RPMs at highway speeds
 

I was driving along today in the 300D, it has 225/60/15 tires all around, and the speedometer was showing 70 mph, and the tachometer was showing 3400 rpms on level ground. Anybody have some sort of equation they use so I can determine the actual speed I was going? Because the 300SD has 205/70/14 tires, and 70 is closer to probably 3100 rpms on it.
-Joe

The formula is:
 

Engine speed=(MPH)(Ratio)(336)÷(Diameter)

or

MPH=(Engine speed)(Diameter)÷(Ratio)÷(336)


Please be aware that the formula works very accurately for a manual transmission. The automatic has slip so you must add a couple of hundred RPM to the result you get for engine speed in the first formula and you must subtract a couple of hundred RPM from the tach reading when you use the second formula.

For the 123 with a 25.6 tire diameter and a 3.46 ratio, the engine speed at 70 mph with the automatic should be about 3375 rpm. So, it appears that the tach correlates well with the speedometer.

Thanks, I'm saving this thread for future reference.
-Joe

Anytime.

Help is what we do here.

You know, that would be a good slogan for the Diesel Discussion forum, or a good signiture for someone.
-Joe

I've copywrited it. :D :D ;)

This may work for you too.

http://www.miata.net/garage/tirecalc.html