FWIW, I thought I'd post this information about what I've experienced recently with my w250.


Since I've owned the truck, it had the stock tires 235/85r16, with an all season / highway styled tread, generally getting 18-20 MPG both highway and tooling around.

Since I installed my mud tires for the upcoming season of "snow" we have here, I've lost almost 5 MPG by just switching to the larger tire size that has a "mud" profile in the 285/75R16 size.

I never realized how much of a difference tires can have on fuel economy.


Have you experienced the same ?

 

Not that big of a difference! Are you getting #1 diesel or is is cold enough yet?

 

I lost a few MPG with new same size tires on a 4cyl dakota, they where heavier. Also keep in mind with different size tires the speedo is off so MPG will be off also.

 

On a 500+ mile trip, I averaged 23.3mpg with a M/T 33x12.5x16.5 tire.

Still average 19-20 combined.

I also have 4.10's and I cruise around everywhere 55-65mph which is 1600-2000rpms on the tach. I am right in that sweet spot for optimum torque and any bit of resistance the turbo spools slightly quicker.

If you have 3.54s, I can see you lugging the engine slightly with not only larger tires bringing the RPM's down but now you are adding additional rotating mass. The auto aint helping one bit either.

 

My truck gets 20mpg with a 5 speed and 33's and 3.54 gears, that's a mixed city/hwy

With the 35's I get 17.0 at worst.

5 mpg down sounds horrible.

 

I don't know what I would run with a mud tread...but with my BFG Commercial T/A's I get around 21-24 MPG mixed Hwy and local. Straight Hwy I can see 25-28 MPG.

 

During the last few tanks I have ranged from a low of 17.5 and a high of 18.8. That is also correcting for my odometer that is short by 5 miles for every 100 miles driven. Verified consistently against GPS. This MPG range is with 90% freeway miles at 60 MPH and driving VERY conservatively. To be honest, I am dissapointed in those numbers and would like to be in the low 20's like so many others here. However, with a 4.10 geared 4x4 dually, I don't think that is in the cards. I run 235/85/16 Toyo A/T tires, a stock motor (with exception of a 366 spring) with 104k miles on the clock. The increase rolling resistance and mass that comes with a dually is a lot. Not to mention greater wind resistance by a wider body/fenders/tires. I would imagine my RPMs at 60 MPH is somewhere between 2,000 and 2,200on stock tires and 4.10 gearing? I know a 1/8 timing bump might increase MPG a bit but I can't imagine much due to truck configuration.

NJTMAN - I see you went from an 85 to a 75, shorter tire. At a given speed your RPMs are now slightly higher with registered miles being artificially higher at the odometer. Is your 5 MPG drop calculated with an adjustment for change in tire height?

 

No,

I simply filled up the truck, reset the odometer, and ran it down 3/4 tank, filled it up again, and divided the mileage by gallons to fill.

Probably not the most accurate, but a 25% drop seemed a lot for a tire change. I attributed it to increased rolling resistance in the MT tires

How do I adjust ?

What formula would be correct to adjust ?

At some point, next year, I would like to drop the trans and install a converter to make the slushbox a little more efficient. Hopefully by then, I can get someone to do the work for me that knows what they're doing, and doesn't just hand me a bunch of bologna as to why they can't do what I ask.



T.

 

I used a BFG AT as an example, but their 235/85/16 tire is 31.7 inches tall, weighs 49 lbs and has a section width of 9.4 inches.
Their 285/75/16 is 32.8 inches tall, weighs 59 lbs and has a section width of 11.4 inches.

Those are huge differences, especially the weight since thats all unsprung weight- a total of 40 lbs. That's like driving around with 400 lbs in the bed all the time. Then add in the tires are 2 inches wider and the aggressive tread and some mileage decreases are expected. If the speedo isn't being corrected, that would explain the severity of the drop.

This formula will help you get the mileage right, or at least explain how it changed (sorry about it being a miata site)
http://www.miata.net/garage/tirecalc.html

 

a 285/75 is NOT shorter than a 235/85 at all.

The 85 and 75 does not even DIRECTLY relate to the height of the tire- It's an aspect ratio and because the 1st number is so big the tire is actually almost 33" tall.

The only reason i could see him losing mileage is the WIDTH of the tire, Skinny tires get better mileage. could also be the weight of the tire.

IIRC, Tman has the "Definity Dakota M/T" tires that Pepboys sells. I believe they are made by COOPER

 

According to the calculator, your speed and mileage would be 3.5% lower. You need to take the number of miles driven and multiply by 1.035 to get your true miles driven, then divide that by the number of gallons to get your corrected new mpg. I keep a spreadsheet with my mods and my mpg, and going from 235/85/16 highway tread to the same size Goodyear Wrangler Duratracs I lost 1 mpg.

 

Those are real good numbers even with the Imperial gallons, ... mine runs about 3mpg less on each number with all A/Ts.

 

jersey,

It tells me that there is a 3.5% difference in tires..

What does that mean, and how do I apply it to the results ?

Never was really that great with math....



T.

 

mk1013 covered the math properly above.

 

Here is a calculator that will tell you the exact difference in your tires. Sidewall height is an aspect ratio, but it does relate directly to the overall height, thereby the overall circumference of the tire which relates to miles per rpms turned, etc........... Anytime you change tire height without changing speedo gearing you get an inaccurate reading. the only way to know what your true MPG is, is to know how all these changes affect your speedo. This calculator will do that for you. But you have to understand that tire changes relate to gearing changes, which relate to how well your engine works in a certain RPM zone. Go over or under and you pay at the pump! http://www.discounttiredirect.com/di...foTireMath.jsp