Mileage help for my f250. 6.0

[gibbons]

f250 fx4 Crew Cab. 6 inch lift with 3.73 rear end and 35X12.5X18 mud grappler tires. Ive have this truck for 8 months and now have 9,000 miles on it. My mileage runs 10-12 mpg and I have never got better then 12.5. I hear of people getting 18 with this truck and am wondering if the lift and tires make that much difference or is something wrong. The truck really pulled big when I first got it. Think the boost was up as it would spin the tires if you punched it. The the dealer did the recall and power went way down and mileage stayed the same. After complaining the tech redid and tuned everything and the truck runs real smooth with a little less power then when I first got it but much more then after the dealer recall, All the while im still at 11 to 12 mpg. I was hoping for 16+. Any thoughts?

[Logan]

First off,



To POWERSTROKE.ORG



Second off, I bet the tires are throwing you off a good bit, but, have you had your speedo and all that good stuff set up for the new rubber? If you think it is close, get on a good stretch of highway and mark 10 miles on the road and on your trip meter. See how close you are. Then you can go from there. Also, have you tried any additives? Make sure your tire pressure is where it should be and not where the shop put it. Hope this helps.

And third off, when you get a chance, fill in where you are from, get some pics of that beast in your garage and fill out your sig so we don't have to ask what all you got in the future. And be careful, the place is very addictive. Have fun with it and if you got any more questions, just ask.

You need gears. Use this calculation. New tire height in inches devided by original tire height Multiplied by original gear ratio. It will give you a number. Round up to the next common gear ratio. Boom, you will be back into your original rpm range.

[Logan]

Originally Posted by Stroketechintraining

You need gears. Use this calculation. New tire height in inches devided by original tire height Multiplied by original gear ratio. It will give you a number. Round up to the next common gear ratio. Boom, you will be back into your original rpm range.

OK, he knows more about it, do what he says

[Clay Henry]

Welcome to the site Gibbons!

[720Deere]

Your 35" tires are about 12% larger than stock thus your odometer is reading 12% less miles. If you factor in the correction, you are getting about 14 mpg when you have calculated 12.5. That's not so bad for being way out of the optimal torque range with the 3.73 gears. You really do need 4.10 gears to run a tire that tall.

[gibbons]

Thanks so far for the responses. Tires and gearing were set at the dealer an Im assuming that they recalibrated the speedo but am now thinking I need to check. Does anybody know what the optimal rpm/speed range is. If I get in the optimal rpm range can I get up to 16mpg?

[Marty]

What year is the truck?!

If it's an 01 then the stock size tire is 265/75/16
If its 05 on, it could be any # of tire sizes... so this will determine how much % error your OD is. I'm assuming it's a 6.0 since you're talking about flashes so it could be from 03 on... so let us know.

This is what you want to use to figure it out after you get your stock size tire settings. Look on your driver side door for stock sized tires.
Wheel and Tire calculator

Do you want better hwy miles or better in town miles? Say your best ratio would be 4.30. If this is the case, and you want best hwy mileage go to 4.10's. IF you want better around town and towing then go with the 4.56's.

Also remember when you have larger tires they are heavier and are more resistance so usually if your final # is 4.30 to go back to a stock "feeling" truck you'll need to increase by at least a tenth. Ie: 4.40. This has been my exp with 2 4wd's...

Obviously if you have a 4wd then you need both rears regeared.

[720Deere]

Originally Posted by gibbons

Thanks so far for the responses. Tires and gearing were set at the dealer an Im assuming that they recalibrated the speedo but am now thinking I need to check. Does anybody know what the optimal rpm/speed range is. If I get in the optimal rpm range can I get up to 16mpg?

You should at least check your calibration to see if it's correct or not. Like Brrs3633 said, a good simple check is to travel 10 miles of highway and check your odometer vs. the mile markers on the highway. You can also use this one to check your speed accuracy. For speed, just maintain 60 mph for those 10 miles and time yourself. It should take you exactly 10 minutes to cover 10 miles at 60 mph.

You will lose some mpg because of the extra weight, resistance and circumference of the bigger tires. I would doubt that it should be more than 2 mpg for 35" tires. My factory 275/65/20 tires measure about 34" tall by 11" wide and my truck averages about 15.5 mpg overall w/3.73 gears. I really wanted 4.10s but had enough trouble finding a truck that matched my wants as well as this one did. I haven't done any heavy towing yet, but I guarantee you that I will wish I had the 4.10s. The extra size of the 20's makes the effective overall gear ratio more like 3.55:1. It's nice on the highway (1900 rpm @ 70 mph) but not as friendly at speeds less than 60 mph!

[gibbons]

2005 4x4 f250 6.0 diesel crew cab. Orig tires265/70r/17 which i calc to be 31.60 inchs. new tires are 35X12.5X18 so I have 10.7% more tire then factory. Only original gear ratio offered in the f250 4x4 non dually was 3.73 which is the gear ratio i have. So if I went to a 4.10 gear ratio this would put me back to original. need to see what my orig crusing speed rpm is but it sounds like ill be added also 200 rpm at a fixed speed.
Questions:
1. do you guys think that will help my mileage running at a faster rpm?

2. Anybody have an idea on what rpm/speed combination on that 6.0 diesel maximizes MPG ?

3. With my current setup am I bogging the motor?
4.Do you guys agree with my calculations?