Here are a few useful calculators I have used in order to help me determine the appropriate diesel engine size for my vehicle and to convert Imperial MPG to US MPG.
#1.
Calculate HP For Speed
#2.
Fuel Consumption Conversion Calculator
The figures for calculating HP for speed will obviously not be completely accurate but it will give you a general idea or a starting point that is for figuring out your design goals.
For the TIGON my inputs were =
Coefficient of Drag (Original Goal) = .13
Frontal Area of Car = 14.5 Sq/Ft
Weight of Car = 1,100 Lbs
Velocity of Car 70 MPH
Results Example = Using your vehicle Speed of 70 MPH takes 4.41 HP to overcome air drag.
Rolling resistance is 2.67
For a total of 7.08 HP to run 70 MPH
Based on these results the 13.4 HP Yanmar 2 cylinder should have more than enough power to get us moving at freeway speeds believe it or not
However it may take a good while and hills may be a challenge but this will be determined on the initial prototype.
The original GT6 had a frontal area of 14.9. by reducing the height of the car and cleaning up the front profile I should be able to reach the 14.5 figure.
The .13 Coefficient of Drag will be extremely difficult as the original GT6 was much closer to the .32 area. So I will be designing a completely new body shell to try and accomplish this .13 goal. I have been using the Fiat Turbina, Opel eco-speedster, and Panhard LM64 suggested by Neil Blanchard in a previous post for inspiration along with my own ideas in sketching up the initial rough drafts!
The weight goal can definitely be achieved now with the availability of Lithium and Super caps for energy storage as well as lighter composite materials and hybrid components. There is also many weight saving performance upgrades available for GT6 OEM parts.