Tire size/mph

[alfert]

I put a set of Kuhmo 155x80x13 tires on my S3 on stock wheels and they fit fine. They have a diameter of 22.8in. and 914 revolutions per mile. The original 145x80x13 had a smaller diameter and 914 revolutions per mile. For arguments sake, the new tire is 10% larger therefore making the rearend (3.77) taller(?) and the result is the speedo is reading about 10? lower then actual?? so 54 mph on speedo is really 60mph.? Is that correct?
Thanks,
Brian

[alfert]

I guess it is about 5%, not 10%, but question is the same.

[Tahoe]

The difference is 2.8% so when driving at 60 MPH your speedo will read 61.7 MPH. Not much of a difference if your speedo is accurate to begine with.

[AHM]

The difference is 2.8% so when driving at 60 MPH your speedo will read 61.7 MPH. Not much of a difference if your speedo is accurate to begine with.

shouldn't the speedo under read ?

[Chancer]

Both being 914 revolutions per mile should give identical speedo readings for a given speed, or was one of the 914's a typo?

[TroonSprint]

Wasn't 155/80 x 13 the standard size tyre? If it was then your speedo should be correct now, having been 2.85% fast before with the 145 tyre. Use this tool to work it out http://www.etyres.co.uk/tyre-size-calculator.htm
Mike

[Tahoe]

The difference is 2.8% so when driving at 60 MPH your speedo will read 61.7 MPH. Not much of a difference if your speedo is accurate to begine with.

shouldn't the speedo under read ?

Yup, got it ass backwards. I was very tired last night, and it seemed to make sense when I wrote it. Anyway, the 155 was probably the correct size to begin with, and knowing how accurate our speedos are, I don't think you'll notice the difference one way or the other.

[alfert]

Sorry, the oe tires were 949 revolutions per mile. The 155x80x13 are 914 revs/mi. About 5% difference.....
Brian

[elansprint71]

Do you have a portable GPS Sat-Nav? That will give you a far more accurate answer than anyone on this forum.

[AHM]

Wasn't 155/80 x 13 the standard size tyre? If it was then your speedo should be correct now, having been 2.85% fast before with the 145 tyre. Use this tool to work it out http://www.etyres.co.uk/tyre-size-calculator.htm
Mike

If the tyres weren't crossply... They would have been 155/13 which is I believe 82.

Also in that period speedos like like BHP was read at the upper end of the scale!

[Tahoe]

155/82-13 was the correct profile back in the day, but the difference between the two 80 profile tires is only 2.8%. The difference between a 155 80 or 82 profile is approx 1%, so again even if you were doing 100 MPH your speedo would be off by only 1 MPH when comparing the old and new 155 tires.