Originally Posted By: jmac
Originally Posted By: ProfPS
Originally Posted By: FordManVT
Originally Posted By: ProfPS
This will happen but it is not accurate. Running larger tires (larger diameter will yield a larger circumference) will cause your RPMs and speedo to be higher resulting in actual lower mpg; not odo calculated mpg which will falsely calculate a higher MPG. This is because your engine RPM is running faster and your odo will be clocking more miles than actual.
Exactly wrong. Taller tires make the gear ratio effectively higher (lower numerically). That means at the same actual speed, RPM and speedo reading will be lower. Any increase in true MPG is because of the lower rpm. But taller tires will also weigh more, and have more drag. odometer miles will be less than actual miles. As long as you compare actual miles, your MPG will change very little. If you use odometer miles, your MPG will drop, as the odometer recorded less miles.
This is all because all speed systems count the revolutions of the tire, and bigger tire revolve less in a mile than a smaller tire.
Actually speed systems are a gear and cable connected to the transmission displaying revolutions of an internal component in the transmission, not a sensor at the wheel.
Depends on the vehicle. (Some) Ford vehicles use vehicle speed sensors located at the wheels or at the differential that track actual revolutions of the axle or wheel. I remember when all cars did have the gear and cable to the tranny, but do not know if most have gone away from this in newer cars but would expect that they would have mostly all moved to electronic speedos.
Cannot think of any tachs nowadays that aren't electronic taking inputs from a sensor or coming off a distributor coil.
I believe most older vehicles had the sensor on the output shaft of the transmission, which STILL would show a higher speed on the speedometer compared to actual speed with a larger diameter tire, regardless of gear ratios. The output shaft rpms are dependent on tire size and axle ratio and nothing else. That in turn, along with gear ratios in the trans, affects engine rpms. Take it to the extremes. Imagine driving a lowrider with the teeny tiny little tires. At 80 mph actual speed, The speedo probably shows 150 and the rpms are at 7,000 with the valves floating!
Or conversely, having 30 inch dubs on the same car. 80 mph actual would probably show 30 mph on the speedometer and the engine would be loping around. That's why monster trucks have to have 100 to 1 axle ratios or higher.
