Guest rperry74 Posted February 18, 2005 Report Share Posted February 18, 2005 those who think they are faster than they are. It goes like this: establish you gear ratio number by taking your rear gear and trans gear and multiply them. (ex: 4.11 rear X .7 5th gets you 2.877) Next plug this into the following equation, RPM X Tire Diameter, divided by Ratio X 336, now you have the MPH at the given RPM. This does not take into accout traction or drag issues, so this is purely theoretical. So for a complete example, I will use my old Mustang: 6250 X 26 = 162500 2.877 X 336 = 966.672 81250 / 966.672 = 168.01 MPH Try this out, its pretty fun and it will shut up some people who think they can go 200 with a stock Civic. graemlins/thumb.gif Quote Link to comment Share on other sites More sharing options...
Guest Silvrbmw1 Posted February 18, 2005 Report Share Posted February 18, 2005 thats a bit to much math for me. but good to know just for the hell of it! Quote Link to comment Share on other sites More sharing options...
Science Abuse Posted February 18, 2005 Report Share Posted February 18, 2005 simplified: RPM x Tire O.D. / (final drive x top gear) x 336 = MPH@ given RPM Its easier to wrap ones mind around the equasion when its not surrounded by paragraph. Ther ei s one significant variable in this, tire Outer Diameter. Its constantly changing with wear, and effects your reading. On top of that you need confidence in your tach's acuracy. Quote Link to comment Share on other sites More sharing options...
Nitrousbird Posted February 18, 2005 Report Share Posted February 18, 2005 In an easier equation to read: MPH = (RPM x tire diameter in inches)/(final ratio x gear ratio x 336) The equation is very useful for finding gearing you need for the 1/4 mile. One thing it doesn't take into account is convert slip (for autos) and tire growth, which isn't much of an issue until you are running pretty large diameter tires. Convert slip is a BIG issue though. Best gearing for the 1/4 is to have it geared to hit your max HP RPM in your 1:1 gear at the finish line (or whatever will take most advantage of your peak power curve). This is where I am suffering; w/ my RPM range and running 26.2" ET Streets, my 3.73 gears are too much....they would be great if I was trapping 140 (but at that speed, I'd probably move up to a 28" tire, making that gearing even worse!). 4.10's would be the perfect gear for me; allow for some growth but much more ideal all around. But when I got my rear end, I didn't have the extra 1000RPM to play with. Quote Link to comment Share on other sites More sharing options...
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