Bigger tires more hwy mpg?

[A6t8vw]

Just picked up a 23 3500 HO dually with the 4.10 gearing, I'm considering 255/80R17 (33") or 255/85 (34") "least aggressive tread" tires on the stock wheels. Has anyone had experience with increased hwy MPG with the reduced RPM? I'm assuming the added weight and city driving will offset any gains but thought i would ask what others have experienced.

*edit , I would feel better lowering the RPM @ 70-80 mph in general. With my current setup i only tow about 10k but i have a 3500lb Lance camper in the bed and around 900lb tongue weight towing a 20' enclosed trailer with a jeep inside.

thanks,

 

[Docwagon1776]

No. RPM isn't a measure of fuel economy, as different amounts of fuel will be required to maintain the same RPM under different conditions. Your RPM doesn't change (without a shift in gears) going up an incline vs going down a decline, but obviously your pedal position and fuel consumption will change.

Larger tires are larger levers, requiring more energy. Larger tires *tend* to be heavier tires, requiring more energy.

You will see mpg reduction across the board.

 

[SpeyRam]

Very useful website to calculate your effective gear ratio with changes in tire sizes and its effect on fuel economy etc.
https://tiresize.com/gear-ratio-calculator/
I've referred to this often.

 

[jeffn]

I have thought about doing the same. If I could run 75 at 2,000 RPM my mileage would improve considerably.

 

[DrTron]

I have thought about doing the same. If I could run 75 at 2,000 RPM my mileage would improve considerably.

Like others have said, probably not. Higher rpm are often associated with lower mpg because the engine needed to downshift to produce enough power to keep the vehicle at the set speed, resulting in higher rpm's.
So the higher rpm's are only the symptom you can see, but not the underlying cause.

Just imagine going up a hill with and without a trailer. I can guarantee that with a trailer, the rpm's will be higher (due to a lower gear being used).

 

[Dave01]

I have thought about doing the same. If I could run 75 at 2,000 RPM my mileage would improve considerably.

No it wouldn't, not by getting there with larger tires, as a few others have also said. MPG would likely go down.

It would be different to drop rpm at highway speed with different gearing. Lower gear ratio might, repeat might, increase mpg. IMO would not be worth pursuing this on a heavy 4wd vehicle, very expensive and results might not be what you want.

 

[Docwagon1776]

Also, wind resistance is a huge factor in MPG on barn door shaped trucks. Taller doors catch more wind, and air going under the truck is aerodynamically 'dirty', increasing drag.

 

[Great White North Eh]

I dunno … back in 05 I put 35‘s on my truck when it was fairly new and I swear I got better mileage. Diesel’s are torque so lowering overall gear ratio should help… in theory anyhow. That truck was a standard so I controlled the rpm so that might have been the determining factor. That barn door was one of the most fuel efficient vehicles I ever owned. Towing and right foot episodes not included.

 

[phatboy64]

I put 37’s on my 2023 HO Limited with 3.73 and my mpg went down by ~ 1.5 mpgs on the HW. I have thought about putting 4.10 on but so far I can’t justify the value vs. cost, I just don’t think it is there for me.

 

[Blythkd1]

Mileage probably has more to do with the tire style than the height/diameter. I'm guessing most guys buying 35's and up are buying M/T's or similar. Big squishy tires with deep squishy tread kill fuel mileage. H/T's are usually best for fuel mileage although you probably can't find any that are 35" or taller, nor would very many folks be interested in them anyway. Seems like A/T's have become pretty popular for a reasonable trade off.

In summary, if you switch from a stock height A/T or highway tire to a taller M/T, good chance you're losing fuel mileage.

 

[IndyRamMega]

He said on the highway fellas....at a constant speed....a taller tire means lower rpms....yes your mileage will improve ....all you'd be doing is effectively lowering your gear ratios to around 3.9 which these trucks also come with 3.73's...and we all know 3.73 gets better mpg's than 4.10's....so yes...it'll improve....slightly....it's not going to be a huge increase....iirc when I went front the stock 32's on my HO drw with 4.10's up to 33's I picked up a half a mile....if you go to 34's...some where around 1 mile a gallon is where I'd guess you'll be....

 

[ClawSS]

I'd say Improve on paper, then the deltas come into play and it decreases. Improve "in a vacuum" based on effective ratio, but then the aero worsens, wheel tire weights increase, rolling resistance increases, extra chicks in the cab due to "them big tires", well...you get right back to where you were or worse.

My 4.10s were ordered because I knew I'd be going 37's which is about like buying a 6.4L with 3.73s once the tires are on....and they are HEAVY! I saw 15-16mpg stock on the DIC, but rarely see 14 now. Same drive day-in day-out. No extra payload or ever towing.

 

[phatboy64]

Thats what I would have thought but that is not what I have actually experienced (see my post above) and I put Wildpeek AT3 tires on and they have a very mild tread design, no more aggressive then the stock Firestone ATs..

 

[DrTron]

back in 05 I put 35‘s on my truck when it was fairly new and I swear I got better mileage.

There's another factor in it and that's break-in.
I put 35's on mine right after I bought it, and was seeing upper 13s in mpg. Now the truck ('22 PW) has 12k mi on it, and I'm frequently seeing 15. Same use case as before.

 

[jeffn]

Like others have said, probably not. Higher rpm are often associated with lower mpg because the engine needed to downshift to produce enough power to keep the vehicle at the set speed, resulting in higher rpm's.
So the higher rpm's are only the symptom you can see, but not the underlying cause.

Just imagine going up a hill with and without a trailer. I can guarantee that with a trailer, the rpm's will be higher (due to a lower gear being used).

Diesels are different than gassers.

Peak torque is 1,800 RPM (1,000 lb-ft in HO.) With 4.10 gears in a DRW, the engine turns about 2,150 at 75 mph. The only way you will get the RPM down, other than slowing down, is to change the effective gear ratio at the wheels. 5th and 6th on an Aisin transmission are already overdrive ratios. When I drop down to 2k RPM, my mileage improves by about 1.5 MPG over 2,150 RPM.

There is a lot of noise in this thread. Tire size, differential gear ratio, transmission gearing, and engine torque curve all operate as a system. It always comes down to how many rotations the tire is making in the sweet spot for power output.

 

[Dave01]

There is a lot of noise in this thread. Tire size, differential gear ratio, transmission gearing, and engine torque curve all operate as a system. It always comes down to how many rotations the tire is making in the sweet spot for power output.

I don't think it's noise, just lots of people with lots of real world experience. The usual differences of opinion are interesting to read.

You mention size, gears, tranny, and torque curve. You leave out what Blythkd1 and others mentioned, the style and weight of tire. Huge factor.

 

[Docwagon1776]

He said on the highway fellas....at a constant speed....a taller tire means lower rpms....yes your mileage will improve ....all you'd be doing is effectively lowering your gear ratios to around 3.9 which these trucks also come with 3.73's...and we all know 3.73 gets better mpg's than 4.10's....so yes...it'll improve....slightly....it's not going to be a huge increase....iirc when I went front the stock 32's on my HO drw with 4.10's up to 33's I picked up a half a mile....if you go to 34's...some where around 1 mile a gallon is where I'd guess you'll be....

Changing gears changes *one* variable. Gearing. You will likely see some gains due to lower engine rpms having less pumping losses, reducing the overall energy requirement to move the truck at a given speed.

Changing tire height changes multiple variables. Many of which require additional energy to overcome. There is no set amount of fuel required to maintain a set rpm, again think of pedal position to maintain 2500 rpm up a hill vs down a hill. Taller, heavier, squishier tires require more energy. That energy comes from somewhere, in this case more fuel burnt.

 

[Rockcrawlindude]

I am enjoying reading some of the responses from people that have no clue but decided to answer anyway.

 

[ironmike]

Very useful website to calculate your effective gear ratio with changes in tire sizes and its effect on fuel economy etc.
https://tiresize.com/gear-ratio-calculator/
I've referred to this often.

View attachment 64379

I am at the 2202 spot with 37's & 3.73's wouldn't this lead us to believe this would be getting the better MPG based on the pattern from yellow to blue?

 

[jetrinka]

Why is it assumed that higher RPM always equates to more fuel used? Light throttle to maintain speed keeping an engine in the optimal part of its power band is always going to be best - as opposed to a slightly heavier foot (read longer injector pulse width) to maintain the same torque levels/speed.

A diesel's EGT gauge tells the whole story. More fuel = more EGT with a diesel. What happens when you lug a diesel up a hill with a load on the back? EGT climbs. Downshift and let the engine speed up a bit and EGT comes down. Of course there's a bit more to the story than that but the principle applies.