[tracer]

I'm getting a bit tired of driving at 58 mph That's how I spec'ed the truck myself and my CAT does 1300 RPM at that speed; or 1350 RPM at 59 mph. I have 295 low pro tires and 3.73 rears with a 13 speed Eaton. I want to basically shorten my work day a little bit and I can either switch to 24" tires or change my rear axle ratio to something like 3.55 or 3.30.

I figure with 3.30 ratio and current lo-pro tires the engine will turn at the perfect 1,315 RPM at 65 MPH

BUT

at 60 MPH it'll turn only at 1,214. No way I'd be able to pull any sizable kind of a load at that RPM in the top gear.

So, my question is this: if your truck has a very fast ratio like the 3.30 I'm using in this example, how do you manage to drive thru areas with 55 or even 60 MPH speed limits?

I'm pretty sure my fuel mileage will be in the toilet if I drive thru Michigan at 60 MPH in Gear 12 (8 low) instead of Gear 13 (8 high)...

Any comments will be appreiciated.

PS: I'm based out of Ontario, Canada and often drive through PA, NY, OH, MI, IN, IL.

[solo379]

My old truck 550 3406E, 24.5 13, use to have 3,36 rears, it was a bit short, if you want to run 60 mph in a top gear. Sweet spot for it, was around 1400 RPM, and that was around 67 mph. I do run a lot of OH, so i specsed my current 2005 Acert T6, the same, but 3.55 rears.

I do not have any problems whatsoever, running 59-60, my only problem is that i have to keep it around 62 mph for the fuel efficiency, and some times, it's not an option. If I'd do it how, I'd go with 22.5 and 3.36. I believe, that would be the best compromise, for my situation. And 99.9% of the time, 60mph in a 55mph zone, is not a problem....

About 40% of my mileage is in PA, 25% in OH, and the rest split between east of Mississippi...

[tracer]

Thanks, Solo. That's my problem too - I run thru too many different regions. It's hard to pick the best speed. I sure don't want to run any faster than 60 in MI - their cops pay special attention on trucks with Ontario plates. Sometimes they send us to Northern Ontario where the speed limit is 90 km/h (56 mph) and especially in winter you wouldn't want to drive there above 60, that's for sure. So I guess one just have to figure out the AVERAGE SPEED LIMIT one have to deal with consistently ... I guess for me it's between 60 and 65 MPH.

[specialkay]

Just go with 11R22.5 tires when the low profiles wear out. It'll help your rpm's out and you might get something for your casings when you change tires.

[Copperhead]

Originally Posted by specialkay

Just go with 11R22.5 tires when the low profiles wear out. It'll help your rpm's out and you might get something for your casings when you change tires.

True, it would help with RPM's but 11R's have a higher rolling resistance to the low pros and will cost a little in mpg's. With so many trucks putting on low pros, not sure if you will get the best bang for the buck on the casings.

I've got 3.42 rear on my ride and at 60 I'm turning 1300. 65 I'm turning 1400. When I get in those situations where I have to keep it around 55, I usually will drop to 12th and be turning 1400 at 55. Not critical on a light load, but on a heavy one, it almost has to be done.

[tracer]

Originally Posted by Copperhead

True, it would help with RPM's but 11R's have a higher rolling resistance to the low pros and will cost a little in mpg's. With so many trucks putting on low pros, not sure if you will get the best bang for the buck on the casings.

CAT on their website recommends to go with low profile tires for fuel efficiency and it's possible to get LP in 24.5 size. They do same revs/mile as 11r22.5 but have lower rolling resistance: e.g. YOKOHAMA TY517 285/75R24.5 has a diameter of 42" and does 496 revs/mile). Also, at least on my International 9400 using low profile tires have improved steering and handling - our tire guy put Michelin 275/80R225 XZA3 on the front and they steer great. So, I am pretty happy with low profile tires.

Originally Posted by Copperhead

I've got 3.42 rear on my ride and at 60 I'm turning 1300. 65 I'm turning 1400. When I get in those situations where I have to keep it around 55, I usually will drop to 12th and be turning 1400 at 55. Not critical on a light load, but on a heavy one, it almost has to be done.

60 mph at 1300 rpm? that's great. that's what i want. with my setup it looks like if i go with that LP24.5 tire in the future (496 rev/mi) and switch my axle ratio to 3.58, i'll have 1296 rpm at 60 mph. which is pretty good. 3.42 will be too fast for conditions in MI, IL and OH.

[specialkay]

You must like to spend money. You have a truck with 22.5 rims on it but instead of going to 11R22.5 tires you'd rather buy 8 rims and go low pro 24.5's ? In Ontario you'll get nothing for 24.5 low pro casings. You can run normal tires on the back and low pro on the front if you are bound and bent to stay with them. BTW a lot of trucks run 11R22.5 all the way around and drive fine. My truck had 11R24.5's on it all the way around. I got a good deal on used low pro's for the drives so I tried them for 8 months. Wasn't impressed with the tires didn't see any improvement in fuel mileage. I cut one and spent hours trying to buy a used low pro and finally couldn't get anything from numerous tire shops for casings (Michelin tires) I went back to 11R24.5 and won't be switching again. Rant over.

[sgreer78]

Split the difference and go with 3.42's. A lot of trucks are running them now with good results.

1350 would be 63.38mph

60mph would be 1278

65mph would be 1384

70mph would be 1491

[boneebone]

Originally Posted by specialkay

You must like to spend money. You have a truck with 22.5 rims on it but instead of going to 11R22.5 tires you'd rather buy 8 rims and go low pro 24.5's ? In Ontario you'll get nothing for 24.5 low pro casings. You can run normal tires on the back and low pro on the front if you are bound and bent to stay with them. BTW a lot of trucks run 11R22.5 all the way around and drive fine. My truck had 11R24.5's on it all the way around. I got a good deal on used low pro's for the drives so I tried them for 8 months. Wasn't impressed with the tires didn't see any improvement in fuel mileage. I cut one and spent hours trying to buy a used low pro and finally couldn't get anything from numerous tire shops for casings (Michelin tires) I went back to 11R24.5 and won't be switching again. Rant over.

I received $60 apiece for my 24.5LP Bridgestone M726 casings from the T/A in Ontario, CA. Didn't have a problem at all.

[specialkay]

He is from Ontario Canada. I live about an hour south of Tracer's area so that why I have a good idea of casing values.