Tag Archives: RPM

Did I run the motor too hard?

So I wanted to calculate the RPM to see if I ran the engine too hard.  From what I have calculated, I don’t think so.  The speedo was 11MPH off according to my VZW Navigator GPS.  I checked the tire size on the bus VS the tires that came stock.  The stock tires were 6.40 – 15, which according to a vintage tire size conversion they were P195/75R15, or P205/70R15.  The tires that are currently on the bus are P215/75R15.  Why is this important?  We need to understand the what the tire diameter is in order to know the speedo correction. By using the handy-dandy tire size calculator tells me that the im either 4.5% or 7.5% too slow.  Which tells me something else is wrong with my speedo, or there is another variable I’m not considering.

Now for the calculation.  The variables i need are the following.

tire diameter – 27.7

4th Gear ratio – 0.82

Ring & Pinion – 4.125

Reduction Gearbox Ratio – 1.26

Constant – 336

RPM = (MPH*336*Drive Ratio)/Tire Diameter

Drive Ratio = (ring & pinion * Gear * RGB Ratio)

Given that I kept the speedo at 50 to 55 MPH, which translates to 61 to 66 MPH.  That equates to 3153 and 3412 RPM. From what I can gather, as long as I keep the engine under 3500 RPM I should be fine.  However I have read that the best cooling is around 2800 to 3000 RPM, so I was probably pushing it some, but I don’t think I ran the motor too hard unless the tuning was off.  That I’ll never know at this point.

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Posted by on July 21, 2008 in Maintenance, Uncategorized


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