Unfortunately this is incorrect for most automobiles. I say 'most' because I'm not 100% certain that there isn't a car that signals the speedometer by wheel speed. The Mustang has a sensor in the transmission that feeds data back to the computer for MPH calculation. This is the very reason people are stating the computer needs to have a variable changed for the altered gear ratio to display the correct MPH.

The Mustang can also have alternate tire diameter's input into the computer for calculation, since different Mustang models come with different sized and diameter tires, thus allowing for a more accurate reading.

Secondly, using wheel speed for a speedometer is not accurate or reliable source. While your logic is sound, it only applies once the speedometer is calibrated to a specific tire diameter. If the tire diameter changes after that initial calibration (either from the installation of a larger/smaller diameter tire, or through tire wear), then speedometer will display an incorrect speed again, necessitating the speedometer be re-calculated to the new tire diameter. Things like bicycles use wheel speed due to their simplistic design, but most vehicles and street motorcycles do not.

To throw another wrench at the speedometer using wheel speed is the motorcycle tire. The motorcycles tire diameter is infinitely variable with the tire's curvature and lean angle of the motorcycle, which is aggravated more so by the extreme tread surface curvature of sport bike tires and the sport bikes capable lean angles. A motorcycle tire's measured diameter is accurate on when perfectly perpendicular to the road surface, but once leaned over the effective diameter is reduced and in constant flux at different lean angles.

 

I'm interested in upgrading my gears as well I currently have 2.73's

 

If you got forza for xbox. try this.
I took my real life GT to the strip and took that time and built my virtual GT on forza to run the same time as I did in real life. My car has 3.31 in it now but when I put 3.73's on the virtual car I was like wow. there is a difference in feel and it makes the car feel lighter as far as running thru the gears but it only improved my virtual time by .05 sec, likely due to controlling launch is more difficult with the hi gear

 

am I the only one regretting having the 3.73?

I get like 14mpg and have to start in 2nd gear... very annoying

 

14????????

 

I went from 2:73 to 3:73 in my V6. One of the best mods i did.

Never fear the gear.

 

Unfortunately, this is now on hold for me until at least next year....my wife now needs over $5,000 of dental work and my out of pocket is over $2,500....

 

She can eat soup, right?

 

How much did that cost you?

 

Interesting - appreciate the details!

 

No regrets here

 

Including the gears, approx $600, but remember you need a tune as well to set the speedo correctly.

 

Thanks that sounds like an alright price

 

No regrets here...

 

While it is true that changes in tire circumference affect the speedometer, it is also true that those changes have the same effect whether the speed is measured at the wheel or at the transmission. So using a speed sensor at the wheel is just as accurate and reliable as using one at the transmission. Changing the circumference (number of revolutions per mile) of the tire has the same impact as changing the gears - not only on the speedometer but also on the torque multiplication.

Most motorcycles still measure speed at the front wheel and not at the transmission. This is especially true for chain or belt driven bikes because it means that changing sprockets (a common practice among motorcyclists) has no effect on the speedometer. Harley Davidson is a notable exception - they have a magnetic sensor in the transmission. But even shaft driven bikes use front wheel measurement. The Honda Gold Wing (arguably the most sophisticated touring bike made) still uses a front wheel speed sensor. They're often not the gear and cable setup of old but the magnetic sensor is still on the front wheel hub.

It is true that leaning a motorcycle changes the effective circumference of the tire but the difference in circumference between an upright tire and one that is leaned is not enough to significantly affect the speedometer reading. Neither the curvature of the tread nor any extreme lean angle capability of the bike make any significant difference in speedometer calibration. When the tire/bike is upright, the circumference of the tire is a circle at the center point of tire tread. When the bike is leaned, the effective circumference of the tire is a circle part way up the sidewall. No matter how far the bike is leaned, that circumference can never be any smaller than the wheel the tire is mounted on. So the effective diameter changes by no more than an inch or two when leaning. For the lean angles used on the street, the difference isn't even that much (and racers that lean more aggressively don't generally care about speedometer readings). Besides, once again the difference would affect the speedometer regardless of where the reading is taken - at the wheel or at the transmission.

Motorcycle speedometers are notoriously inaccurate anyway (and they ALWAYS read optimistically because there are significant penalties in Europe for motorcycles that show less than actual speed). It is not unusual to have a motorcycle speedometer read 5% or more higher than actual speed. BMWs are known to read as much as 10%-12% above actual speed.

 

I average about 16-17 mpg's all around and without much highway. I've never reset my trip-B and that is saying 17.1 mpg in almost 14K miles.

I've only done one short highway trip last year of about 100 miles and got 22.3 with cruise control on 70 mph.

And I use first gear all the time. I find it more annoying having to give it more gas to take off in second gear instead.