how fast am i going
Freshman User
Joined: Oct 2003
Posts: 25
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Willowbilly is right - I measured my tire diameter and it was 28.1" instead of the 28.9" calculated by the Excel spreadsheet. I plugged this in and got 61.4 MPH at 2800 RPM, pretty much what my speedo says, so assuming my tach is accurate the formulas work pretty well.
The calculators NM5K mentions sound better but this one seems not too bad.
If you want to use a measured tire dia. in the Excel calculator just replace the value in cell B15 with whatever you measure. I'd copy and paste the formula in B15 first to another cell at the bottom of the spreadsheet and update the cell addresses (they change when you copy and paste) so that you've still got a record of it.
To get an MPH for any RPM you want, just pick another cell at the bottom of the spreadsheet and enter the formula =B18*[RPM/1000]. You can enter the RPM directly in this formula or enter it in another cell and have the formula reference that cell location.
Opiewan, I assume you're using the downloaded version of the Excel spreadsheet - that way you can make whatever changes you like and save them.
The calculators NM5K mentions sound better but this one seems not too bad.
If you want to use a measured tire dia. in the Excel calculator just replace the value in cell B15 with whatever you measure. I'd copy and paste the formula in B15 first to another cell at the bottom of the spreadsheet and update the cell addresses (they change when you copy and paste) so that you've still got a record of it.
To get an MPH for any RPM you want, just pick another cell at the bottom of the spreadsheet and enter the formula =B18*[RPM/1000]. You can enter the RPM directly in this formula or enter it in another cell and have the formula reference that cell location.
Opiewan, I assume you're using the downloaded version of the Excel spreadsheet - that way you can make whatever changes you like and save them.
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Elder User
Joined: Aug 2003
Posts: 952
Likes: 1
From: iowa
hell now i cant even get the damn thing to work lol. can you email me the one you got ? i can change the tire values and gear ratios myself. i should learn more about these damn things
Post Fiend
Joined: Jun 2003
Posts: 8,209
Likes: 12
From: Black Hills of SD
Thr basic formula is simple, Just do the math yourself. Determine the diameter of your tire and use the gear ratio to figure out how many times it goes around in a mile. you have to convert miles to inches. Example. 3.55 gears and a 29 inch tall tire. Multiply 29 times pi (3.14) to get diameter,( or measure around your tire for a more accurate measurement) hence 91.06, 91 is close enough. Turn miles into inches, 5280 x 12=63360. divide tire diameter into that. 63360 divided by 91=696.26 tire revolutions per mile. Multiply that times the gear ratio. 696.26 times 3.55= 2471.723 which would be your rpm at 60 mph. take it further and divide 2472(rounded off) by 60=41.2 revolutions per mph. Now you can just multiply whatever mph you want by 41.2 and get your rpm at any given speed. These figures are for a 1:1 high gear. If you need to know the rpms for a different gear just multiple the rpm times the ratio of the gear. For instance if you had overdrive with a .80 ratio you would be turning 1977.6 rpms at 60 mph. It's all just simple math.
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