I worked with the head R & D person when doing work on the 1/12th scale GT-40's by GMP (www.gmpdiecast.com
) when they were creating the tooling for the 1/12th scale GT-40's.
There are 3 different styles of the GT-40 done so far by GMP
1967 Ford GT-40 MK IV #1
1966 Ford GT-40 MK IIB #2
1969 Ford GT-40 Mk I #6
This involved a lot of work and a lot of pictures and measuring of the originals then trying to get these to be understood by the factories in china.
Here are 2 such animals in my collection at home shown next to an Autoart Subaru 1/32nd scale slot car for size comparison...
The black one is a preproduction prototype (hence the Chromed wheels rahter than the Gold ones) 1966 Ford GT-40 MK IIB #2- McLaren/Amon- 1966 LeMans Winner done to scale that is used for a test mule for engineering parts. It is a final preproduction sample just before the car was completed. The red one 1967 Ford GT-40 MK IV #1- Foyt/Gurney- 1967 LeMans Winner- Nbr Ltd Ed of 900 is a production version. Both highly detailed.
While this in no way says I am an expert, you have to understand one thing in so far as the racing versions go. The actual shapes are in no way the same as they once were when first raced. Every one of the cars underwent some type of body modification for better aerodynamics and/or downforce as time went on. While the general body shapes were retained, they are not exact as one would think.
My opinion is that while neither are 100% correct, the Scalextric versions are closer in body but the Fly wheels are pretty much right on the money. The bodies on the Flys are much to flat across the hood and tops of the fenders...especially in the Fly GT-40's done after the Gulf series with the bad chassis. Tire and wheel detail on the Fly is far superior to the Scaley though. From a scale perspective, the overall size and shape of the tires on the Fly Gulf liveried cars is right on the money with the wheels being perfect in their design from what I can see.
Just my opinion.