MPG Drop

I averaged about 13.5 mpg between city and highway driving before leveling my truck and changing the wheel and tires. After I installed king OEMs and 17" rims with a 315 70 17 MT I dropped to 7.5MPG. A drop of 6.5MPGs seems extreme? Is there anything I might have missed that is contributing to to my awful MPGs?

 

Miles driven / size of the gas tank. For example hundred sixty miles divided by 21 gallons is approximately 7.6 MPG.

 

Very low tire pressures?

 

I fill the tank, and reset the trip odometer. The next time I fill up I divide the number of gallons I jut put in by the number of miles driven since the last fill up. This will get you really close to actual mpg, and will be different from the onboard mpg computer.

 

Speedo recalibration

 

That's how I do it.

 

Shouldn't be significantly low about 35 PSI if I remember correct

 

I know the spedometer would be off after changing tires and the odometer wouldn't read 100% accurate but I didn't think that it would result in that significant of an trip odometer reading.

 

Ok, well Cement Pro may be right on the recalibration though it still seems like a huge drop

 

Does this thing work?

 

I hope that's it but I have my doubts considering I went from a 32 inch to a 34 inch tire it doesn't seem like it would warrant that kind of drop.

 

The speedo difference only makes like .5 mpgs difference. It’s not that. Why are you dividing by the size of the tank? Are you running it all the way down to 0 miles left and refilling 21 whole gallons?

 

You are traveling further than indicated so you may need to manually calculate your distance traveled.
As other said a speedo healer could solve this part of your equation.

 

That seems like an excessive drop for sure.

 

21 gallon gas tank. Used 1/4 so 5.25 gallons, divided by 40 miles driven. 7.6 MPGs.

 

That's what I figured. I assumed I would see a 2 MPG drop.

 

try with 40psi + on each tire.
I run KINGs and 295/70/18 Toyo A/T (41 psi) and my average is 14 mpg.

 

I'll give it a shot thanks! I know the MT will redcue it more but I wouldn't think that much more.

 

You need to divide mikes driven (corrected for tire size/ odometer being off) by how many gallons your tank. Guessing 21 or portions of a full tank isn't accurate.
I would imagine you'd lose a few MPG but not almost half. That's crazy

 

So you're estimating how much gas you used? That is very inaccurate. You need to do it the way @k9shag recommended, and get a larger sample size. Like maybe the whole tank. Fill up, zero the odometer, use most of the tank, then get an accurate amount of gas used by seeing how much gas goes in on next fill up.

 

It's a fresh trip odometer. And as far as the accuarcy of estimated gas used I know it's not 100% but I frequently montiored my MPGs even prior to putting the lift on and what it read at 1/4 would be very close to the final tank. I'll see how it goes until I put the gas in and get a final number to get a 100% accurate calculation.

 

To go along with the post above, if I'm not mistaken, these are actually 26 gallon tanks. At least on my truck, my owners manual says it's a 26 gallon tank, however I have had the gas light come on and it takes less than 20 gallons to fill up. Now I'm not much on pushing my luck to test it, but I've read elsewhere that these trucks have large reserves. It sounds like though that your calculations are off. Use the number of gallons you put back in the truck at the pump as how much was used.

 

The hash marks on the gauge aren't going to give you an accurate reading at all. Plus, when did they start making a 21 gallon tank? Mine is 26. I know the newer ones have a 38 gallon option, but I am not aware of a 21 gallon option.

Best bet is to actually see how many gallons you put in at the pump. Use that number for your calculation.

 

You need to run real numbers on your math, not guestimates.

Fuel, reset trip, drive, fuel - then do the math. Don't guess.

 

I know the tank contains 26 gallons, but each fill up is 20-21 after the light comes on. Again I know it's an estimated number, but it's fairly accurate from what I've experienced in my truck this far.

 

They're 26 but the light comes on at 20 meaning they count 6 gallons as reserve and not really factored into the gauge.

 

Thanks for the replies and help everyone. I'll know for sure when it's time to refuel, but i just wanted to see if anyone else was in this area of MPGs.

 

the thing is that he used the same mpg calculation method and now he finds a lot lower. maybe a bad batch of gas ? was it just one fill up or has it been the same for couple tanks already ?

 

This is the first fill up and one of the only times at a 76 station I thought that might be it.