Hi all,

I have a 7.3L Diesel EX, I have lifted it 6" and put 37" tires on it. I re-geared it from a 3.73 up to a 4.30 to get it closer to factory ratios.

I have stock air/exhaust no chips. Nothing really aftermarket other than the lift kit/tires....

Currently I'm towing a 6,000-7,000 pound travel trailer at 29' long, and my MPG is not great, I live in California and nothing is flat, I go up the Grapevine, Tehachapi passes backside 395, Sequoia, Yosemite, The coast, etc.... Lots and lots of mountains and grades, all the time, all mountains all the time......

I get about 7-9 mpg towing, 10mpg in the city, and 11.5 hwy while not towing.

I'm pretty sure the 37's are killing me, but after the re-gear I was hoping for better.... I'm tempted to put in some more after market components in, but honestly at this point I think that unless I bring the tire size down nothing will matter...

For a while I tried 35" tires and I was hitting pretty high RPM just going 60mph up a hill, like something like 2200 RPM, the turbo kicked in even at 60

Now with the 37's and the regear I can go 65 MPH what I call "cruise mode", where I'm just under 2,000 RPM and crusiing at 65MPH. (no turbo)....


Anyway just looking for some advice and to see if my numbers seem normal to anyone else....., I know my MPG will tank with big tires but 7MPG while towing on a diesel seems a little crap.....

I will also mention I have no idea what PSI to run on the tires now that I swapped everything, if I should use doorjam pressure or max PSI per the tire.... I have Toyo open country R/T tires they say I can go to 65PSI but per the "doorjam" I'm running 50 PSI in front and 55 PSI in rear....

Any advise anyone has is appreciated, if chips, tuners, or something else will help my MPG on long trips....

I spent $700 in gas in 10 days and at that price, I was considering buying some small 16" rim 285s or something and just keep them in the garage for towing and road trips.......

Any advise is appreciated, sorry for the novel.....

 

Sounds like your up-pipes might be leaking and costing you turbo efficiency. This costs you fuel efficiency in the long run. Have you checked them for exhaust leaks? How much boost are you making on these long pulls up the mountains? What is your top boost number at WOT? The lift is likely hurting you more than you think, as well -> aerodynamics. Big tires equal more rolling resistance and negate the benefit of the bigger circumference you would think you are getting (final gear ratio).

I live in Central Texas...small hills lots of flats. Recently towed 11500lbs and got 11mpg while towing it. I have lots of mods...basically, everything has been modded on my engine. I am running stock 32" tires with 3.73 gears. I run 1900 rpms at 65mph.

My MPG averages are 15-16 when wife is running kids around. On trips to the farm 480 miles away, I routinely get 20 MPG highway if not pulling the stock trailer.

You should ONLY be running E rated tires on your X. They handle up to 80 psi and have a much stronger build than the D rated it sounds like you are running. I run my E rated 32" tires at 65 psi rear and 60 front WHEN UNLOADED and NOT TOWING.

I doubt a tuner would help you much without an injector swap, intake upgrade and exhaust upgrade. If you upgrade injectors, it would be prudent to upgrade the turbo as well. You see where this is going....$$$$

 

Hey thanks for the reply, you still think I should get 20 MPG with my 37" tires and 6" lift?

For sure they are E rated.... it's the Toyo open country R/T , 37x13.5x20

 

The '99 in my sig with 6" lift and 37's gets about that same mileage towing and not towing, and my trailer is 31" topping off at around 8k lbs. The only slight difference is I get a tad better MPG's towing, and a tad worse MPG's on the highway not towing. City is the same though.

Nope, that's about right, based on my own results.

Tire size and lift, but that's not something you can undo unless you plan to re-gear back down to 3.73's.

Your turbo isn't an on/off thing, it's always on, pushing air down the throat of the engine.

With a 6" lift and 4.30's, going back down in tire and wheel size is no longer an option.

Stewart

 

That's not what he said. He gave you his results for his situation and equipment.

They are not E rated. I wonder if anyone even make an E rated tire in a 37" tire size.

Stewart

 

Thanks so much for the replies stewart, it's good to know I'm at least in the ball park with numbers.....

I've looked at some other threads and 6% grades for 5 miles ++ looks like even 20mpg trucks are tanking into 6's and 7's mpg

I guess these toyos are crazy, I triple checked them and for sure they are e rated.... I even double checked the website..

https://www.toyotires.com/tire/patte...-terrain-tires

Anyway, thanks so much again for the replies!

 

Oh you know what, I assumed you have 18" wheels like me, are yours a different size?

Half awake and my brain didn't engage and I made the cardinal sin of assuming you're running a 37x13.5x18 like I am. Those are D rated tires.

Stewart

 

I looked at the link you supplied and one thing stood out: The little logo to the far right shows these make for terrible fuel economy. They are big, squishy, and smother the ground. Gearing down to match the tires means you are very committed to this tire size... unless you are OK with throwing the accuracy of the odo and speedo out the window with a smaller tire size during towing. Lifts kill economy, even when using stock rolling hardware. You have a macho truck, and it take a macho wallet to operate it.

 

there's a couple things that stood out to me. A soft squishy tire is going to suck up gas mileage versus one that is solid and rolls with less resistance. I run 75 PSI in the front and typically 70 to 80 in the rear with my 8000 pound trailer on my truck.
Regarding tire wear, you can do a chaulk test. Get some sidewalk chalk like your kids would use, and draw lines across your tire and then drive in a straight line about a half a block. This will tell you where your tires are touching the pavement, i.e. Chalk will rub off. If there's no chalk in the center of your tire there's too much air in them. If there's no chalk on the outside of the tire and the inside still has chalk your tires are cupping.
The same goes for your trailer, if your tires are soft and squishy on the trailer that's also going to take up fuel to get them to turn. I run an E rated tire on my trailer too, and I run 70 psi in them. Is your trailer and truck pulling Level ?

We had a saying when I was in the military, Ounces equal pounds, pounds equal pain. Meaning little things add up and can cost you big-time in the long run.

I tow through the Rockies, i.e. Colorado, I get 11-13mpg towing between 7K to 12K in elevation, this is also affected by traffic speed, strong winds etc.
I'm geared at 4:10's with stock tires, I also pay close attention to my gauges and RPM's to keep the truck in the power band.

What gauges do you have and what numbers are you seeing? How would you rate the overall condition of your truck? Any exhaust leaks etc?

As others have already stated, you're kind of in a position with your gears that you have to stay with that size tire BUT running an AT versus an MT tire will help too. Good luck.

 

How was the odometer corrected?

 

Thank you for that advice!!!! My trailer is very level, (I measured on a flat road front and rear), but my excursion does sag in the back, I don't have any air ride suspension, or anything on the rear axle to help, not sure how much that will help.

Also I will try the chalk test that's a good idea, I thought about raising the pressure as well, 50 and 55 might just be to low for those tires......

Stock tires are 31" mine are 37", I was even thinking about a 33"-34", maybe 35" set of tires that are not offroad, built for towing, really hard, even if it gets me 20% more efficiency that's $200 per trip extra, and maybe even more on a longer trip.....

Anyway like that other user said, you macho up the truck, you basically just pour out the money.....

 

Heres a picture of the setup...... trailer is level, but excursion has some back sag, will that mess up my fuel con significantly??




And a closeup of tire specs

 

No problem Stewart you are very knowledgable, so I triple checked.... I have a 20" rim, (city boy rims)

so that may explain the rating difference

 

Yeah, Tugly I think you may be right......that's why I'm thinking a set of towing tires is a good idea....., my 35's cruised at 60 mph, in California, the speed limit for trailers is 55 anyway, so as long as I can cruise at 2k rpm I should be getting maximum mileage, I still have my old excursion 16" rims, and I can get 285/75/16 tires all day highway for like $125-$150 each..........

I feel like that would be my biggest improvement 13.5" wide vs 11" wide on the 285 alone would make a big difference, I don't think the height of my ex matters a lot since I'm towing a trailer that is taller....

Anyway thanks a lot everyone you have all been very helpful

 

It looks like you might benefit from tweaking your weight distribution (WD) hitch setup. It won't improve the fuel economy but it will make it safer any more enjoyable to drive. If the WD bars are tightened up it will level out the truck some and put more weight on the front wheels. If you measure the front fender height with nothing hooked up and then WD you want to return the front (down) to 50-100% of the height when nothing is hooked up. Never more than 100%.