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77' 300D NA Auto: Optimum Speed for Best MPG?

863 views 7 replies 4 participants last post by  uberwasser 
#1 ·
I have a 1977 300D Non-Turbo with automatic transmission. This car has no tachometer. I'm wondering what is the speed that will deliver the best MPG?

BTW, South Louisiana = no hills.

Thanks.
 
#2 · (Edited)
Keeping it under 60mph will help. Not sure on the exact best speed.

Despite not having a tach it's pretty easy to figure out engine RPM based on a given speed. You need to know four things -

The speed you are traveling.
The diameter of your tires.
The ratio of the gear you are in at the time
The final drive/rear end gear ratio.

The first one will change, but let's call it 60mph for now.

The diameter of your tires will depend on what you are running, but many are running 195/70/14 tires which are 24.7" in diameter. You can find your tire diameter online.

The third, trans ratio, changes depending on the gear but in this case we're undoubtedly talking about 4th gear which on these cars is a 1 (aka a 1:1 ratio).

Finally, there is your final drive ratio. The non-turbo 300D's have a 3.46 to 1 ratio.

The calculation is:

((Axle Ratio x Vehicle Speed x Transmission Ratio x 336.13) / Tire Diameter) = Engine RPM

So it would be ((3.46 x 60 x 1 x 336.13) / 24.7) = 2825rpm

Top speed on these early non-turbos according to the internets is 92mph, which equals just over 4300rpm by the calculations.

Fun stuff!
 
#6 ·
Close with the 55 quote. According to the ratios/tire diameter and torque info supplied by Uber the formula says 50.9712 is the speed at which I will be at the peak torque. I think I will round that off to 50!

Thanks for the info!!

BTW Uber, I still use your great iFixit tutorials even when I know how to do the job just to make sure I'm not forgetting something and have all the tools and supplies I need. So, thanks for those too.
 
#8 ·
BTW Uber, I still use your great iFixit tutorials even when I know how to do the job just to make sure I'm not forgetting something and have all the tools and supplies I need. So, thanks for those too.
Glad to hear it!

As you go through them never hesitate to let me know if you find something I've missed.
 
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