2010 FX4, 5.4, 35's on 20's, 6" lift. Otherwise stock, no nprogrammer.

Lousy gas milkeage, but I expected that. 14 (maybe on highway) 12-13 on a full tank of mixed driving. But the thing that is bothering me is this truck downshifts to get up a freeway overpass. @ 70, @ 65, @ 60, ANY speed, it has to downshift to maintain speed. Do theses tires and lift make that much of a difference? I traded in my diesel for this, maybe I'm just spoiled with having tons of power on tap, but DAMN!

Any ideas? Or is this just the nature of this beast?

 

Yes, your combo does make that much difference. you are changing the diameter on the tire by 4 inch or so. That makes the truck move forward, further for every turn of the wheel. This requires more power and torque. to get to speed and maintain speed. Also, your speedometer is way off if you haven't recalibrated it for the larger tires. So when it says 60, you are going closer to 70.

If you want it to stopped downshifting, you need to get the computer reprogrammed for your tire combo, and change you gears to 4.56s to get back your bottom end power. Otherwise, you will always have the lackluster performance.

Also, the lift kit does create a lot more drag, because you are knocking a much larger hole in the air than you were before. Also, those large tires are heavier and take more power to turn, and they probably have a much more rolling resistance than the stock tires.

So all in all, it makes perfect sense why it is acting this way, based on your mods. Need to finish the mods, by changing the gears and recalibrating the Computer to change the shift points and speedo.

Like the pic. Looks really good with that lift and wheel tire combo. Just go the extra mile on your mod and you will be happier with it.

 

Not uncommon with the 09-10 tranny.

 

Did you update the PCM with the new tire size? If not, your truck' torque management strategy is totally confused; not to mention your odometer and speedometer are off.

Have you calculated your new effective gear ratio? With 35s you might be shocked to see where you are.

And the 6" lift darn sure didn't help your aerodynamics.

 

Can you correct how the speedo reads by updating the PCM? I hate asking stupid questions, but I haven't been good with keeping up on this stuff.

 

Yes, kind of. The PCM uses the tire size to calculate the revs/mile which is used for the speedometer and odometer.

However, the dash display will always read 2-3% higher than the actual speed.

In other words, the odometer will be dead nuts on and the speedometer will still read high.

Clear as mud?

 

You are used to your diesel, which didn't need to downshift as often. The power in the gas engines is at higher RPMs. My truck will lug up hills below 1k RPMs without shifting.

 

My 5.4 never downshifts. His is doing it because the motor is now out of it's power band because of the larger tires. As soon as he gets the proper gearing installed, he will have his truck almost back to the way it was. Although the tires, wheels, and lift kit add allot of weight.

Wonder if anyone on here has ever weighed a truck before and after a lift kit?

 

Thanks for the clarification, Charlie. My big concern with changing tire sizes is the odometer only. I kinda like that to be dead nuts; I don't really care if the speedo is off, as long as I'm logging the correct number of miles.

 

If you've changed tire sizes and haven't recalibrated the PCM, your odometer will be off.

 

I'd say with that much lift, your mileage isn't bad, especially with only 3.73s and 35"s. The new 6 speeds do shift a lot more than the older 4 speeds, but that should be expected. As mentioned before, you have really placed a lot of extra load on the drivetrain, and need to change gears/re-program to compensate. That is the price of vanity (I say vanity because 6 inches of lift isn't going to do much for capability...)

 

The speedo was recalibrated. Checked it against a GPS and it was off by 1-2 mph. I'm going to check the calibration and make surte they did it right. But I guess I'm just stuck with it. I doubt I'll be changing rear geares in it.

Thanks to all who answered!

 

My truck with the NTGATs (34x11.5) is off @ 1/10 per mile, so when I filled up Thurs I was at 400 mi x.1= 440 mi I was still at 16.5 mpg mostly rural roads- not much stop and go but not quite hiway either. 5.4 w/3.55's, I can live with that.