I have a 2005 EB 6.0 and just moved from the stock 265/75 R16s to 285/75 R16s which have added an inch or two to the overall height of the truck. My mileage on the lie-o-meter appears to have gotten worse by a mile or two based on the last few months.

The thing is that I just towed my camper out of storage - 32 foot Jayco BHS. Usually I get horrible mileage towing it. On a calm day (wind) it looks like I may have added 1-2 mph overall. It was on level ground and I was using the stock tow program. I am not counting on just a half day of driving to figure this out but it looks promising. Does this seem possible?

Did the tires push the engine/rpm's into a sweetspot?

I have not thought about it much - final drive ratio, how bigger tires impact the lie-o-meter, how those things might be fooling me... The tires did move my speedometer from being 2 mph slow to now being dead on.

 

I'm guessing it's a math error.

 

I have a Jayco 32bhds I tow with my 7.3. My mileage varies alot from trip to trip. I think there are many variables involved so I wouldn't count on getting better mileage from here out.

 

Throw the lie-o-meter calculations out and do some real hand calculations. I would bet that you will find that your mileage has actually dropped.

The taller gear ratio won't overcome the added rolling resistance of the wider tire.

(I lost 1 mpg going to 285/75/16 tires.)

 

Don't forget revolutions/mile change as you change tire size. Thus, your odometer is now undermeasuring your distance travelled.

 

Use your gps to determine how far off your odometer is, mine was off 1%...

 

On my truck the gps showed my speedometer was more accurate with the 285's. Don' think it's a huge difference but 285's were closer.

 

I know i will get crucified......but I know my trip mileage increased with bigger tires....not in town ....but highway put me under 2k rpm.......which is sweet spot for a 7.3L

 

No worries mate, you look good up there!

now where's that 'burb?

 

I knew I would get crucifed !!....the burban was not as advertised.....still dealin for a lower price......aparently "runs and drives" means something different to some folks.....

 

I have 285/75s on my 02 X 7.3. ive found that my best FREEWAY miles are at 57mph on my speedo,about 1700rpms' or lower puts me right around 19 mpg, not by the lie-o-meter, basic math and i round down, otherwise it comes out as 19.7. all stock X with Stanadyne mixed with fuel, Diesil 911 hurt my mpg by 3 to 4 mpg, delo 400le every 2000 miles. had an edge evo turner....... hahahahahah HAD one!

 

Who drive 57 on the highway?

 

me do

 

I'd cause a 100 car pile up on the I5 if I drove 57mph there!

 

Keep in mind I am using the Lie-o-meter for both calculations. I estimate its about 1.5 difference for both calculations only in the OPPOSITE direction.

All are relatively level ground (I live in MN) and driving around 63mph on average.

265 tires
Non towing freeway - ~17.5 mph
Towing Freeway - ~12 mph

285 tires (bigger)
Non towing freeway - ~16 mph
Towing Freeway - 14 mph

Difference in moving from 265 to 285
Non towing freeway - ~1.5mph worse mileage
Towing Freeway - ~2 mph better mileage

My understanding is that trucks are geared for alot of things but ultimately the cruising speed for the freeway tries to find the optimal engine revolutions with the least amount of fuel. There is a sweet spot where the amount of fuel burned reaches the bottom of the curve based on the amount of resistence on the engine. It makes sense to me that not only does the MPH change when there is more resistence - but the curve may also change meaning that the sweet spot may change. Tire size also changes the curve.

Think about it this way. If the truck were geared perfectly for towing a 32 foot travel trailer (its like pushing a garage door down the road) then it may not get optimal mileage when it is not towing. Since vehicles are sold based on optimal mph when not towing this is what they gear it for.

Not sure if this is correct thinking though.