How much does the speedo lie?

[TriNode]

I was driving to pick up my latest ebay purchase, and when my speedo said 80mph my GPS said 74mph, is it that far out?

So probably when it's saying 30 I'm doing 27?

Reason I'm asking is, I've just today fitted new alloys, going from:
155/70/13 to 165/60/14
if what I'm thinking is right that's a radius of 273.6mm -> 276.8mm
or circumference of 1719.08mm -> 1739.18mm or a 1.17% increase.

So is my speedo going to be more accurate (slightly) in total?

Will the shorter sidewalls make an improvment to the handling?

Also if it helps others I found this:
http://www.clubpolo.co.uk/articles/005.htm
it lets you compare any tire size to your current one (it's what I used, others may find it useful)

Thanks,

Anthony

 

[sonic]

clubpolo have taken the table from this site http://www.chris-longhurst.com/carbibles/tyre_bible.html and are in breach of copyright.

 

[TriNode]

Wow, that was a quick spot. you're right the exact same tool is on the link you posted.

 

[SplanK]

mine is bang on 10% out... however im gona sort it out using my emanage

Dont use a table as defo 100% though - other factors are in play - the best way is to get a GPS unit that will tell you your speed, and work off that...

 

[TriNode]

Ahh, so my speedo will now be more accurate as it was over estimating my speed, now for every turn of the wheel I'm going 2cm further, so it'll be closer to the truth.

I always thought they were more accurate than that though.

 

[59 bhp is more than enough]

my speedo is 1/2 mph out at 111 with 195/45/15's on

which is nice

 

[D4R73N]

mines 1 mile out according to the sat nav... or maybe the sat nav is 1 mile out...lol

 

[TriNode]

mine was 4 to 6 miles different to what tomtom said at about 80 I must have the most inaccurate speedo, or a really crap tomtom!

 

[Arnold]

Mine is miles out. Just use GPS and ignore the car's speedo