Wheel/tyre size differences from std fit- inaccurate speedo??

[trekbiker]

I'm currently running 265/30/19 on a B6 avant , which i have been told by an experienced employee of ATS are 10mm bigger in overall circumference than the standard fit of 205/55/16 or 235/45/17 which would normally be standard fit.

these wheels were fitted by an Audi dealer from new. I have 2 questions:-

1. Would the speedo read slower than i'm actually going and by how much(is there a calculation?). would the mileage also be affected as my MPG, which i have worked out by brimming the tank against the mileage(supposedly) done?

2. Would a smaller tyre, say a 235/35/19 be better? /ubbthreads/images/graemlins/noidea.gif

Any help please!

 

[AndyMac]

Have a look at:
Calculator

 

[trekbiker]

thanks Andy

So my gearing has definitely been altered. What i need to know now is the speed variation for the MM measure difference!

 

[AndyMac]

Well it's the same as the percentage difference in rolling diameter.
i.e. your setup is shown as out by 1.58%, so when your speedo hows a speed og 100mph you're actually doing 98.42mph. As all speedos are 4-5% out the other way anyway it will actually make the speedo more accurate.
I run 225/40/18's which gives me a 5mm increase in rolling diameter and according to my Road Angel my speedo is now only 2% out (shows faster speed than actual).
Those tyres must cost you a fortune, aquaplane city!
You'd be better off with 245/30/19 's which will be 2mm out i.e. less than the difference between new tyres and worn ones, but personally I'd go for the recommended (& cheaper) size of 225/35/19 to avoid tramlining & surfing.

 

[simon_golf_tt]

Are the 265's on the front as well as the back? I've noticed that some of the US guys run 265's on the back and 225/235 on the front. They usually have 1" wider rims on the back too - 8.5" front and 9.5" back. Some even run this on Quattro and don't seem to have any problems. I bet the 265's look excellent but I'm not sure how it effects handling - how does it feel?

As Andy says - new tyres will hurt!

 

[trekbiker]

[ QUOTE ]
Well it's the same as the percentage difference in rolling diameter.
i.e. your setup is shown as out by 1.58%, so when your speedo hows a speed og 100mph you're actually doing 98.42mph. As all speedos are 4-5% out the other way anyway it will actually make the speedo more accurate.
I run 225/40/18's which gives me a 5mm increase in rolling diameter and according to my Road Angel my speedo is now only 2% out (shows faster speed than actual).
Those tyres must cost you a fortune, aquaplane city!
You'd be better off with 245/30/19 's which will be 2mm out i.e. less than the difference between new tyres and worn ones, but personally I'd go for the recommended (& cheaper) size of 225/35/19 to avoid tramlining & surfing.

[/ QUOTE ]

mucho gracias.. it need 2 new boots soon so i may change the lot and go for something more common and cheaper. Michi pilot sport at trade was going to be £500 a pair!. It handles Ok, but i have only had it for 2 months and we aint seen much rain here in Mk. IT seems to manage the monopoly of roundabouts here fine, though not as compliant as my TDI Q i had last year(this one's FWD)

 

[AndyMac]

Ouch! Goodyear F1's 225/40/18 are only £230 a pair

 

[domcj]

235/35/19's i run there are about the right size fromn what I can make out speaking to a few people from various garages

 

[AndyMac]

235/35/19's are even further out - 15mm bigger.