Change rear gears or go to 11R24.5?

The trucks I am looking at are spec'd with 3.55 rears. I have heard they have some with 3.36 but I haven't verified that and I am waiting on an e-mail. I have exhausted my search on here to find where someone has changed rear gears and posted how much it cost them. The truck is an old TMC truck so they run 62mph. I want to be in the "sweet spot" between 68mph and 72mph. According to eaton's gear calculator, I can get the RPM's between 1400 and 1500 in that mph range with either 3.36's and 11R22.5 or 3.55 gears and 11R24.5. If they do have 3.36 trucks available then that is what I will get. If all they have are 3.55 trucks then would it be cheaper to buy 10 24.5 wheels and 10 11R24.5 tires and change it all out or keep the low pro tires and just swap the rears to 3.36 rears and run it and change to a 11R22.5 or 11R24.5 later on when it needs tires? I've looked at tires and wheels and it looks like it will likely be in the $5,000 range to swap them out. What do you guys think?

 

I like the 3.55 rears. I run them with 11R24.5. The truck gets best fuel mileage between 72-74 mph. 3406b motor. 3.36 rear swap will cost you about 4,000$ more then switching wheels and tires. Depending on if your doing labor yourself or not.

 

Depends.Does the truck need new tires ? Rims look OK ? Any info on the mileage on the rears ? Pulling tall trailers / loads ? Might not be able to raise height.

Me ? I would leave them alone until I have run the truck a while. Then MAYBE change, but I doubt it.

 

Good questions. I should have added more info. 2007 pete 379. They all have around 900k miles and they all have the C15. Not my choice of motor but CATs are good engines. They come with new virgin tires and aluminum wheels. I will be pulling vans and my loads are normally not over 12,000lbs. I'm not as worried about the mpg as I am running the engine at a high RPM to keep it at 70mph. I don't want to go around at 1600RPM just to be able to run 70. That is a slung rod waiting to happen.

If I changed the wheels and tires, I would obviously sell the other wheels and tires. It shouldn't be too hard to sell 10 brand new virgin tires for something like $2500 just to get rid of them and regain some of what I would spend on a new set of 24.5 wheels and tires. I wouldn't be looking to make some huge profit off of them. Just offset the cost of the new ones.

Before anyone says something like, "why do you want to run 70, you can get better mpg at 62" or something like that, The place I haul for is a major car manufacturer. They place orders for parts and have to have them asap so putting around at 62 is not an option. It has to get there before they need those parts on the line. Plus, we have some Texas runs and being able to do 75 in the 75mph zones would be nice. With the way the trucks are set up I would be between 1700 and 1800rpms to hit that.

 

changing the rear gears would be more effective than changing tire size. IMO if you went to 24.5 you would lower rpm slightly but gain more sidewall resistance that would probably cancel out any benefits. Another thing you could consider if you went with the TMC truck is a less resitrictive exhaust system and a PCM tune for better fuel economy.

 

That is what I was thinking. I was planning on doing the exhaust work and I hadn't thought about the tune. As far as the gears go, I figured changing the gears instead of the tires would be more effective. I guess I could always change to a 3.36 (if they don't already have some available) and change the tire/wheel size when it comes time to change tires or when I have some disposable income. Petes just don't look right with 22.5LPs anyways.

 

22.5LPs are a poor set-up with 3.55 rears if your hauling highways speeds with lighter loads. I would must def dump them. 3.36 with 24.5LPs is a very nice set-up for the c-15 which is why it's so popular. Plus later on if you want to explore high rubber when you need new tires you can. If your hauling Texas and wanna run 70s without screaming that motor day and night get rid of those 22.5LPs for sure. Plus looks wise the 24.5s will be much better. Cheaper now to change the wheels then if you still want to change rears later you have time to shop around and find the best deal.

 

Well I'm really hoping they do have 3.36 trucks. Someone said the 2007's had the 3.36 rears and the older ones had the 3.55. They have 06 and 07s on their site but both say 3.55. I dont know if maybe they have some 3.36 trucks but since they have a lot more 3.55 they just made the same page for every truck and just changed the mileage. I e-mailed them earlier. Maybe they will e-mail me back this week. If not I guess I'll have to call and check.

Would it not change the height of my truck/trailer if I went with 11R24.5? The places we run aren't in small towns or cities or anything like that and I don't think I've been under a bridge that was under 14ft yet but I would hate to find out that my truck was taller than I originally thought by shaving a bit off the top with the bottom of a bridge.

 

whats pcm tune?

 

Pretty sure he meant ECM tune.