remember the rumble in the rockies and how poorly ford did pulling loads in the mountains??? well here is a factor, my truck has 3:31 gears the stock tires were some puny 245's I think they were the 17" wheel. I upgraded to the stock 18" wheels that came on the lariet along with some 275's which are also the stock size on the lariet. this change made me loose about 7mph pulling the same load up the same large grade we have in our area called parley's canyon. only thing that might have been different is last time I checked it, it was alot cooler outside like 80 degrees today vs 40 with the old tires, but on this pull I turned off the AC, surpising how much difference a tire size change can make. don't worry I am still pulling the hill 7 or 8mph over the posted speed limit and passing most gasser cars who are gasping for air. just not doing so in excess of 80 mph anymore.

 

Sounds like a normal occurrence.

Did you get your PCM recalibrated with the new tire size?


biz

 

A couple of guesses come to mind :
1) you basically changed your rear end gearing, so you would expect some power changes...
2) and as stated, your speedo has not been corrected for the new tires so it may not be what you think it is

 

If you've got a Garmin or other GPS unit, hook it up and check your actual speed on the GPS. You may have only lost MPH on the speedometer since the truck hasn't been flashed for the new tire size.

 

I thought the net tire sizes were the same..

use the discount tire calculator to compare two and see of there is any speedo correction required
Tire Dimensions Made Simple - Discount Tire

Sam

 

Between this post and your power curve post, I'm starting to worry about your torque converter.

I have read your posts long enough to know that you would probably identify this as a possibility and watch it closely? Aside from obvious slipping, I'm not sure what the other symptoms would be, but I'd bet you could figure it out.

I did a search, there are three threads about new torque converters. Here is a good one.

https://www.ford-trucks.com/forums/1...-the-6-7l.html

 

245-17= 30.6 dia
275-18= 32.1 dia

 

70 profile is 33.1 inches

so the correction is almost 6mph.. speedo=65, actual = 70.73

 

my speedo was 3 mph fast with stock tires now I am about right on maybe 1 mph slow, I accounted for that in my posted numbers. I don't think I have any TC issues truck runs fine and shifts fine for the most part since I had the flash done over a year ago. no slipping either. the actual final gear ratio with tire diameter must have taken me just enough away from the torque curve to cause the drop in speed.

 

Then a downshift is in order

mine pulls WELL at 1600 rpms... even at 23k total weight... most of the time I'm in manual 6th so the turbo and torque pull most of the 2-3% little hills in the southeast...
now at the higher rpms, it doesn't seem as strong,
not that it's bad up there, just that it's so good down there

but probably because it was designed to be stronger down low?

 

I grew up in texas, there is nothing in texas that simulates parleys canyon with a climb to over 7500 feet of course your truck will pull great on texas hills. remember even with lost speed I am still 7-8 mph beyond posted speed limits before the tire change I was a locomotive night train in excess of 80 mph, leaving honda civics in the dust gasping for air.

 

Chill - I did say 2 -3% grades...


ok - good luck with your quest... I'm out...
driving my nice truck