interesting fact.. maybe

[tufferoza]

on the way from Canberra to Bendigo i found where the old man had put his GPS and forgotten where it was in the bus.
i had a little play with it to figure out how to set it up along the way so it would show current speed, with the 31" tyres my speedo reads 6.2kp/h slower than what the GPS says i am doing. the GPS is spose to be far more accurate than a cars speedo?
later down the track this speed was confirmed to me when i went under one of them speed check doobies that show your current speed in a big display, while i was doing 80kp/h the doobie said i was doing 87kp/h and the GPS was still around the 86.2kp/h mark.

dunno if anyone else has ever checked by how far the bigger diameter wheel puts the speedo out??

[Goatse.AJ]

The amount your speedo is out will be a PERCENTAGE, not a fixed amount.

Easy way to work it is that your speedo is out by around 10%, so when it reads 100kph, you're really doing closer to 110. At 60kph, you're really doing around 66.

Gotta be real careful with ya calculations in Victoria, the Speed Camera State.

[tangwyn]

What about 30x9.50R15 / 245/75R15 tires? Do they throw off the speedo/odometer too?

http://www.warfs.org/content/view/26/63/

[Goatse.AJ]

With 30 x 9.5 tyres my speedo reads about 6.5% low, so when it READS 100kph, I'm really doing about 106kph.

[senergy]

i checked my speedo last night with my gps last night. 65km/h with the gps and 60km/h on the speedo.
The radius is approx 29.5" which equates to almost 10% larger then stock rolling rubber.

So using the 10% rule seems pretty accurate.

[Mudsierra]

MMM? Funny about that, i run on 30x9.5 on stock rims, and when i do 60km, went past one of those speed sensor in NSW one time, did say i was going 60km on the dot. Don't know what the gsp would say about that.