First, drove my xj stock for 15 years before lifting it. With stock tires I found the ave mpg display to be a bit generous compared to real world. Ironically, when I went 31s, it seemed to be nearly right on. Would this mean that with 33s this gage would be showing less than what I’m actually getting?

hoping to real world test at some point. I just always forget to keep track.

 

The on-board MPG calculator has been generous on every vehicle I've ever driven. They're okay for a ballpark estimate, but that's about it.

Are you accounting for the difference in distance per revolution with the different sized tires? Figure you gotta take the number you can and adjust for the greater distance per rev that the computer is not accounting for.

Most accurate way is to hand calculate. Fill your tank, reset the odometer. Drive until you're ready to fill again. Divide the number of miles driven by the number of gallons pumped. Adjust mileage driven to account for non-factory tire size.

 

Ideally, I would recommend replacing your speedometer gear to get an accurate reading of how fast you are going then hand calculate the MPG numbers with miles driven divided by gallons of gas pumped when you fill up.

You can't compare stock (smaller tires) to lifted (bigger tires) correctly without replacing your speedometer gear because you have no idea how fast you are actually going when lifted (bigger tires).

 

You can multiple by the ratio of the tire size. Speedo/odometer are calculate by distance traveled per revolution of tire. You travel the distance of the circumference of the tire with one revolution. Circumference is calculated by 2*pi*radius, or just pi*diameter.

Stock tire size on these is 225/75r15, which calculates to 28.3". Ignoring the fact that most tires are not true to size, you can multiple your speed or milage by (tire size/28.3) to get a more accurate value.

In the case of 33's, actual road speed and distance traveled will be about 17% more than the speedometer and odometer show (assuming factory differential gears and speedometer gears).

 

I understand you can calculate it very close, but like you already mentioned, most tires aren't true to size. You would need an exact measurement of the tire to be spot on. We are talking tenths or hundredths though so not a big deal lol.