hydro's truck is playing hard to get

 

My tires are the stock Michelin LTX A/S 255/65R17. They have an OD of 30.1", are 7.7" wide and weigh 34 lbs a piece. They are good for fuel economy, but goofy looking on such a large truck.

That's why I'm moving up to the factory 20's with the Pirelli All-Terrain Tires. Plus, since it is a 4x2 I need to have some decent tires to do anything in the dirt. Otherwise it would be next to useless offroad.

The 20" rims should be heavier than the stock 17's, but I have no idea by how much. Maybe I will weigh them without tires and post the results. The Pirelli Scorpions are 275/55R20, OD is 31.9", 9" wide, and weigh 42 lbs a piece. On the plus side, they can be inflated to 44 PSI instead of only 35 PSI like the michelins. So there should be some economy loss, but I am hoping it isn't much and should only really become evident around town. The increased diameter should make my 3.55 final drive feel like I have 3.35 differential. Maybe that will offset the increased rolling resistance at highway speed.

Only time will tell. But man do those 20's look so much nicer. They are the gunmetal FX4 wheels.

 

If the new setup is taller, you will need to have the PMC reflashed to recalibrate the speedometer.

 

Is that easy to do? I was just planning to do hand calculations to compensate, but that would be much better.

 

Your dealer will need to reflash the computer with the change.

 

Good to hear the MPG numbers are so good... I'm going to have to show my dad this, cause his Dakota is only getting 16-17 MPG and it's a V-6...

 

You need to get it reflashed, it's not just the speedometer/odometer that will be off, the system will be using the wrong speed and it won't run quite right.

 

What would cause it not to run right? Does the computer use the vehicle speed as a feedback for something? I would think that it would only affect the mileage calculation.

 

You bet - especially for the transmission behavior.

 

The EB I now have, which I spoke of above, replaced a 2007 F-150 XLT that is basically identical except it has the V6 that Ford used in that year. The best I could get with that truck was 18-19 (same conditions, on the same highway as my above numbers).

Forgot to mention in the previous post that the EB now has about 5100 miles on it.

 

It will run fine. He is only changing 6.2% that's not enough to mess up anything or even make a noticeable change in shifting.

ie reading 35mph will actually be 37mph.. 60mph will actually be 63.7mph

Run it, keep in mind that your speedo is off, flash the PCM for $90 if you want to.

 

Thanks for the advice. I may just do it so everything is legit and I dont have to wonder/calculate everytime

 

Update. I've got 6000 miles on the odometer and I've had the 20" FX4 wheels with Pirelli's for about 1000 miles. Average economy since the tire swap has been 17.5 MPG with 60% highway/30% city/10% trips less than 2 mi to work on a cold engine. Those short trips really kill the mileage.

Just got back from a trip to Las Vegas from Orange County CA. We had 4 guys at about 200 lbs each and a bed full of gear and beer. I averaged 20.5 MPG for the whole ride there and back. That included 7.5 hours of stop and go traffic on the way, some off-roading to bypass traffic near state line, and 4.5 hours of cruise control at 70 MPH on the way back. I think the way home by itself would have been almost 23 MPG because I remember the average being around 18.5 when we arrived. This is all with 10% ethanol 87 octane fuel.

I'd say she is breaking in nicely...

 

Cool, I'm going to V-town late this month. I have your exact set-up and leaving from lakewood, CA (same drice as you). What was your highest speed? I'm thinking to stay with the slow traffic flow I'll need to be around 75mph (fast lane flow at 80-90mph is too much). We're leaving early a.m. so traffic won't be an issue. I'm thinking I'll get around 16mpg for the trip @ 75mph. My truck sucks fuel like crazy, currently averaging 13.5mpg on this tank. Although, I'm driving it like I want to, not fuel econ mode

 

I used to always check the mileage and wrote down the odometer on my gas recipts. With my 07 5.4 flex screw 6.5 bed I got about 17 mpg on highway, 12-17 on mixed hiway/ city and rarely 19-20 on best conditions (tailwind twice). I'm looking forward to 2-5 mpg better on my new eco (still on order). crossed fingers !!!