Speedometer readings exaggerated

[famousfochabarian]

Hi guys, just bought my first VW campervan, a 1984 1.9 T25 not sure of the campervan conversion, got it from ebay, I know not the best idea to buy a van unseen, but as it turns out the condition is not that bad, some rust on bodywork, interior needs some tlc and a very big bodge repair on the driver's side floor (bolted plate in lieu of welding - interesting) will have to be looked at before the next MOT, hopefully nothing my basic diy mechanics skills can't attempt to fix.

Anyway on to the problem I need some help on, the speedo reading is way exaggerated from the actual speed being travelled e.g. driving back I managed to hit 100mph on the M6, if only! it has had alloy wheels and non-standard tyres fitted as follows: -

Front - 195/50/R16
Rear - 205/50/R16

I realise this could effect the reading but could it be by this much? if so I assume my best option would be to fit tyres of a similar circumference? Is there anything else that could cause this problem?

One concern I have is that the speedo cable coming through the dust cap has lost it's cir-clip fixing and has been bodged with a lump of weld on its end, is this meant to be fixed solid to the dust cap as it does move separately? Does anyone have a picture of what this should look like?

Would appreciate your help on this guys.

Thanks

[horns]

I think they're all a bit on the optimistic side. Mine reads about 15-20% high on standard tyres.

[tinman]

Have a google for a tyre size calculator.
It looks like your front tyres may be too small, My calculations when your speedo reads 70 your only doing 64.5.
But I may be wrong

[andisnewsyncro]

tinman is right, your speedo is over reading, the tyres on yours have a much smaller rolling radius than "standard" factory tyres.

Have a look at this http://www.kouki.co.uk/utilities/visual ... calculator

[tinman]

Standard tyres are 185/14. But what profile it don't say on the tyre is it 80?

[andisnewsyncro]

Standard tyres are 185/14. But what profile it don't say on the tyre is it 80?

As I understand it, if no profile is stated its going to be 80 (or more correctly 82)

[silverbullet]

Front - 195/50/R16
Rear - 205/50/R16

Your tyres are far too small. The fronts - which drive the speedo, not the gearbox - have a (theoretical/new) diameter of only 601mm, the correct size is 654mm! That's 8% small - a full inch on rolling radius
See this: http://www.brick-yard.co.uk/VehicleSpec ... ltwhls.htm" onclick="window.open(this.href);return false;
Thanks to Mr Baxter as always