Tires are hurting PSD towing performance

[Maverick]

I just put a set of 315/75r16 tires on my '02 7.3 PSD eXcursion. I LOVE the tires (2nd set of BFG AT KO's), love the traction, love the ride, hate the drop in towing performance, and even day to day driving. It downshifts more often and the mileage has dropped. And I mean dropped. Where I used to get 11, now I get 7 to 9 (towing and around town). Highway mileage is about the same, maybe 1mpg lower at any given actual speed (not what the speedometer says).

The performance with 295's was fine, with 285's even better. As far as looks, the 315's don't look that much bigger than the 295's.

What to do?

Swap/trade tires with someone for 295's
Get a chip (looking at DP tuner 60 tow, 80 econo, 120 fun)
Swap axles for 4.30 gears

 

[Chevycrew]

You might just start with having the speedo recalibrated. With the tire change, the VSS is telling the tranny the wrong speed to shift at.

 

[waynehartwig]

You might just start with having the speedo recalibrated. With the tire change, the VSS is telling the tranny the wrong speed to shift at.

Exactly!!

Then if that doesn't work, go with 4.56 gears with your new 35" (34.6) tires....

What gear ratio does your truck have stock and what size tires were on it stock? If you regear correctly, it will fix your speedo issue as well...

285/75-16 (32.8")
295/75-16 (33.4")
315/75-16 (34.6")

With that information, your truck likes the RPM range of the 33.8" tire the best. So with that I can tell you what gears it would like with the 35's.

EDIT: fixed my typo of tire size....

 

[Bart]

It's still worse mileage, no matter how you figure it. I found the same thing when I put 315s on. I've lived with it and love the way the truck looks, but I may drop a size the next time around. I do have a programmer that helps.

 

[mbryson]

I think I'm pretty happy with my 285s. I still haven't even put my leveling kit on. If it's going to be warm for a few days, I might venture into putting that on PDQ. If I do go to 315s, I'd like to move to a 4.10 ratio, but don't know that I want the expense on the tow rig.

Mine gets it's best fuel mileage below 2000 RPM. It drops significantly every 100 RPM above that. I'm thinking about 2 mpg per 100 RPM.

 

[Maverick]

Exactly!!

Then if that doesn't work, go with 4.56 gears with your new 35" (34.6) tires....

What gear ratio does your truck have stock and what size tires were on it stock? If you regear correctly, it will fix your speedo issue as well...

285/75-16 (33.8")
295/75-16 (33.4")
315/75-16 (34.6")

With that information, your truck likes the RPM range of the 33.8" tire the best. So with that I can tell you what gears it would like with the 35's.

I certainly appreciate the offer to help with the math.

The 3.73 equivalent to 35's is 4.21's...so 4.30's would be a bit overkill. (assuming 31's are stock) Since it likes 33's, (both 285's and 295's), 3.95's would be the equivalent. ( btw, 285's are 32.8)

Don't know about the VSS....for example, when it shifts into OD at 45 I'm actually doing 49ish...and that's actually too soon since the load is more than it can handle. Redo the VSS and it would shift even sooner, when I'm actually doing 45. I would think that that kind of shifting would be more "load based". I don't think it would hurt, and it would help in other ways, but the load is the load, unless the computer is smart enough to adjust for bigger tires. All that is gonna do is tell it that it is going faster than it thought it was.

And that's not the issue with shifting anyway. I'm talking about being on the freeway at 65 to 70. In this range, towing, it will stay in OD on all but the steepest hills or if I totally floor it. Now, with the 315's, it will downshift on slight grades or with an increase in throttle (not floored).

I really don't want to regear ($$). If a chip will help, that's what I'll do. If not, I'll try and swap for some 285's or 295's or even 305's.

 

[mbryson]

I certainly appreciate the offer to help with the math. If you knew me better you'd know why that was funny. btw, 285's are 32.8.

The 3.73 equivalent to 35's is 4.21's...so 4.30's would be a bit overkill. (assuming 31's are stock) Since it likes 33's, (both 285's and 295's), 3.95's would be the equivalent.

Don't know about the VSS....for example, when it shifts into OD at 45 I'm actually doing 49ish...and that's actually too soon since the load is more than it can handle. Redo the VSS and it would shift even sooner, when I'm actually doing 45. I would think that that kind of shifting would be more "load based". I don't think it would hurt, and it would help in other ways, but the load is the load, unless the computer is smart enough to adjust for bigger tires. All that is gonna do is tell it that it is going faster than it thought it was.

And that's not the issue with shifting anyway. I'm talking about being on the freeway at 65 to 70. In this range, towing, it will stay in OD on all but the steepest hills or if I totally floor it. Now, with the 315's, it will downshift on slight grades or with an increase in throttle (not floored).

I really don't want to regear ($$). If a chip will help, that's what I'll do. If not, I'll try and swap for some 285's or 295's or even 305's.

You guys working on the green on Sat. @ Ian's? We should swap tires and see what you think of my chipped truck before you plunk down the cash. (I'll try to get my kit on before coming down or I'll need some new fenders)

 

[Meat_]

You are already taxing your transmission more with the bigger tires, if you chip it to fix your lack of power you can count on rebuilding your trans soon. The best way to fix bigger tires is to regear. wtf is up with 11mpg? Something is wrong there....

 

[mbryson]

.... wtf is up with 11mpg? Something is wrong there....

Yeah, I get that towing my camp trailer, kinda faster than I should.

 

[Kiel]

Yeah, I get that towing my camp trailer, kinda faster than I should.

Yep, I get that towing uphill doong 80 mph w/37's. I put 4.56's in mine. Changed it night and day with shifting, egt, and trans temps. I would suggest going back down to a smaller tire if it was me. I only tow the buggy with mine and drive it everyday, so towing weight was not a major concern it just have to works.

 

[Maverick]

I think I'm pretty happy with my 285s. I still haven't even put my leveling kit on. If it's going to be warm for a few days, I might venture into putting that on PDQ. If I do go to 315s, I'd like to move to a 4.10 ratio, but don't know that I want the expense on the tow rig.

Mine gets it's best fuel mileage below 2000 RPM. It drops significantly every 100 RPM above that. I'm thinking about 2 mpg per 100 RPM.

The only reason I'd do the level is for looks. The 315's barely rub. I don't want the expense of regearing either, including just swapping in V10 axles.

If someone had some 285 or 295 AT KO's with only 5000 miles, I'd trade straight across.

Maybe we should trade tires for a week, or the next time you tow (soon) and see how yours performs.

My mileage has more to do with speed than RPM, at speeds above 60 anyway. I've never charted it vs RPM, though it's easy enough to calculate

I've posted this before, but with 285's, this is what I got:
55=25ish (1571rpm)
60=20ish (1714rpm)
65=18ish (1857rpm)
72=16.5 (2057rpm)
78=14.5 (2229rpm)

The rpm is based on 70mph at 2000rpm. The stopwatch and mileposts tell me it's accurate with 285's. My mpg "lie-o-meter" is usually dead on, verified at the pump, painfully topping off the tank.

It got even better mileage with 265's when I very first got it (70,000 miles)

The 315's have dropped my RPMS at a given speed, but the mileage hasn't gotten any better. In fact where I would get 16.5 at 72mph, now I get 15.5 at 72 (speedo reading approx 67) even though the rpms are lower. That's also corrected for tire size.

 

[Maverick]

You are already taxing your transmission more with the bigger tires, if you chip it to fix your lack of power you can count on rebuilding your trans soon. The best way to fix bigger tires is to regear. wtf is up with 11mpg? Something is wrong there....

Yup, I don't get 22mpg towing like everyone else with a turbodiesel.

With 285's/295's on my 2002 excursion 7.3, I get 11mpg around town (stop and go on surface streets) or towing my jeep to Moab. That includes timing lights, coasting, driving mellow, etc. At the same time and under the same conditions, I can go get on I-15 and do 65 and get 18ish.

btw, when I'm towing, I'm going wtih the flow of traffic, maintaining 70 over Soldiers.

Good points on the tranny...that's something that has been hanging in the back of my mind.

 

[waynehartwig]

The 3.73 equivalent to 35's is 4.21's...so 4.30's would be a bit overkill. (assuming 31's are stock) Since it likes 33's, (both 285's and 295's), 3.95's would be the equivalent. ( btw, 285's are 32.8)

EDIT: I'm an idiot.. I see what I did, I mistyped 33.8 for 32.8. I'll leave the following in case others want to know how calculate it...

Width x Aspect Ratio = Section Height x 2 = Combined Section Height + Rim Diameter = Tire Diameter

So
285/25.4 = 11.22 (this is the tread width)
(11.22*2)+16" = 8.42 (this is the distance from the rim to the outside of the tire)
(8.42*2) + 16 = 32.83" (obviously this is the overall height)

So that makes it equivalent to a 33x11.50-16 tire.

 

[mbryson]

The only reason I'd do the level is for looks. The 315's barely rub. I don't want the expense of regearing either, including just swapping in V10 axles.......

My 285s rub enough to take my splash guards off. I'm sure I'll need the leveling kit for 315s

 

[Milner]

I run 4.10/4.11 with 315's a chip and injectors.....towing at 80mph I get 11-12, drop to 65-70 and I get 15ish....I think the 4.10's are a good match for 315's....but I drive fast I would not want to run the rpm any higher than I do....
I hope to gain a little when I get around to to puttin the intercooler in.

 

[Maverick]

I run 4.10/4.11 with 315's a chip and injectors.....towing at 80mph I get 11-12, drop to 65-70 and I get 15ish....I think the 4.10's are a good match for 315's....but I drive fast I would not want to run the rpm any higher than I do....
I hope to gain a little when I get around to to puttin the intercooler in.

IF I went with gears (big IF), who would you recommend to install them?

 

[RockMonkey]

IF I went with gears (big IF), who would you recommend to install them?

Carl.

 

[RockMonkey]

I'd just go back down in tire size. I know you use yours a little off the pavement, so the 315s may make sense. To me it seems the vast majority of the full-size trucks on the road never leave the pavement. Mine really only sees dirt when I'm parking to pull the trail rig off the trailer. I'm sticking with stock size tires and keeping it as economical as possible.

 

[Maverick]

I'd just go back down in tire size. I know you use yours a little off the pavement, so the 315s may make sense. To me it seems the vast majority of the full-size trucks on the road never leave the pavement. Mine really only sees dirt when I'm parking to pull the trail rig off the trailer. I'm sticking with stock size tires and keeping it as economical as possible.

ya, I LOVE the 315's in the snow, especially the deeper snow, but the 295's did pretty decent too. They were certainly enough to get me the places I needed to go.

If I could find someone to swap, I'd probably do it.

I've been wanting a tune for some time...but I think the Green is gonna take most of the green from my wallet for a while.

 

[Milner]

Carl.

Yep, or down to Kevin at 6States in Orem....