When choosing gear ratio’s I notice that many folks are too conservative. The popular calculations out there will show you the RPM change at a given speed, but that is not the whole answer.
The size of tires varies greatly. If the sidewall says 35” or 37”, it is just an estimate. Reputable tire manufacturers like BFG, have more detailed data on their websites for each tire, but even that is still an estimate.
Inflation pressure and vehicle weight compress the bottom of the tire. It is different for every tire on every car. The number that is most accurate is called Static Loaded Radius (SLR). That is the distance from axle CL to the ground when the car is loaded, and the tire is inflated to a given pressure. You can measure this yourself in your driveway. Multiply that measurement x2 to get a more accurate tire diameter for your calculation.
The other factor to consider is that tires rolling on pavement create friction and drag. Larger tires create more friction and drag. Lift kits also increase frontal area that adds wind resistance.
Some folks think that if the engine turns slower they will save fuel. Not necessarily. The engine is designed to be most efficient in a given power band. The factory makes this decision to conform to Federal fuel economy laws (CAFE). If the ratio is too high (low numeric number), then the engine will lug, and burn more fuel. The transmission will also spend less time in OD, and hunt in and out frequently. Also bad for fuel economy.
When you do your calculation, you are better off having the engine spin somewhat faster than stock (higher numeric gear ratio, aka lower gear). That will maintain optimal fuel economy, and overcome the wind drag from higher body height, and rolling resistance of the bigger tires on pavement. It will also make the car far more fun to drive. It will feel lively and responsive.
