Last summer i went from the stock mich's 265/70/17 to the tara-grappler highway 285/70/17 with 3.73 gears. I wouldnt think changing 2 sizes (from 31" to 32.8") would be much of a deal but i have realized it now is. Between 45-55MPH the engine lugs like crazy, even with the smarty on a low setting i am blowing a good bit of smoke. I either have to put it in tow/haul mode until i hit 55 or just punch it to get the turbo spooled up. I am really thinking about going with the 4.10 gears to bring me back to stock RPM ratio for tire size.

What do yall think?

 

I think something else is wrong.

 

Must be something else wrong cuz Ive heard guys say they never even noticed a differance...are you pulling anything?

 

I agree, something else is going on. Removing the Edge J/A would be the first thing I would do for troubleshooting. Also make sure the MAP sensor is clean.

I didn't notice any change other than cruising RPMs going to 285's.

MikeyB

 

I did the very same thing, 265s to 285s on 3.73 and while I noticed a slight change, it's not to the point of lugging and blowing smoke. Especially not at 45-55, for me, the truck's just starting to take off on her stride. I normally run in TH anyway since I'm in the city.

 

You have to figure that Cummins Dodges made before 03 were running around with 3.54 rear axle ratio's and didn't have a problem with power. Without out doing the calculation, I would have to think your still around that same area now with your effective rear ratio since you added the larger tires. Something else is wrong, that small of a jump shouldn't make a huge difference.

 

not pulling anything, just cruising around the highway. anything under 50 i usually run tow/haul anyway, but if im riding around on the highway and traffic slows below 50 i dont want to have to put tow/haul on. I would rather the tranny just shift down but i have to get on the gas to do that. I will start with removing the edge and bring the smarty back to stock. see what that does.
Im going to do a RPM test today and report back what i have at certain speeds and see if those seem right.

 

I just went from 265's to 285's on my truck which has a 3.73 a week ago and have not had any issues. In fact, with the different threads, it seem to ride better, no change to acquire my desired speed, maintaining my speed or change in mpg's. There has to be something else affecting it.

 

I assume you have an automatic 48RE. With the 260 tires and 3.73 gears you should have been turning 1800 rpms at 65 mph. With the 285 seris tires you should be doing 68 mph at 1800 rpms.

Going back to the 260 tires it would take a 3.55 ratio to get you to 68 mph at 1800 rpm's. So your effective ratio is now 3.55 with 285 tires on the truck.

Not that much differance, but some. Might be messin with computer for fueling as it is set up for the 3.73 gears. Speedo should be off a little as well.

When I changed my rear from 3.73 to 4.10 I took it in to the Dodge Dealer to reset the values. Don't know if that is necessary in your case.

Dave

 

Here are my speeds / RPM's. All these were read from my edge attitude while checking my tach to make sure they are the same. this was with tow/haul off and my TC locked.

50mph - 1390rpm
55mph - 1480rpm
60mph - 1650rpm
65mph - 1790rpm
@2000rpm my speed was 74mph

I have had the dealership adjust my tire size/speedo cal already. But these speeds were referenced by GPS and not speedo. As you can tell, 1400rpm at 50 is low and can see why my engine lugs. I didnt have this problem before my tire swap and noticed it right after. I thought i could get use to it, but i have noticed my fuel mileage suffer 1-2 mpg if i do a good bit of driving between 50-60. most of the time i cruise on the highway at 65-70 but there are times where 50-60 is done because everyone in the Seattle area takes up all 5 lanes doing those speeds, but that is a different topic.

 

When I look at my spread sheet and compare to want you posted its really close to what I see here. You've got the equivalent to a 3:55 gear with those tires.

If I were you, I would run in 3rd gear to keep the engine r's up some. Get into OD on the big road.

Take it from a Boat Driver, changin tire sizes is like changing wheel sizes, a
20x18 will turn 2100 rpm, while a 18x18 will turn 2250 rpm's. So you put some bigger wheels on it.

Dave
CWO4 (BOSN) USCG RET.

 

Dave,
I do usually run with tow/haul mode while putting around town and as i pick up speed. I think going with 4.10 will balance me back out to normal RPM's. going off the below chart, im on the lower end of fuel economy. since i do mixed driving I think i need to be a little closer to the green. the 4.10 gear will be on the lower edge of the green and i think will bring my fuel economy back up.

http://www.offroaders.com/tech/gear-ratio-chart.htm

 

My buddy runs a 35 and just switched to a 37 inch tire on his cummins with 3.73 gears. He says that his truck doesn't even have that much of a problem getting up to speed. Did you adjust the tire/speedo setting in the smarty? My truck is 4.10 gears with the stock 265 for now and on the highway at like 75mph I run over 2000 rpms easy. I think when I go up in tire size it will actually become a better truck to drive on the highway. Are you sure you have 3.73 gears in your truck and not 3.55 or equivalent.

 

My spread sheet says with 4.10 and the 285 seris tires;

1800 62 mph

1900 65 mph

2000 69 mph

Stock tires with 4.10 is 2100 at 70 mph. When I changed from 3.73 to 4.10
there is only 200 rpm difference at 70 mph. 1900-2100.

Dave