Understating speedo.

[Donkey]

I'm no mechanic but in my experience speedometers have always overstated your actual speed. Well, I have one that understates my speed by around 10% across the range as compared with our satnav. and that's a little bit worrying!

Any one any thoughts/observations?

Donkey

[mole42]

Depends whether you have the correct diff ratio and what wheels & tyres you are running....

[Donkey]

Thanks Richard,

I made made my original query based on the assumption that every thing is as originally specified. The wheels and tyres are correct and apart from the previous owner replacing a "quill shaft" bearing (?) and some maintenance on the back plate, there's no record of anything else carried out.

Donkey

[Lokisdad]

Mine's the same - about 10% under reading. I have 185/80 x 14 tyres and as far as I know a bog standard diff. I tend to rely on the sat nav or a speedo app on the phone.

Harvey

[V Mad]

We should swap. Mine reads over.

[Radenac]

Thanks Richard,

I made made my original query based on the assumption that every thing is as originally specified. The wheels and tyres are correct and apart from the previous owner replacing a "quill shaft" bearing (?) and some maintenance on the back plate, there's no record of anything else carried out.

Donkey

Hi Donkey,

Just out of interest is the speedo MPH or KPH - could this make any difference?

Rob

[Donkey]

Hi Rob,

Both measurements were MPH as was the original flagging by my mate in his car following on the Auto route.

Looks like Harvey and I have strange ones.
I have e-mailed Don Maxwell to see if he can throw any light on the situation.

Donkey

[Ian F]

Donkey

I would suspect that either the speedo head has been replaced or the diff may have been swopped for a 3.45 : 1 diff?

I have a manual/overdrive with a 3.45 diff and, guess what, the speedo under-reads ( or at least under read) by 10 %. I have since replaced the wheels and tyres with 205/55-15 and it now "only" under-reads by about 5%. So I would suspect the diff has been changed or the tyres are oversize?

Ian

[Donkey]

Thanks Ian,

What a can of worms this is!

I suspect there'll be a few Stag owners doing speed checks this weekend.

Donkey

Well officer, my speedo read 50, honest!

[Radenac]

I was wondering whether it might be a speedometer conversion to km, but no joy there. I can never see the little km figures anyway, so I often go by the rev counter - it is much easier to read!

The good thing is that round here I haven't seen a gendarme in months and there are not that many cameras either thankfully.

Rob

[Rubce]

My Stag under reads by about 10% and has done for the past four years ever since I fitted a 3.45 diff.

Regards

Bruce

[stagling]

Hi Donkey most classic cars have speedos that read slow by about 10 mph. this is down to old age and can only be cured by fitting a recon speedo

[v8ken]

Just bend the needle a bit,,,ha ha.Or put a ''return set''in it,,as I used to do with pipework ,in my plumbing days.
I have an old spare speedo and I had read somewhere that you could remove the needle ,turn it slightly and refit it.So I'll try it first before doing it on the working one,which under reads about 10%.
I like my tyre profile and the diff I have,so a speedo calibration might be the only other option.
Ken.

[pagan]

I'm guessing the 3.45 ratio is out of the 2500.....?
Being if the stag and the 2500 gearbox are the same or similar..
Is the 2500 speedo drive gear in the gearbox a slightly different ratio?

[stag8manod]

My Stag speedo reads spot on (Km) with TomTom, the drive train is standard (man od) as are the alloys and tyres. I have a 3.45 diff out of a S saloon to fit so the results will be interesting. As an aside our Mazda 3 over reads by 4k at 100K on TomTom.
Cheers Ian A