after putting on my 35 from stock tires my mph isnt off well my speedo and gps are right on together but i never tired the gps with the stock tires or in another car does it make sence that it would still be the same is it even possiable thanks

 

Mine is the same as other cars now as if it was slower before and when it said 60 I was really going 55 or something. Like a frickin scam so I go the speed limit lol now it is pretty well spot on I cruise control at 60 and it's really 60

 

That's weird you say that cause my 05 with stock tires always feels like it's going 55 when I'm doing like 60-65, and when I'm doing 55 in a 55 mph zone people are getting pissed for me to move and I feel like I'm only going 45. Maybe these trucks were meant to have big *** tires!!

 

The spedo in the F150 reads about 3-4 mph faster than you are actually going. If it says 60mph, you are actually going about 57mph.

With that said, it makes sense that the 35's put you right on with the correct speedo reading. The 35's makes less rotations per mile than the stockies, so that equates to a lower reading than your actual...so now the speedo reads 60mph, you are actually going 60mph.

 

That's Odd - my gps was dead on accurate with my speedometer with my stock tires. Now since I've put on 35's, I'm about 5-6 mph faster than what the speedo says according to gps. I got my speedo recalculated at a dealer and it brought it closer to being accurate, but I'm still off about 3-4 mph and it gets further off the faster you go.

 

maybe it has something to do with the fact that yours is newer then ours?

 

How much didi it cost a stealership to get it recalculated

 

$0.00 (I work at one) If it was a customer, it would probably be around $30-$40 depending on your area and that particular dealerships labor rate.

So just for your guys' infomation - when I manually went into the module that controls the tire size, I typed in a 315/70/17 since that's pretty close to a 35x12.50 and it calculated the revolutions per mile at 606. Unfortunelty, the stock programmable limits start at 644 and go up to 7xx. So I programmed it to the 644 since that was the lowest I could set it at. It did get the speedo closer to being accurate but not perfect. When the speedo shows about 62 mph, I'm actually doing about 65-66 mph according to gps.

 

So a 305/70 tire might be dead on with the 644 limit?

 

Maybe?? I honestly don't know how to calculate it. In another week and a half I'm putting the stock wheels and tires back on to make a trip to Colorado so I was going to put the speedo back to stock for the trip. I could punch in that tire size to see what it says the revolutions would be and let you know unless someone else knows how to calculate it???

 

It will be lower - more like 613 revs per mile for a 305/70/17.

I got this number from www.tirerack.com You can enter a tire size, then have it find the tires that fit that size. Check the specs for the tire - they will show revs per mile. Different brands and tire models of the same size can vary slightly . For example, some of the 315/70/17 range from 601 to 603 revs per mile.

 

Cool, thanks for the info. I will check there. My next set of tires will probably be 285/70/17 anyhow. Dont want to crank the t-bars, but do want bigger tires.

 

I Just Put On 305/65/18 And My Gps Reads About 5mph Faster Than My Speedo. Which Makes Sense. Bigger Tirs= More Revs Per Min= Which Means Youre Goin Faster Than Wht Your Speedo Reads

 

my 35 threw my speedo off about 8mph by gps