How does tire size effect your speedometer? - I tell ya EK,I haven't been excited about putting tires on a vehicle since I was in hgh school.And that was quite ...

I tell ya EK,I haven't been excited about putting tires on a vehicle since I was in hgh school.And that was quite a few years ago. The 285s I put on gave the truck an appearance and feel that made it stand apart from stock and,...well quite frankly, makes it a bit more manly if I may be so bold. There are several choices out there in a 285/65/18 and after a couple of weeks of comparing overall I settled on the BFs as they more closely fit the billet along my intended use and appearance intentions. I personally went with a single block aluminum spacer(.750" which is 1 1/2" lift) under the front strut bracket as a leveling means and it seems very solid and I have no creaks or pops.Maintained a 1/2" rake for the ocassional trailering and load carrying. Handling and factory ride was not negatively affected either. In fact ,I liked it better from the get-go.I drove with the factory tires after the level for a few hundred miles before the tire upsize. I like it even better now.

Without consideration to the mileage and tranny wear.....your initial calculation is slightly flawed. Assuming the engine is turning over the larger tires at the same angular speed (RPM, regardless of gearing), the new tires will be traveling much further than the difference in the heights of the tires. The traveled length will correlate with the circumference of the tires, or around 3x the height difference. This will be a varying percentage of the original speed (a greater difference at slower speeds, less at higher speeds).

Probably need to recalibrate...enjoy the new tires though.

Yeah, what he said. Allmost 8% diff from factory rubber to my new tires. Recal , for sure for me.

I'm an engineer....I have to over think things!!!

8% for StarsnBars, but not for my scenario, right?

Nah... My speedo is 2mph off at 60. Checked it with GPS. Tire comparison chart lists the change from 265/65/18 to 285/65/18 at 3.14 %. Plus the BFGs I'm running aren't the rock magnets like the factory Bridgestone Duelers.

While reading this thread, I was curious why it hasn't been mentioned that with the speedo being off, so will your ODOMETER....while a 3-5% in speed (depending on your tire size) might not be too bad, a 3-5% mileage difference on say a 75K mile truck may be something that folks may not have thought of...Just my .02..

Let's say you changed your tire size the day a truck came off the assembly line. At 100,000 miles, the odometer would say approximately 97,000. I don't think that is significant. Not to the owner, and not to a prospective buyer.

Here's a great little site that can help with tire size and speedo corrections.

http://www.miata.net/garage/tirecalc.html

Here's a shot of my truck after the 1 1/2" leveling and 285/65/18 BFGs.