration may be out and may need to be adjusted via the pots on the back of the speedo.

 

Nah, it's a UK Spec Z so should be ok as standard.. Might try adjusting the pots though, didn't think of that! ;)

What size tyres on your 18s

What size tyres on your 18s

 

285/35.. When sat next to my Brothers M3 from ground surface to the top my overall wheels look bigger and his M3 sits on 19's but there only 265/30's...

Thats not to bad, must be the speedo thats out.

What Tom TOm was it??

 

I have just got a 'one' and im fitting new 18s tomorrow, can i use it to calibrate the speedo?? I haven't seen a screen where it gives out your speed.

What Tom TOm was it??

 

I have just got a 'one' and im fitting new 18s tomorrow, can i use it to calibrate the speedo?? I haven't seen a screen where it gives out your speed.

 

It should display speed at all times, if not then you need to turn it on under teh preferences.

Ah cool thanks,

I believe the GPS systems are accurate. I have a Roadpilot Camera Warning system mounted to the top of the dash - far right, and I use it as the speedo... And, with the conversion on the Jap clocks (97NA) it's good up to about 80 when the clocks start to read a tad slow.

 

Good luck

 

Jack