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Discussion Starter #1
I recently added 16" eagle alloys and 245/70-16 Bfg's. They forgot to swap in the tire pressure sensors into the new wheels so the service tpm light comes on all the time. My problem is the gas mileage has dropped to like 15mpg. This was not a z71 truck and the new tires are only 1" taller than the stock tires. The speedo does not seem to be affected by the slight tire change according to pacing someone and gps? Dealer said the the speedo is off so the miles are off so it looks like i get bad mileage? WTF.. I'm getting terrible mileage i never had to fill up twice a week. Could the Tpm's effect somethign else? About the same time the dealer replaced the front oil seal maybe they left a vac hose disconnected? Dealer said if they did that the engine light would come on?

P.S. The new 16" eagle alloy off-road wheels are junk. The finish pitted in like two weeks.
 

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the dealer is correct. you need to correct the speedo before you can figure your mileage. or atleast correct the mileage.

I'm guessing you have a 4wd Z85 that came with 235/75/15?
 

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Discussion Starter #3
Yes that's what it came with. Could they flash the computer with Z71 Specs or do i need to purchase a new speedo thing.
 

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ok, its not a very big difference. 2.12%

so if you got 300 mi to the tank, you'll need to multiply 300 x 1.0212 = 306.39 mi.

what bfg's did you get? its probably a combo of extra weight, odo being a bit off and maybe a more aggressive tread pattern
 

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COBRACON said:
Yes that's what it came with. Could they flash the computer with Z71 Specs or do i need to purchase a new speedo thing.
they could do that, but your tire size is inbetween Z71 and Z85 so it'd still be off a bit.
 

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Discussion Starter #6
Huh? how did you figure that mileage difference. I like that. I got about 222miles and filled up with 15 gallons. I used to get about 325 miles per tank at about 17 gallons. I lost around 3 mpg? I got the all terrain tires.
 

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Stock tires are 28.9"

New tires are 29.5"

difference. 1.0207

so it's really 2.07%

I was going on generic size first, but these numbers are accurate based on the specific tires.

Your new tires are about 15 lbs heavier...PER TIRE. thats a lot of rotational mass. Seems like a big drop in mileage though. Was it just the first tank? i would go through a few more and see how it goes.
 

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Discussion Starter #8
wow, i wasn't aware they would be 15lbs heavier each. it's been about the 4th tank of gas. it's staying consistently at around 15-14 mpg. The temperature dropped, the dealer repalced a part and the tires were changed out so i really can;t pinpoint what it might be. I could see losing a little but it seems a bit drastic.
 

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probably the cold + tires. I wouldn't expect more than 1-2 mpg drop from the tires.
 

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To figure out your speedo error you will need original tire revolutions/mile (divided by) new tire size revolutions/mile x given speed = actual speedo error
 

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Discussion Starter #11
thanks...and to do that i need a tape measure, math teacher, white marker and a dude to roll the tire down the road a mile?
 

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NVR2LOW said:
To figure out your speedo error you will need original tire revolutions/mile (divided by) new tire size revolutions/mile x given speed = actual speedo error
you can use the diameter like i did, then just divide the speed by 1.0207
 

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COBRACON said:
thanks...and to do that i need a tape measure, math teacher, white marker and a dude to roll the tire down the road a mile?
you dont need to do that.
 

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I think the height of these trucks makes a lot of difference in mileage. When I levelled mine with the TB crank, my mileage immediately went to pot. Everything I've done to it since has made the mileage drop correspondingly. I'm lucky now if I get 250 miles out of a tank! (I should say also, that virtually all of my driving is local stop & go stuff). I agree with what someone above said, it's probably a combination of several factors: increased height (more drag), bigger tires with more aggressive tread (more drag, more rolling resistance, more weight), speedo error, maybe cold weather with reformulated gas too. Your wheels may actually be heavier than stock as well. Every modification has its consequences, some good and some not so much. You just have to decide what those mods are worth to you.
 

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Discussion Starter #15
two point i was just kidding. thanks for your info.

Dirty i forgot about height. So pretty much i screwed up my mileage and it is probably from all those factors. By the way the myth of a Tonneu cover giving you better mileage is bogus. I watched it like a hawk it didn't change. I thought about fabbing a bunch of aluminum plates for underneath to reduce the drag and see if i get killer mileage just for fun.

Dawg did you fix your speedo aftergoing with bigger tires?
 

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COBRACON said:
Dawg did you fix your speedo aftergoing with bigger tires?
Yeah, I sent the brain to PCM for Less and had the whole deal done. They fixed the speedo, firmed up the shifts, and tuned it for mid-grade. I think it made a difference in performance, but I can't tell it helped the mileage any. It's hard to tell what did what because I did the Lift(s), tires, wheels, e-fan, AEM, & PCM all at virtually the same time. But the speedo is good and performance is surprisingly good for rolling 33x12.5s.
 

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Discussion Starter #17
Huh, so i send the brain to PCM and tell them what tires i'm running and they reprogram it? ABout how much did that cost if u don't mind? It must have done something i would think 33's would slow it up a bit. What s the diff between the aem and pcm?
 

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COBRACON said:
Huh, so i send the brain to PCM and tell them what tires i'm running and they reprogram it? ABout how much did that cost if u don't mind? It must have done something i would think 33's would slow it up a bit. What s the diff between the aem and pcm?
aem is an intake, pcmforless is the company that flashes your PCM. two different things.

http://www.pcmforless.com
 

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Thanks 2point. Didn't have time to get back to that one yet. :)
 
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