Oversize tires MPG GPS
With oversized tires and re-gearing it’s a good practice to note for your adjusted speed and mpg. For my little Toyota trail rig I went from 225 to 31″ tires and from 4.10 to 4.88 ring and pinion. These upgrades altered my dash readings. Randy’s Ring and Pinion is a great site to utilize before building your rig to work out your RPM, gearing ratios, etc.
After your modifications are done a simple way to note your dash readings are with a GPS. Couple tips when using a GPS. They are most accurate when moving. And the longer the distance of collecting data the better. Ten miles should be good enough for this application. You can either snap a few pictures or capture some video once moving. This needs to be done at the beginning and at the end of your data collection trip. Or have someone write down the numbers. Ideally this should be done at a consistent speed without any stops. Use extreme caution not to become distracted and get yourself into an accident while doing this!
With this project build I learned my speedometer is indicating approximately six MPH faster than my actual ground speed. I also noted my trip/odometer is reading approximately a tenth of a mile more than what is actually being traveled.
You can then use these numbers to adjust for MPG. Which will obviously go down when building a trail rig. Not only from higher RPM’s and larger tires, but from the additional weight. I.g. going from 225/75r15 to 31×10.5×15 tires increased my rotational weight by almost seventy-two pounds. Adding a winch bumper combo and rock sliders about another two hundred pounds.
With an application like Filemaker you can create a database not only to track your mileage but to calculate your MPG for you. If you have the software on your desktop you can download a free app for your smart device.