Hello everyone,

Well i had to get some new tires since i didn't pay attention to my misalignment - camber and smoked the front tires. I had the stock 245s and switch to BFG 285's. Truck looks great, but now have a egt concern.

I don't romp on my truck, prob avg about 5psi of boost. However, i have several steep hills on my travels which i usually cruise at about 60mph. With the 245's and TS MVP at 2/3's i was at about a steady 800*. Now with the 285's, by the time i hit the top i am creeping up on 925*, empty (full fuel though).

Now this is at around an elev of 500ft. Soon, i will be heading out on my yearly trek which involves crossing a 7500ft elevation pass, hence my concern. Total weight is a 1800lb truck camper and a 3000lb trail with dirtbikes.

I plan on no OD, and turning my chip down (or will that be worse)? Open to all ideas and I know 900* may not seem hot to some, but this pass is a steady 25 mile grade which would be bad news.



Thank you!

 

Well with taller tires on your actual speed at 65 mph is going to be closer to 70 mph now. So that can be the cause, also you have increased rolling resistance which means the truck has to produce more power at the same speed to overcome the greater resistance. This to will increase egts.

 

... more.

Rotational mass. For every 4 pound of rotational mass you add to the tire you require 1 HP more to keep the truck running. Just for a example. I've got a set of 235's and a set of 265's. The 235's weight 62 pounds a piece and the 265's weight 85 pounds a piece so that's a 96 pound difference between 4 tires. That roughly 23 HP more required to twist the 265's vs. 235's which is roughly a 2-3 MPG loss on the 265's so you lost about 4 MPG just upgrading to the 285's. So hopefully your pockets are deep because the cool looking tires will cost you every day you drive the truck.

Rolling resistance. Rolling resistance is based on the contact patch and tread pattern. The more surface of the tire touching the highway plus more aggressive tires you can have upwards of a 20% loss in just rolling resistance. This why true racing crowds run racing slicks they weight much much less (roughly 20-25 pounds per tire) and smooth face tires have nearly zero rolling resistance give the most traction face.

I'm truly a offroad user and run 235's for offroad work... I'm up around 8,000 feet... I climb grade that are 16% here locally...

 

Thanks Mopar Man for the great explanation and truely thank you for your time.

I should have known going to a bigger tires would cause more resistance, but I guess I never figured to account how much the weight would really play into the situation. 23 hp is quite a big difference when it comes down to it.

Unfortunately i believe i might have a boost leak bleed off as well, as one of my clamps appears to have failed. I have not driven the truck since i posted this, as i picked up a new DD and now the truck sits.

1hp for every 4lbs sounds like a good rule of thumb and now you got me curious, so i plan to calc out my tire weight difference between the old and new.

Thanks for the knowledge

 

Even car and driver agrees...

 

That is a very good test to see, as the car was the same throughout all tire/wheel sizes.

I forgot to mention that i am 3.55 gears too for all that read this thread and makes me about 3.05 ratio now with the new tires. A plus though, it will help with OD off for lower RPM's for me, as i always hated to spin the engine over 2k for any long period of time (should be a tad under now)

 

My truck had 285's when I bought it. Took out my hunting GPS and found that there was a 10% difference in actual vs speedometer speed. With my current 265's it's a 5% difference. So it appears that each tire size makes a 5% difference in speed. I believe that 245's were the factory tire size. So make a correction to actual miles traveled when calculating fuel mileage. I add 5% to my odometer when calculating my mileage. With Michaels 235's you'ed deduct 5% from the odometer. But his computer probably accounts for that.