I have a 99 318 with a Hughes big gulp throttle body and Hughes intake manifold, 5" lift with 35's. I know I'd lose some mpg with have the 35's but I don't think it should be this bad nor getting worst 6 months later. When I first got the truck I was averaging 11/14 city/hwy and now it seems like I get 8.5 no matter what I got 7.6 mpg on my last tank. I just did a full tube up a month ago. Could it be from using the champion plugs? I replaced my plug wires too and they ended up being snug compared to the old ones could that affect it? Any ideas would be awesome thanks guys.

 

Somehow methinks the old numbers of 11/14 were off. That's what I have always gotten in my 318 whether 245s, 265s, or 285s. If I tried to run 35s (would never get out of the parking spot) I would see 8mpg too. My worst tank was towing heavy at 9mpg. Guess check the cat to see if its clogged. Also, is the speedo corrected for the larger tires?

 

Did you fix the speedo?

 

Plenum fixed? What gears in the diffs?

 

[QUOTE=HeyYou;3272062]Plenum fixed? What gears in the diffs?[/QUOTE

Plenum is fixed, stock gears. I haven't done the soeedo yet. How do I do that?

 

Have a shop do the speedo. That may be your mileage issue. Wrong readings on mileage.

 

Stock gears? As in 3.55s? If so, that's a large part of your problem.

 

I have 33's with stock gears and mine reads 12. But if you calculate actual rotation speed and miles to the proper tire size it's more miles than your speedo reads. Mines approximately 6 miles an hour higher at 100mph. So every 100 miles is an extra 6 the speedometer doesn't read

 

Yea 3.55's, what gears should I look into? I'll look into the speedo thing.

 

For 35s probably 4.56 gears should be good