Speedo difference with 37" tires
JK Newbie
Joined: Mar 2009
Posts: 94
Likes: 0
From: Warner Robins, GA
All you need to do is take actual tire size (37), divided by your original tire size (convert from metric to standard), take your answer and multiply it by what your speedo says. Then that's how fast your going 
JK Super Freak
Joined: Apr 2007
Posts: 1,866
Likes: 3
From: Eastern Long Island, NY
On stock tires my speedometer agreed with a local "Your speed is xx" meter, then on 35s my speedometer now reads 4-5 lower than same meter, which jives with the above posts on 37s.