Hello, I have an '04 F250 6.0 diesel which still has stock wheels/tires. I want to upgrade to 20" wheels wrapped in Nitto Terra Grapplers 325/60/20. Will I see a big decrease in fuel mileage?

I currently get around 22 mpg freeway and 17.5 mpg average.\


Thanks

 

Yes, any time you increase tire size (note I said tire, not wheel), then fuel mileage will drop. The amount of decreased mileage depends on tire size, tread, driving style, tire inflation, etc. Remember that you are raising the truck's height by putting bigger tires on, plus you are increasing the rolling resistance with the larger and heavier tires.

 

I went with 37" which is 2" bigger than you want and mine dropped 2-3mpg highway and city. Hope this helps you!

 

thanks fellas . . . this is what I was looking for!

 

My fuel mileage dropped at least 2 mpg going to 35x13.50's and a 5" lift.
More rolling resistance and alot more wind resistance will cause the drop. I switched gears from 3.73 to 4.30 which brought back the power that was lost, but only helped a little for the mpg's.

 

At highway speeds of 70-75, my mpg increased when I went to the 35" tires. The reason is, that I now run at less than 2000 rpms at those speeds. Before, I was at 2300 rpms. I have 4.30's and I had to go 60mph to maintain 2000 rpms. So on long trips I get better millage. In the city it is worse, since it takes more to the tires up to speed. This is all manually calculated.

 

Don't forget that a larger tire will show lower speed and less distance traveled over the same distance as a smaller tire. This has to be taken into account when calculating mileage. If you put a tire on that is 3.5% bigger in diameter then you have to multiply your MPG by 1.035 to get an apples-to-apples comparison to MPG with the smaller set. The same applies to indicated speed. When I went from the stock 265/75R16 tires to the Nitto 285/75R16 I thought I would have to make mental corrections for speed and mileage. I tracked indicated vs. GPS numbers over a 900 mile trip and found that the new tires were within half a mile over that distance and speed was dead on. I guess that's close enough for me.

 

I would agree with GwizRC - the larger tires should actually increase your mileage at highway cruising speeds. I went from 18" to 17" wheels on my '05 and the mileage went down 1-1.5 mpg, probably due to higher rpm. I switched back to the 18's recently but haven't really driven it far enough to verify the mileage is back up.


Of course, tread design comes into play as well. If you are going from an all season radial to an aggressive m/s tire than the extra resistance will probably eat up any gains.

Either way you shoud expect your stop and go mileage to drop due to the extra engine work required move the larger diameter tires.

 

I always use my gps to determine speed and mileage. I just did a 725 mile round trips with trailer in tow and averaged 14.5 mpg. The trucks lieometer said I did 660 miles and averaged 13.2 mpg. Average speed was 62.3 mph with a max of 74.1 mph.

 

Your mileage may go up or down, depending on a few things. It isn't all black and white.

First, larger tires usually have more rolling resistance. Your mileage will go down (all else being equal) by using tires with a higher rolling resistance. Using a skinnier tire will decrease the rolling resistance, but most guys want the wider tire look.

Second, larger tires will change the effective gearing. This may increase or decrease your mileage, depending on your engine. If the new effective gearing puts your engine into a sweet spot at cruising speed, you'll get more mileage. If the gearing puts the engine at an RPM where it has to burn more fuel to make the same power, mileage will go down.

 

[QUOTE=thedaddycat]Don't forget that a larger tire will show lower speed and less distance traveled over the same distance as a smaller tire. This has to be taken into account when calculating mileage. If you put a tire on that is 3.5% bigger in diameter then you have to multiply your MPG by 1.035 to get an apples-to-apples comparison to MPG with the smaller set. The same applies to indicated speed. QUOTE]

This is very important - If you don't make this correction, either with math, or by recalibrating your speedometer/odometer, your numbers will be meaningless. And you may be driving faster because your speedometer is not right, making the comparison even less apples to apples.

 

Has anyone kept the same width and just went with a larger tire? I want to go from a 285/65/R18 to a 285/75/18. Has anyone does this and what were your results?

 

Redford hit it on the nail. The only thing I would add is that there is a HUGE difference depending on what your driving habits are. Mostly highway driving you might see an increase in MPG, but mostly city driving, and it will be a fuel hog with a drop for sure.

 

This is something that I have yet to find so far. From what I've found, if you want to go up in tire size you have to go wider as well unless you want to change wheels. I currently have 285/75R16 Terra Grapplers which are 32.8" tires. I could go to a taller and narrower tire(33.2-33.6" depending on brand and tire) if I went with 19.5" wheels and got 245/70R19.5 tires. Going to 265/70R19.5 (stock width) would get me in the 34.0-34.5" range and 285/70R19.5 (my current width) would jump me up to 35.1-35.4" tires.

If I went all the way up to the tallest tire I would change effective gearing from the stock 3.73 to 3.46. This would drop engine RPM from my current 2000 RPM at 70 MPH to 1855 RPM at 70 MPH. Now this might be a little low for me as I usually don't drive over 70 but if I regularly ran in the 75-80 range it might be OK. Going with 34.5" tires yields effective gearing of 3.55 and 1900 RPM at 70 MPH.

 

If you're getting 22 MPG on the hiway now, I would do everything I could to keep your effective gear ratio exactly where it is. You're at about the top of the game now.