So I am asking this question because...well first I just want to know and second I am getting terrible gas mileage and was wondering if this is because of my gears...tires or something!

My odometer is off slightly...well slightly being 5-10mph at 70 MPH

I have a 1998 Dodge Ram 1500 with K&N 14x3 intake and super 44 flowmaster exhaust. I run royal purple oil and always keep my fluids up.

I did have 265/75R16 10 ply tires. Now I am running 285/75R16 8 ply now...thought that might but I am still getting like 12-14 HWY, what am I doing wrong?

Nothing feels or seems wrong on the truck but it frustrating! One car is broken so wife drives it in the city for work and I have to fill it every week...this starting to **** me off!

Any help or insight would be great!

 

Them 285's and the 3:55's you got in there is what is killing it during city driving.But, you get 2 mpg better than me and I have 265's. And BTW, if you don't have the speedo/odometer calibrated for the bigger tires you'll never get an accurate MPG reading.

 

It wasn't accurate for the 265/75R16 I was running previously but like I said those were 10 ply so I am not sure if that makes a difference.

 

Maybe go see if the dealer will reprogram the speedo for you or get a Hypertech or something. However, 12-14mpg seems decent as I don't get more than that. I get 12-13mpg usually. More like 12 usually but sometimes 13 or so. This is mixed driving driving it fairly easy. However not sure what you are actually getting since your speedo is off.

 

To see what rear end gear ratio you have is fairly simple. Jack up the truck and support it on jack stands. Put a chalk mark on the tire at the six o'clock position. Put another mark on the drive shaft and align it with another mark on the differential housing. Have someone turn the tire slowly while you count the turns the drive shaft makes before the tire mark comes around to it's original position. That will give you a fairly accurate gear ratio. Make sure the tire is turned SLOWLY!

 

edit - apa beat me to it.

i think i've got this right.

assuming that you have an open diff, raise one tire off the ground and leave the other one on the ground so it cannot turn. get under the truck so that you can eyeball the driveshaft and reach the tire off the ground (or get an assistant). turn the tire exactly two revolutions and count the drive shaft revolutions. so if two tire revolutions gives you about 3 and 1/2 drive shaft revolutions, then you've got a 3.55

the reason that you do 2 revolutions on an open diff, is because of the spider gears, and how all the power goes to one wheel. if you don't block one wheel, you'll get some random revolution of the other wheel, and it'll mess up your count.

if you've got an lsd (that works), or some other lock up method, so that both tires turn equal, then jack up both wheels and do one tire revolution instead of two.

there's probably nothing wrong with your mpg. with some speedo calibration errors in play, 14 hwy at 70 mph is not bad. 10 mph error at 70 is about 15%. 15% of 14mpg is over 2 mpg error.

 

Thanks guys...I really need to get myself a good jack! and also a hi-lift for those oh s@*# did I just do that?