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I am a new happy owner of a 155 1.8 8v 93` from Greece.Anyone knows the default alloy and tyre size.My 155 has 15" new alloys with 195/50 tyres,but i think my speedo is reporting to many km/h.I`m running at 140 but my experience tells me that i`m running at 120-125 km/h.Any suggestions
 
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In the UK a 1.8 8v would have been running either 14 or 15 wheels.

185/60/14
195/60/14

or

205/50/15

So....your set up is ok.

What do you mean "your experience tells you"? Could be the Alfa is travelling a lot faster around a bend ;)

Welcome to the forum :D

wrinx
 

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Sounds to me like your experience is about right. Your tyres are a slightly small but ok. But, I've yet to see an accurate 155 speedo. I'd say mine is about 6-8% over and that appears to be about average.
Shame, the speedo was officially verified the other day at 86MPH, but I forgot to check the speedo :cheese:
 

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I find that those red LED "speed warning" signs that flash up your speed are a good way to calibrate your speedo'..

My standard wheels - indicated 42 is actually 40 mph.

205/40 x 17 (slightly bigger rolling circumference) indicated 40mph is dead on. :D

So.. get some big alloys and it'll fix your speedo'.

Ralf S.
 

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When my 1,8 8V shows 140 km/h, The sat nav shows 132 km/h :) - I´m also riding on 15" Alloy
 

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I have a GPS based RaceTechnology AX-22 performance logger that I use for track days, its supposed to be pretty accurate.
I tested the speedos of of a few different cars (all on original rims) to see how inaccurate they were and the result were very similar across different makes of cars.
All speedos were pretty accurate at about 40mph,
then off about 4-6% off at about 70 and rising by about 3/4 of a percent very additional 10 mph. Some were out by up to 9% at higher speeds.
In all cases the speedo was overestimating the speed as you would expect, have seen this frequently mentioned ono the web as well.
So apparently this is a pretty common issue..

Well at least you are less likely to be Gatsoed by accident :)

Cheers,
TB


jorila said:
When my 1,8 8V shows 140 km/h, The sat nav shows 132 km/h :) - I´m also riding on 15" Alloy
 

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I imagine one major reason for the speed overestimation is that the speedo doesn't know how much tread is left on the tyres, and this can affect the rolling diameter by up to 10mm (2 x 5 mm wear) for worn tyres.
So basically I am surmising that your speedo will overestimate your speed more when your tyres are worn since I am sure that it must be conservatively calibrated for a new set of tyres.

Not too sure why they get much more inaccurate at higher speeds, I
would tentatively guess that there is some tyre slip even in normal road usage at higher speeds.

TB
 

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Right enough, there will always be differences between cars. Tyres play a significant part becuse there sizes are nominal, and do vary slightly. Temperature during the test will also make a tiny difference, and remember that the higher the speed rating of the tyre; generally the less it will grow at high speed whilst trying to withstand the effects of the centrifugal force; will play its part also.
That is generally why a decent speedo is fairly accuarate at the low and high ends of the scale. However, manufacturers provide a correction to make them more accurate in the mid sector which throws the rest all to pot. :D
Producing a very accurate speedo cheaply just can't be done it seems.. That is why the all tend to err safely.. ;)
 
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