Have a new 2006 F150 Supercrew 2wd with 5.4, factory tow and changed the rims and tires to P285/50R20; the original stock tires were P255/65R17.
Does anyone know how to compute the difference bewteen the two tire/rim packages in terms of either gas mileage / odometer delta?

Also I here a lot stuff about K / N filters, exhaust changes and or superchips in terms of improving MPG. Anyone out there tha can validate before I go out and spend some $$$. Seems like the K&N filter is a low cost no brainer, but the exhaust and chip?

 

 

Nice truck!

Not sure on the tires. I have seen a formula for it somplace though.
Exhaust is a plus, use caution with the oxygen sensors.
Don't skimp - get a quality system.
My opinion on chips, is that the inprovment in HP/MPG is not worth the cost, or
the problems that arise afterwards. (CEL,CEL,CEL )

 

To compute the difference with the tires, you first need to find out how many RPM (revolutions per mile) each specific tire has. You can usually get this info from the tire manufacturers websites if you look hard enough. So lets say that your original tires were running 450 RPM, and your new ones are running 425 RPM. Divide 425 by 450 = 94.4%. In other words, your new tires are making your speedo off about 6% from your OEM tires (you are actually going 6% faster and further than the speedo says). So if your speedo says 60 MPH, you are actually going about 63.5 MPH. Of course this is just an example - you would have to find the actual RPM's for each tire to get the true percentage/difference. Hope that helps.

 

If you go to a tire size calculator such as

http://www.miata.net/garage/tirecalc.html

you can put in your sizes and it tells you the difference,,,for your situation your old tire was at 672 RPM and your new one is 646 RPM. This is a 3.9% difference so when your speedometer reads 60 mph you are going 62.3 mph

Hope this helps

 

Air filter will give .5~ MPG, exhaust likely nothing, and proper tuning .5-2~ MPG depending on your habits.