Fibbing Speedo

[bigharty]

I have a speedo with a replacement face ( miles per hour but with odometer in Kms). It is miles out! 8mph! The speedo reads a law abiding 30mph, the TomTom says 22mph. Seems to show the same error at any speed, (e.g. 60mph = 52mph). I know that all speedos read high but this much? It is testing my mental maths at times I can tell you.

I confidently cruised past a speed camera van last week with the speedo showing 36.

Does the odometer share the same input as the speedo? If so then I guess that could be inaccurate too.

Just returned from my first Bongo trip and managed 26.5 mpg from my V6 (500 miles) but these figures will be wrong if the odometer is out.

[Aethelric]

When I replaced the face on mine to a mph one, I wrongly removed the needle. Not a good move. Its a push on fit and can push on anywhere.
So to get it right I used the sat nav and drove around with the cover off and kept stopping and adjusting it until it agreed with the sat nav. Time consuming, but not hard, and almost certainly necessary for the MOT.

[dave_aber]

With speedos, there are 2 factors to consider with calibration. Offset and Gain.

An offset error is where you have the same error at all speeds (as you have).
A gain error is where you have the same percentage error throughout the range (1.e. 10% out, so 1mph error @ 10mph, 10mph error @ 100mph)

The gain error can be caused by incorrect tyre overall diameter etc. No idea if this can be corrected on the bongo speedo, but it would be an adjustment inside, either by turning a pot or changing a component value etc.

Offset error can be adjusted by removing the speedo needle (lever off gently - 2 spoons has been suggested as the best tools for this), and refitting it in a slightly different position. It is a fiddly job - and be aware that there are 2 wee weights inside the black centre boss on the needle which will try to fall out. If you are trying to reduce the needle position, you may have to remove the 0mph endstop to get the needle back on initially in the right place. You might get it lucky first time.

10mph over-reading isn't uncommon on lots of cars. However, no need to put up with it!! I recently fitted a GA5R speedo, so everything reads in miles / mph, and it reads within 0.5mph of my GPS. Distance is within 0.1 mile after a 10 mile run. Since your gain seems right, your odo should be pretty accurate too.

[.co.uk]

My Speedo reads out as well. It is out by about 6mph (66mph on speedo = 60 mph).

[dunslair]

My Speedo reads out as well. It is out by about 6mph (66mph on speedo = 60 mph).

Mine too! I like to know I have a little margin for 'human' error on any speed cameras when I try to follow the speed limits

[Doone]

I was once told that speed cameras allow a plus/minus10% margin, but I don't know if that's true.

[bigharty]

Thanks everyone. Think that I will investigate yanking the needle around a bit. Good tip.

[rwill19050]

Mine reads 8 mph low at 60 (actual speed 52 mph), as before checked against TomTom on a straight run with no hills.
I wondered why people were getting the hump with me when I was doing (I thought) 30 mph, my actual speed being 25 mph.

[David Edwards]

What tyres have you got on it, if you are..like me..running on 215's all round then yes it will show as you have described, just like mine does, a handy safety margin with speed cameras,lol.

[jaylee]

I suspect mine is pretty much lying through its white dialie teeth too..

But, i'm not entirely convinced the sat nav is on the level either...?

Only when i size it up with the "law abiders" passing through the speed traps does it all seem to tally up on the mark?

Mine reads 8 mph low at 60 (actual speed 52 mph), as before checked against TomTom on a straight run with no hills.
I wondered why people were getting the hump with me when I was doing (I thought) 30 mph, my actual speed being 25 mph.

I put that down to the fact i'm in a automatic..? I fancy it's a different style to a gear shift driver.. If you are below the speed limit?? So what!

[splitpin]

What tyres have you got on it, if you are..like me..running on 215's all round then yes it will show as you have described,
just like mine does, a handy safety margin with speed cameras,lol.

If your tyres are 215/65/15 then they are the right size so shouldn't affect speedometer reading. I had standard sizes on ,195 front and 215 rear and speedo was about 7mph over. Have 215's all round now and it is just the same.

[scanner]

Time consuming, but not hard, and almost certainly necessary for the MOT.

The speedo isn't tested for the MOT - it's only the odometer reading that is recorded and if yours is recording in km's you should tell the tester or it will be logged by them as miles and that could affect the value.

[chipvan]

doone,ive been told no,the camera has to be tested,if tested you can get done at 31,my dad did.

he got sent on a driving course,there he was told its MAX speed not 31,32 0r 10% 30=30 when it comes to camers,despite what people make up...

if you do over 34 in a 30 you will not get the offer to remove points in exchange for the one day course

[Rhinoman]

I was once told that speed cameras allow a plus/minus10% margin, but I don't know if that's true.

Usually they are set at some arbitary point above the speed limit, the camera has a degree of inaccuracy so the trigger point must be set above that. Personally I would have challenged a camera reading of 31mph, based on my prior experience designing radar systems.
The law regarding speedos is a little complex but it is illegal for a speedo to read low so manufacturers set them a little fast to guarantee that with manufacturing tolerances and tyre wear the speedo will always be legal, 5-10% high seems to be the norm. It therefore follows that a low reading speedo would not be accepted in mitigation.

[Alacrity]

I put that down to the fact i'm in a automatic..? I fancy it's a different style to a gear shift driver..

How does being in an auto change what speed you are doing - or feel like doing??? I would guess the 12 - 16% of people who get done for speeding (camera's or otherwise) will have been driving automatics as that is appropriately the percentage of automatics on the road.

My Bongo is auto & it has been known to break most speed limits at some time or other!

Nothing I hate more than a long que of cars behind an ignorant dawdling driver that won't ever pull over to let the que past. Living in the super congested retirement area that I do those of us that want to 'get on with it' spend our lives in this situation. Why can people drive at the posted speed pleeeese. Or pull over every now & again as I used to do when driving my 1919 Model T - I usually ended up catching the que up again though with it!!

Rant over . . .