Hey Guys. first post on the forum. Figured I would throw this out there since my dealer can't help with nothing. And the female service manager knows more about fighting with her family on the phone then dealing with customers.

I got a 2008 1500 4x4 mega cab. Hemi motor. I am the original owner, and had the local dealership do the every 5000 mile service. Right now it has 70,000 on the clock. Problem is, around 50,000 the stock muffler rusted from the inside out. No biggie, just replaced it with a 3" magnaflow muffler and good to go. around 55,000-60,000 I replaced the tires to 285/70/17. I think they are 1 or 2 sizes above stock.

Issue is the truck is only getting 7-9mpg around town and 12-13 mpg on highway(65-75mph). Before the stock muffler rotted out, I was getting 15-17 on highway and 10-12 around town.

Anyone come across anything like this?? I love the truck, but its getting crazy in gas. I have always ran 87 or regular in it. Replace fuel filters/air cleaners, had injectors cleaned, new plugs twice so far(every 30k).

Thanks for any help

AJ

 

Welcome aboard!!

The muffler failure didn't change your mileage. Part of the problem is wrong fuel. The Hemi wants at least 89. 91 is preferred if you can get it. The computer has pulled the timing back to keep detonation from occurring. That's killing your power.

Your odometer and speedometer are now reading low. You may be getting better mileage than you think, since your odometer is ~4.3% low (assuming you did go up two sizes - check the door placard for the stock size). The throttle body is long over due for cleaning, especially with too-low octane. The PCV valve could probably stand a cleaning.

 

I have a mega cab as well Im running 35 inch tires and my HUD says i average 11 mpg but recently i did a manual calculation of mileage and fuel i was actually getting about 13.5 highway I have seen my HUD drop as low as 9 mpg

 

MikeTHally hit the nail right on the head with his comments. Bigger tires=wrong reading in mpg. Buy a programmer to fix this. Also changing the plugs would probably help unless you meant to say a 50,000 mile service.

 

I figure out the MPG by hand, I never go by the HUD. I was running highway today 70-80MPH for 400 total miles 200 on regular 87, then on the way home I went with 200 miles with 89. Gain 2 mpg right there. So next I am going with the PCV valves, and TB clean.

My speedo to gps is only off by 1-3 mph from 50 mph up, At 80 on GPS I am reading 77 on speedo, any thing slower it still reads dead on.

Thanks for the tips!!! I will be happy to just get back to what it was getting. Also I ran a Cap, that did nothing for mpg, off or on same mpg.

 

First off, it'll take several tanks of the proper fuel. At least you're headed in the right direction, but 2 mpg on the first run is unlikely. Perhaps you were more conscious of the change and feathered the loud pedal a bit?

Second, your speedometer/odometer can't be right at any speed. It just doesn't work that way. 4.3% just isn't discernible when it's only 1-2 mph. My speedometer reads 76 at 80 with 285/70x17s and my odometer is 4.3% low (the best I've ever gotten was in the winter, driving in rain, fog and mist - 16.1 mpg) I couldn't drive my usual 80 - had to stay in the 60s. As far as buying a programmer to fix the error - don't bother. Just be aware of it.

Actually, a cap or tonneau can help a tiny bit. They reduce the wind drag a trifle. Also, there is only one PCV valve. Since they're kinda pricey, clean it a few times before you replace it. If the check ball moves when you shake it, you're good for a while.

Pretty sure you can stretch out the plug change interval. I know the book says 30K, but I think 50K is acceptable. When I changed the plugs in my C at 46K, they weren't too bad.

 

Dealer has changed plugs @ 30k, and 60k, they do what ever the book says. I don't want any issue with the lifetime warranty.


Thanks for the help, It will get back to were she was.

 

I have an 08 mega with 42k on it and the hemi as well and I average about 10.5 according to the display with 325/65/18's (I think that is something lik 34.5x13). I don't do much highway driving but get around 14mpg when I do. I've seen some people post that they get unbelievable mpg with theirs but I've never seen great mileage.

 

I used to run 285/70/17 on my mega cab and i don't think i ever saw anything over 13mpg on the overhead! I got a 5" lift and 35's on it now and lately its been sitting around the 9.3 mark, which i have always assumed was a few MPG's low because of the tires! Cant always believe what ya see on the H.U.D.