calculate speed of tires

[BLHFUN]

So for my truck... I have 22" 285 nitto terra grapplers. These tires are almost 2 inches taller than standard ford specs and reduces the speed on my speedometer by 8 MPH at 65. When my speedometer states at I'm driving 65, I'm actually driving 73/74 due to the truck calibration from the original tire size. My GPS confirms this and so does the ticket i got in Oklahoma. The question is valid, but you would need to check the speedometer to a GPS and see if they match. If not, do the math to keep your trailer tires at the suggested speed rating. Had I not known my speedometer was off, I would have been dragging my trailer around at 8 to 10 MPH over the rated tire speed, which is where the OP was going.. IMO?

 

[JanAndBill]

When I changed out the tires on my truck my SCT controller allowed me to input the correct tire size. You can get all the information from the manufacturers website. Highly recommend recalibration as doing so will recalibrate your speedometer, transmission(shift points) and engine for the bigger tires. If you don't have a controller then most dealers will reset it for a fee. You've already spent the big bucks on tires and rims, why not spend a few more and finish the upgrade.

 

[BLHFUN]

When I changed out the tires on my truck my SCT controller allowed me to input the correct tire size. You can get all the information from the manufacturers website. Highly recommend recalibration as doing so will recalibrate your speedometer, transmission(shift points) and engine for the bigger tires. If you don't have a controller then most dealers will reset it for a fee. You've already spent the big bucks on tires and rims, why not spend a few more and finish the upgrade.

True. Wasn't concerned with the speedometer so much, but you bring up a valid point about the transmission shifting. it's headed to Ford on Monday for a Front left brake that is dragging. I'll ask them about it and report back. Pretty sure it's a flat 50 for any programming.

 

[JanAndBill]

The electronic diesel is a highly adaptable machine. The ecm will adjust the fuel, timing, boost, and transmission based on everything from load, to atmosphere, but like a computer bad data in - bad performance out

 

[JohnD]

Sorry, I've lost track again. Who's on first ?

What's on second.

Where is third

 

[fritzwell]

Where is third

I don't know

 

[danemayer]

Someone's made it easy to find out the error on your speedometer if you upsize your truck tires. Here's a link.

 

[BLHFUN]

Someone's made it easy to find out the error on your speedometer if you upsize your truck tires. Here's a link.

great Link. I suspect this was exactly what the OP needed.

 

[JanAndBill]

Someone's made it easy to find out the error on your speedometer if you upsize your truck tires. Here's a link.

Interesting, though not totally accurate as it assumes all tires of a given size have the same circumference which in reality is not the case. One thing it reminded me of though is that the difference is progressive as your speed increases.

 

[hoefler]

If your looking for revolutions per mile:
with tire at its proper inflation and load
mark the given tire vertically at the center of its contact patch on the ground and mark the ground at this point
pull vehicle forward and stop when the mark makes one full revolution and mark the ground
measure the distance between the marks, this is your loaded rolling circumference
divide 63360 ( inches per mile ) by the distance between your marks
the results is your revolutions per mile.

or you can simply have someone run beside you and count the revolutions

 

[Bob&Patty]

Whats on third.

 

[rcox]

I have come to the conclusion, as I breathe a deep sigh of relief. the tire states max speed of 65. so the tire will do what it deems necessary to keep the truck from pulling it any faster. possibly a b/o. always slows my truck down. problem solved. I don't have to change the speedometer or put a gps in the camper. alls well that ends well.

 

[fritzwell]

Whats on third.

Mr. Cranky,

What's on second. I Don't Know is on third.

 

[Bob&Patty]

Yep...been awhile since I saw the Abbott and Costello movie.