We're currently on a two week camper trip through AZ and NM Indian Country with my brother John and his wife Krys.
Jeanie and I run a 2020 7.3 F-350 XL SB SC with Northstar Laredo and they have a 2022 7.3 F-350 XL LB SC with 10 foot Alaskan. This was a good test to see the difference between our 4:30 gears and Bro's 3.73 gears on virtually the same truck carrying about the same load.
I gassed up at COSTCO but there was a problem with the pumps on the island I was on. Regular gas was not to be had.
All the pumps had 91 octane gas at the 87 octane price. Great! This is a test. We filled with 91 octane, and what a difference it made! I hardly used any pedal at 65 mph. But with the newfound power came a downside: namely you couldn't stop playing with that newfound power passing trucks *****-nilly and suffering a net mpg loss. Even pulling hills was a breeze, with minimal downshifting. You could accelerate uphill in 10th gear. Bro John thinks the computer advanced the timing up to detonation (ping) and set the timing just below that point. What a ride! But it didn't last. Once the tank was out of 91 octane, the fuel in s.e.NM was 85 octane.
What a difference! The 7.3 was a slug. You had to mash down on the stupid pedal to make the 10K pound truck/camper move and it constantly downshifted, way down to 5th and 6th gears when we hit the slightest uphill. The only good news is the computer was saving me from myself by continuing to deliver very good mpg.
jefe

 

WOW. I can not believe you posted this. You are brave. Many are going to claim you are the crazy one not the truck and octane has absolutely no impact on anything. There will be the waste of money flying from many.

I have ran high test in my tow truck for decades due to it really does allow timing to advance and it gives the computer the ability to advance the timing. It is true octane does not make power, but advanced timing does.

I pull in Dixie and it is hot and humid. I experienced knock sensors errors and kept demanding to have the sensors replaced. My wrench told me the knock sensor was doing its job and to run high test. I followed his advice and never kicked another error on that truck for 22 years of towing.

I will continue to run high test on my truck and tune out the nonsense noise from the know it alls.

 

Sounds like a great time was had by all.

Thanks for posting.

ps: I guess the Big Question (which you alluded to) is could you tell difference between 3.73 and 4.30?

 

Hope you all are getting a tail wind heading east.
Happy traveling!

 

Nothing less than 87 in my truck.




87, 88 or 89 for my driving. Just depends whats at the pump.

Also curious of mpg between the 2 seeing they were traveling together with same conditions.

 

Haven't tried this test on my F350 yet because I still haven't gotten it! But I found this out the hard way on my SUV, a Toyota with the 4.7 V8. When I first bought it, I ignored the note in the manual to not use anything below 87 octane, and to use premium 91 for best performance. I live above 9,000 feet and was told that 85 octane was all that was needed up here in any non-turbo engine.

When I filled up with 85, everything seemed okay at first. Then the next time I tried to start the thing with a cold engine, it wouldn't start! I guess there's a sensor that detects when the engine fires, and disengages the starter. So it would sputter a couple of times and then stall. I would have to start it over and over again before it would stay running. Even after many years, spark plug changes, tuneups, etc..., it will still do this if I ever have to fill up with 85 on an empty tank.

So I flipped to the dark side and filled up with 91. Not only did it start perfectly, but it would downshift much less often on uphill grades, which was so much nicer to drive, and mpg went WAY up. The MPG doesn't go up enough to justify the difference in price, but the drivability is worth the difference.

I don't always run 91, but I make sure the octane always stays above 87. So if I'm in an area with rip-off gas prices, I can fill up with 85 for one tank, knowing that the 91 that's still in the tank should offset it.

 

Has this been posted?
We've been out of internet range.
jefe
edit: Oh,sorry, it finally loaded.

 

Posted fine.

 

It looks as if using 85 octane in the high elevations of Indian Country as we did was a mistake.
Using 87 from Flagstaff home to NorCAL kind of healed itself. The wind blew up a storm for 8 of the 12 days
we were out on the road making a side by side comparo kind of useless. Day after day of 40-60+ mph gusty winds invariably as a headwind,. All in all, John's 3.73 and low profile Alaskan got better mpg at freeway speeds. Not by much, but better. There are just so many variables.
My 2020 XL was bought off the lot as a 5th wheel tow rig with the 5th wheel hitch and the extras that go along with it already installed. My Northstar Laredo hard side camper, narrow as it is, is a windbreak at 70 mph with a 60 mph headwind. A couple of big gusts blew me in the opposite oncoming lane a couple times. Not white knuckle material, but having to drive every second on the road.
The 4.30's are "trailer starters" with gobs of torque to get the 4 tons on the pin moving. I've never used the 5-er hitch. By 68 mph, I see the rpm's moving up in 10th gear and off minimum peak torque. With 91 octane gas, the Super Duty is an absolute beast. The computer changes the tranny shifting pattern with not as many downshifts.
So, bottom line: the 3.73'a were great above, oh, say 68 mph. Good interstate manners.
the 4.30's were better with the approx. 3200 pound load while creeping around on back dirt roads well under 64 mph.
Our average speed, overall, over the 3571 mile trip was about 65 mph on on interstates (at my request) but Bro John was happier at 72 mph. Lower profile/3.73's.
In thinking about the so-called "perfect gearing", I still like the 4.10's as the best compromise with the campers/7.3 gassers/10 speed trans. If I go with 35's, I'll get a defacto 4.10 gearing,or close to it.
If asked what mpg I realistically got, I would have to say between 10 and 14 mpg with the camper fully loaded and the centerline at 12.5 mpg. I"ve gotten better than 14 a few times, and worse into the single digits once on Hwy 40 coming back to CA with everything against you. This is about 1 mpg less than I get with my 2001 24v Cummins TD HO 6 speed manual- but sans the egg beater and a bucket of bolts noise level.
jefe

 

Thanks for the report/ comparison on the trucks.

The 91 octane is making that much of a difference it should be considered when working these engines. I will be keeping it in the tank.

That wind will keep your attention that is for sure.

 

Anything more than 87 gets you nothing.

 

Not necessarily when the computer can advance timing vs retarding it to keep from pinging.

Pull at sea level on 110 degree 100% humidity day. I’ve went around with my mechanic on this and he proved to me the timing is advanced with 91/93 vs 87.

 

Anything less than 87 can also threaten the warranty. When I'm at elevation, I usually need to do midgrade to get above the threshold.

 

This is great info, as I'm coming from a 98.5 24v Cummins 5spd manual. Doesn't seem like the 7.3's MPG penalty is very much compared to our old Cummins rigs.

 

I'm only down about 3mpg from my 2004 5.9 Cummins... and the 7.3 tows the same trailer way better too, so I'll take it.