Originally Posted by Cou78p
The car XC Coup
The engine 351C
diff ratio ??
But what is the best ratio for good top end speed, at the moment I think it is set up for drags as it am only getting 120kph at are 3500rpm before she starts to realy vibrate around 4000rpm. As I live in an area with no speed limits I would love to open her up and fly down the high rather then holding her back and just making alot of music. love the sound a V8 makes.
Depending on your budget, you may want to consider swapping transmissions along with rear gears. There are a couple of ways to go:
Richmond (formerly Doug Nash 4+1) 5 or 6 speed
Tremec (and others) 5 or 6 speed
The Richmond has a 1:1 5th gear in its 5-speed and uses a lower first gear ratio. This allows its owner to utilize a high ratio (lower number) rear end gear and get the same torque/driveability and fly up to higher highway speeds.
Most of the other 5 and 6 speed transmissions use an overdrive (less than 1:1) ratio. Combined with proper tire size and final drive ratio, they make very high speed at lower RPM. However, it needs to be noted that your engine needs to be designed/built to make its best torque at the desired RPM range where the car is geared for better results.
However, these kinds of changes can be quite costly. If you're looking at simply changing differential ratio, you need to find out where you are now. It isn't too difficult to calculate, but the easiest method is to jack the car up and turn the wheels. One complete revolution of the tire should equal somewhere between nearly 3 and nearly 4 revolutions of the driveshaft. If you're good at visualizing the "compass points" around the driveshaft as it turns to stop at the place where the tire has completed one revolution, you can get fairly close by just knowing the common gear ratios for your differential. Another choice is to decode it using the information found on your car, if it exists. This method assumes that it hasn't previously been changed by someone.
A simple thing to do is to figure out your gear ratios based on the transmission gear selected and what you learn about your current or planned differential gear ratio. Simply add the various gear ratios together to get the "total" gear ratio. A real easy way is to use any of a number of calculators on the web. Many are in MPH, so convert to KPH by multiplying MPH * 1.60934.
...near the bottom of the page is a MPH utility. Add RPM, tire diameter and gear ratio and you'll get the MPH. For example, at 2500 RPM, using a 2.70:1 gear ratio, you'll be at ~66 MPH with a 24" tall tire. 66 MPH is about 106 KPH. 3500 will get you to about 93 MPH/149 KPH.
If you were using an over drive transmission where 5th gear was .83:1, you would multiply your results by 1/over drive ratio, or in this case, about 1.2.
If you are using a Java-enabled web browser, you can use the applet on that same page to Calculate Rear Gear Ratio. Ignore the "End of Quarter Mile" stuff and just type in what RPM you want to run, what tire diameter (in inches) you have and what MPH you want to be going. You can then find the rear gear ratio. Adjust the numbers until you get a combination that works fairly closely.