Those who are interested in getting different size tires... (vehicle, fuel, best)

[Innovator831021]

I posted on another forum an equation I use to determine final MPH with particular wheel dimensions and other relevant vehicle parameters. Maybe some of you will find it useful.


RPM X 60 X [1/(Transmission gear ratio)] X [1/(Differential gear ratio)] X [ { ( [ {tire width in mm} x {aspect ratio} x 2 ] + {rim diameter in mm} ) X 3.14 } / 1609344 ]

60 is a conversion factor to go from minutes to hours.
The 2 next to aspect ratio is to get the combined height of the side walls.
The 3.14 is Pi, Pi*Diameter=Circumference of Tire
The 1609344 is how many millimeters are in a mile.


For my truck @ 2000 RPM with the NV4500 in fifth gear, 3.54 ratio differential gearing and 245/75-16 wheels I get the following:

2000 X 60 X (1/0.75) X (1/3.54) X ( [ ( {245 x 2 x 0.75} + 406.4) x 3.14 ] / 1609344 ) = 68.3 Miles Per Hour


Enjoy.

[whiteboyslo]

very nice. here's a calculator for those interested:

Tire size calculator

Mike

[Grandpa Pipes]

Doesn't changing the tire diameter mess with the gearing ratios the computer most cars today use to control fuel metering and emissions?

[cdelena]

Originally Posted by Tightwad

Doesn't changing the tire diameter mess with the gearing ratios the computer most cars today use to control fuel metering and emissions?

If you don't maintain the outside diameter (within a couple percent) it can affect more than just the speedometer.

[caution]

i just changed the gear that reads the speed to match the new tire size. i think the part was less than $20

[M3 Mitch]

Originally Posted by Tightwad

Doesn't changing the tire diameter mess with the gearing ratios the computer most cars today use to control fuel metering and emissions?

Depends on the vehicle. A lot of later model pickups have a programmable feature where you can put in optional tire/wheel sizes.

Although, yeah, in general making your M3 into a "donk" will screw up the handling, and overall running of the car...

Any properly engineered *car* will work best with a wheel/tire assebly rolling diameter close to original, but *trucks* can be modded for increased off-road capability up to a point with bigger wheels/tires. At some point you need a different ring & pinion.