Outer Limits 4x4 Board

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??

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.

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

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

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

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.

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.