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.