An ant's strength has very little to do with its muscles and a lot to do with the exoskeleton. Chitin is stiff but flexible, the energy stored by flexing it can build up to a remarkable level. To lift something heavy an ant flexes its shell until it has sufficient force, then heaves it up.
I suspect the multiple legs lock out in sequence, again leveraging the hard exoskeleton to provide a constant basis to rest the weight on.
Neither property is particularly useful for building something fast. Ants are tractors not roadsters.
Dismissing your muscle car as a horse is understandable but missing the point. A muscle car would have many advantages over a natural animal, like wheels. However, assuming only the engine is organic, you need to bond a shoulder-type assembly to a drive train. Presumably genetic engineering could offer a tendon with a gristle hoop on the end. Wear and tear would be an issue, but human hips generally last a good 30 years past maturity if not abused.
The concept would be much faster and cheaper to realise without digestion; or a vastly simplified version. Cellulose is extraordinarily hard to digest. Outsource the problem to a biofactory. Generate fuel composed of alcohol or simple sugars, doped with amino acid sources for tissue maintenance, coupled with an activation agent of enzymes and bacteria.
It would probably have to be sealed in a box to prevent infection, given the near complete absence of an immune system.
You could do without a nervous system; or rather, have a device like a regular car. That should give any wannabe animal rights protestors conniptions; either it can't feel pain, or computers can.
What have I missed?