Very good bit of troubleshooting the "image" problem, sir! Of course you're right, and it's something I never would have thought of.

Regarding the change in accelerating force, the figure I gave you should be quite accurate. It's simple mechanics. The torque on the axle from the engine does not change, and that leads to this relationship:From that, the new "Force", or "push" on the road from the tires has to be less if the tires are taller. The percentage change is given by:It's the biggest influence on performance when you switch to larger tires. Much more effect than the increased tire mass or rotational inertia.

- Jack

 

wouldn't the easiest solution just be to kick off the o/d when towing up hills? Your tranny will likely thank you too.

 

It is but hills are not the only problem just the biggest. When I'm running straight down the road it will shift to low rpms also, unless I give it more throttle than I would normally (and really would like to) causing the rpms to be higher.

 

Again, setting the part throttle shift and TC values up may get rid of that problem for you..

Off topic, but - Great writeup on removing the starter! What a pain that top bolt looks to be!

- Jack

 

Alright I will try to mess around with that this week. My trucks been sitting all week as the new starter I got Friday had a bad solenoid and I had to wait till Monday because they aren't open on the weekend. Been a big ordeal I'm just glad to have her runnin again.

 

I just now read through this thread. I had a similar situation. Went from my stock setup and 17's to 20's with 305/50/20's. I pull a 30ft travel trailer. I had a lot less shifting in and out with the stock setup versus the 20's and the Gryphon. Even with the OD on the stock setup was not bad. I really was not sure why it changed other than tire size and weight.

I am with Watson in that I don't completely understnad TC lockup and shift points. However one thing I found out with my PHP tow tune is that it likes the overdrive off. I not only have more power but I also get better MPG's while towing. I really don't know why I would better fuel economy when the RPM's are higher but it does. I typically run Bluejay's setup that he's posted on here a few times for shift points, WOT etc.

Anyway, I thought this might help. I'm not unhappy with it, it works out great, but it has had me scratching my head some. Maybe Jack or someone can figure out why it is this way for me?

 

Possibly, with the OD on, you were able to run in 4th gear, but the Torque Converter was unlocked, and you might have been doing fairly frequent downshifting. In 3rd, the TC is likely locked, so there's no "slippage", and the load on the engine is less.

Gas mileage IS related to RPM, but if the load is higher at a given RPM, more gas is used. By going to larger tires and wheels, you effectively "moved" the final rpm of the engine to a lower setting at any gear. You MAY be close to your previous 4th gear rpm/load combination with the new wheels when you are in 3rd. I imagine the load is simply too high now in 4th.

- Jack

 

That sounds like a pretty good explanation! I have thought about going with even larger tires next time around but I also know that I am pushing needing a regear with what I tow. Being that my truck is 5 years old and I typically keep one about 10 years I don't know if a regear is worth it at this point. My current tires probably have a year or two left on them anyway before any change is made. Thanks for the input!

 

I'll add that I've noticed my rpms with the O/D off (3rd gear) and the torque converter locked my rpms are pretty close to 4th gear with the torque converter unlocked. The difference is the engine pulls better with OD off until the rpms simply get too high to be practical.

 

Running at 70mph my RPM with the OD off while towing is around 2300. I will get around 11-12 mpg versus the 8-9 mpg with the OD off.

 

Even though the discussion has moved past, I finally uploaded the picture to this forum so this should help explaining.


From what I have found, my truck does have certain conditions of speed and load that get better mileage. Before I changed my tires the optimal speeds were 50 and 70 mph. I could get 20 easy at these speeds even in hills, any drive time outside of the range would get as low as 16. Still trying to figure out my new mileage zones. I think the 50mph speed has actually increased slightly.

 

Well here I am digging up an old thread of mine again. After the discussion I had planned to let the truck go until I had to switch to my tune again. Just a few weeks ago my dad used my truck to pull the trailer and switched to my stock tires. I reprogrammed for those tires, and then had to reprogram again when I put my larger tires on. As I was going through customizing my tune, I didn't notice any settings to change the torque converter lock point or part throttle shift? Maybe I just overlooked but I went up and down quite a bit to check, the only thing that I saw was where you could change shift points my MPH?

 

Then those were the part throttle shift points. WOT shifting is determined by RPM while part throttle shifting is determined by speed.

 

Only the gryphon has them adjustments?

 

No, I think just about every programmer has that option.