Tire circumference headaches

[DarrenWS6]

I am absolutely sick of this. I had to return to stock for a dealership visit and re-uploaded my circumference of 2556mm apparently is way off.. I have 33x12.5/20" Toyo M/Ts. That is the posted size on the tire. I hate that there is not a metric size which would make this loads easier. Most say they are the same as 305/55/20s but that is not enough for me. I am trying to get my mileage and speed/rpm spot on.

The tires have almost 50k miles on them, because of the low treads, and the vehicle weight, they measure a total height of 31.80" I would say, just shy of 32"s. The 12.5 is still on, and the rolling distance test with tape came up to 99.15/16"s. Just one cm from 100"s. The book says to multiply the inches in the rolled distance to 25.4, which gives me 2540mm.

Now, I go down the road and get to 70mph, i'm turning 1890 - 1905rpms. Way off, stock tune it does 2000rpm at 70mph. Before hand of returning to stock it usually was 1960rpm @ 70. What I don't understand is why with the correct math, why is it not correct in the gauge cluster??

I found an online calculator that told me 33x12.5/20 = 318/52/20. I did the steps in the book to figure that out and came up with 2634mm. 94mm higher than what the math gave me. The rpms were about the same at 70mph! What gives? I made sure it is set at 3.73 ratio and have only adjusted shift points and idle rpm upped 50 just for a little idle sound.

There is just no way a 2007 5.4L 4wd with the 4r75e and 3.73s can turn that low of rpms at 70mph, I feel like I am not at the full speed of 70mph and am adding more miles than I am actually putting on! Any advice?

[Longshot270]

Yeah, that definitely isn't correct.


I'm sure it would be possible to extract a correct tire size based on RPM but that appears to be some math I don't want to do.

Have you tried the map/odometer ratio I posted on F150Online?

[RajunKajun59]

Originally Posted by DarrenWS6

I am absolutely sick of this. I had to return to stock for a dealership visit and re-uploaded my circumference of 2556mm apparently is way off.. I have 33x12.5/20" Toyo M/Ts. That is the posted size on the tire. I hate that there is not a metric size which would make this loads easier. Most say they are the same as 305/55/20s but that is not enough for me. I am trying to get my mileage and speed/rpm spot on.

The tires have almost 50k miles on them, because of the low treads, and the vehicle weight, they measure a total height of 31.80" I would say, just shy of 32"s. The 12.5 is still on, and the rolling distance test with tape came up to 99.15/16"s. Just one cm from 100"s. The book says to multiply the inches in the rolled distance to 25.4, which gives me 2540mm.

Now, I go down the road and get to 70mph, i'm turning 1890 - 1905rpms. Way off, stock tune it does 2000rpm at 70mph. Before hand of returning to stock it usually was 1960rpm @ 70. What I don't understand is why with the correct math, why is it not correct in the gauge cluster??

I found an online calculator that told me 33x12.5/20 = 318/52/20. I did the steps in the book to figure that out and came up with 2634mm. 94mm higher than what the math gave me. The rpms were about the same at 70mph! What gives? I made sure it is set at 3.73 ratio and have only adjusted shift points and idle rpm upped 50 just for a little idle sound.

There is just no way a 2007 5.4L 4wd with the 4r75e and 3.73s can turn that low of rpms at 70mph, I feel like I am not at the full speed of 70mph and am adding more miles than I am actually putting on! Any advice?

Did you solve this yet?

I am runng 275/70-18 33.16" Dia.

the calculations came out to 2634mm but I got my GPS out and played till I got my Guage to match the GPS at 70. Which is 2604. I have 3.73 Gears.

2000 RPMs @ 70.

Remeber our 4th gear OD Ratio when locked is .71 not 1:1.

[fine_style150]

If you want to find the rolling distance of one rotation, wouldn't that be like the circumference of a circle?

32" = 812.8 mm

so circumference= diameter x (pie or 3.14)

812.8 x (pie) = 2553.49 mm