MPG Calculation w/ non-stock size tires

[chitown40]

I've been getting conflicting numbers from other cruiser heads as far as how to calcualte MPG based on larger tire sizes. I also read somewhere that the odometer reading difference is different from the speedometer difference. Any truth to this? I'm trying to tune the beast a little better and MPG seems to be the endall factor. I've got great vacuum ay idle, passed emissions, and no pinging under load, but I need to get an accurate estimate of MPG to see if I can improve on it. I've been calculating a 10% difference roughly with 31" BFGs on her, but I'm not too sure if it's correct. I'd also like to get a true ratio so I can figure out how fast I'm actually going at certain RPMs. Any help/insight, most appreciated. Right now with my 10% calculations I get around 12-13 around town and 14-15 highway. Lots of folks have reported upper teens on the highway and I'd like to get there myself.

 

[Adventure Teq]

I came across a sweet page the other day for this. But this one is similar. Good luck.

So all you have to do is do the conversion then do the normal mpg calculation. Or were you asking something more?

http://www.onlineconversion.com/bigger_tires.htm

 

[Doc]

Or, just do what I did and purchase a nice little GPS unit. Handy for lots of things, including finding your true speed.

 

[lovetoski]

Tires actual size varies from it's listed size. The most accurate way to measure milage is to first determine the correct conversion (for your tires) from the stock odometer.

Find a section of highway with posted mile signs. Use as long a section of road as you can. (Longer makes for more accuracy.)

Lets say you find a 20 mile section of road. Set your trip odometer at zero, and check the odometer when you hit 20 miles. Say that the odo reads 18.2 miles. (With larger tires, your odo and speedo will read low. ) This means your odo reads 91% of actual distance, or that you actually go 9% further than indicated by your odo.

Calc is like this:
18.2/20 = .91 (amount under-reported by your odo.)
(20-18.2)/20 = .09 (amount further you go than indicated by the odo.)

Once you know your actual conversion, simply take your odo reading and either:
divide by the amount under-reported (.91)
multiply by the amount further you actually travel (.09)
Either method will give you a bigger number than your odo indicated. This calculated number is the distance you actually drove. Then do your mpg calc as normal.

Of course, if what you want is to track improvements in milage, as opposed to actual milage, you don't need to do the conversion.

 

[zcruiser]

So... dumb question, since my 62 came with 32's.... what's the stock tire size on a 62?

 

[chitown40]

Ok, thanks guys. Thats pretty much what I was looking for. I had heard that the speedo and odometer would be off different percentages, but after thinking about it for a while, it really can't be. That link Hawk posted up worked well and was what I needed.