Technical Manufacturer of tyres affect on speedo ?

[brocherboy]

I have put new avons on my car this last week and have noticed that my speedo is out by 7mph compared to my snooper +gps ,,i no that a cars speedo is a % on the + side to help prevent speeding and other tyre issues , but if my speedo says 60 gps says 53 ,,i changed tyres like for like being 215/45/17s only brand was diffrent, had pirelli,s on before and it was fine
Went back to tyre fitters and they tell me that fiat do not reccomend avons on the stilo 2.4 but could not say why ,but he did point to this being the reason why the speedo was so far out
So went home pulled out the owners handbook only to find he was right they reccomend pirelli,s /michelin/bridgestone/goodyear
It actually says "size only certified and admitted only if "the above are fitted
Has anyone noticed this much diffrence ,,,i suppose i should be glad speedo is not the opposite way

 

[Ffoxy]

Dont be so anal lol!
The tyres will come back to your speedo as they wear and get smaller!

 

[Andymacselespeed]

Not all tyres of the same size actually are!

All cars overread by a bit, 7mph is a little high, but not too far off the mark. As the tyres wear the difference will get smaller.

As for only certified on this type of tyre, I'd guess those are the ones the car was homologated on. In which case it doesn't matter.

Most important thing in a tyre is its grip. If you're happy, then all is good.

I had a set of Avon ZV3s on my Ford KA, to be honest I didn't rate them. But again, I'm a glutton for punishment who buys Goodyear Eagle F1s in Z rating. They last 8K at best. I'm a little spoilt for grip!

 

[brocherboy]

Dont be so anal lol!
The tyres will come back to your speedo as they wear and get smaller!

so if the speedo had been out the other way ,and bin stopped by the boys in blue i could have pled my case in court ,sorry you honour just fitted new tyres and waiting for them to run down cos not alot of peeps no that (including me) so thanks

 

[scottie2065]

no mate, they'll "do you" for having a "defective" speedo, i tryed that before i told them because i had went for 14's to 17's my speedo wasnt working correctly and they put that one to me, so i admitted "going a little fast" and luckily they let me off.

 

[Argonought]

Dont be so anal lol!
The tyres will come back to your speedo as they wear and get smaller!

All cars overread by a bit, 7mph is a little high, but not too far off the mark. As the tyres wear the difference will get smaller.

Bit back to front you lot aren't you

Error will get worse with wear not the other way around.

The Stilo only knows it's speed from (I believe average) wheel rotation. Therefore at say a constant 60mph then over the years the car would get progressively slower (doesn't travel so far in an hour when speedo is on 60)

 

[StiloBoy]

What Avons did you get? The ZV3's? Because they look good, I wanted them but I found out they weren't rated that highly. I recently got a set of ZV5's and they are lovely. Don't look like much, not a trendy tred pattern but grip like naebodies business.

 

[brocherboy]

Aye have had zv5,s fitted told they had the best grip as roads where i am are VERY windy ,,so much grip i tend turn in toooooo much lolll ,,

if argonought is wright then my speedo will be even more out soon as i do a lot o miles in a week ,will soon mount up

 

[StiloBoy]

Well look at it this way, if you trust your speedo then you'll never get caught speeding, and your mpg will also increase! Both good positives lol. Negatives are it will take you a couple of minutes longer to get to your destination, and a'body will be hammering past you.

I totally agree about the level of grip from the ZV5's I've caught myself turning in too much too lol. Or expecting mild understeer and having 100% constant grip.

Wet roads around Fife today, constant heavy drizzle, I've only put about 100-150miles on the tyres so wasn't expecting them to be fully bedded in but bugga me the grip was immense in the wet too.

 

[Ffoxy]

Bit back to front you lot aren't you

Huh? You'se confused Argo... as a tyre wears, it gets smaller, smaller tyre = higher indicated speed over actual speed, check here :-
http://www.kouki.co.uk/utilities/vis...?wscr=1024x768

205/40/17's on my car instead of 205/45/17's would show me on my speedo that I'm going faster than I actually am.

What I said was correct, as his tyres wear from too big the speedo reading will get more accurate not less.
Error will get worse with wear not the other way around.

The Stilo only knows it's speed from (I believe average) wheel rotation. Therefore at say a constant 60mph then over the years the car would get progressively slower (doesn't travel so far in an hour when speedo is on 60)

Nope it would INDICATE its going faster as it wears (more rotations) but ACTUAL speed would be lower...

Or I'll eat my socks... Salt & Pepper anyone?

 

[Argonought]

Nope it would INDICATE its going faster as it wears (more rotations) but ACTUAL speed would be lower...

That's correct

"205/40/17's on my car instead of 205/45/17's would show me on my speedo that I'm going faster than I actually am." That's correct too

"What I said was correct, as his tyres wear from too big the speedo reading will get more accurate not less." but that's wrong

The OP said his speedo was already over reading with the new tyres so it's simply going to over read even more as it wears (as you've just pointed out).

Maybe you misread the 1st post :chin: but I'm sure you can buy some paper socks from somewhere

 

[Ffoxy]

That's correct

"205/40/17's on my car instead of 205/45/17's would show me on my speedo that I'm going faster than I actually am." That's correct too

"What I said was correct, as his tyres wear from too big the speedo reading will get more accurate not less." but that's wrong

The OP said his speedo was already over reading with the new tyres so it's simply going to over read even more as it wears (as you've just pointed out).

Maybe you misread the 1st post :chin: but I'm sure you can buy some paper socks from somewhere

:nerner::woot: I'd prefer an accurate speedo...

 

[Most Easterly Pandas]

All cars overread by a bit, 7mph is a little high, but not too far off the mark.

A depends on speed. 10% overread from speedo is average, so at 70mph 7mph out is ok.

Do bear in mind that speedo readout and the speed the car knows it is doing are different things.

Sit at an indicated 100MPH for and hour and the car will only clock 90 miles on the odo

 

[StiloBoy]

Well on an old fashioned mechanical Odo you'd still clock 100miles! I doubt they'll make the electronic Odo's clever enough to know your speedo rating is out! So it too would record 100miles! Your Satnav might say different though eh!