I used to figure about 1/2 the the percentage of rpm drop to added fuel economy. For example a 30 % drop in rpm would equal 15% rise in MPG. This calculates if you are getting 12 MPG and drop the RPM by 30% such as a Warner OD you could realize 14 MPG at the lower RPM. You would have to drive a lot of miles before you would notice the difference in your bank account. These figures do not come from any scientific tests. Only observations from adding overdrives over the last 50 years with 50's engineered jalopies.
Last edited by crenwelge; 02/24/2010 2:22 AM.
