I will agree the op needs a better cord. He needs a better cord not necessarily the best. Most serious mechanics have sufficient tools and this is one many might need on occasion. But this guy doesnt sound like he is becoming a pole barn contractor over night.
75 or 100 ft is too light for loaded motor starting at that distance, especially with a cheap possibly older comp, those thoings are notorious anyway especially as they age. At 100 ft a 12 is probably the way to fly especially since he likely cant compare. At 75 a 14 starts most of this just dandy. If the tool comes with a 12,,,, very rare,,, then it needs a 12 but right off I cant recall anything I own came factory with one.
Right off can only think of 3 things even come 14, a chop saw, a welder and a small comp. There might be something else but would have to ponder it a while. And as for the rest of it,, 2 or 3 services worth,, 25 panels, 300 breakers, 400 service (which I could have done fine on 200) the only thing I own and rarely use that would overheat a 10 wire is a 300 Synchro that I rarely use. Dont own a 6 extension cord.
Those ready made 8 welder cords are the way to fly with that. The price is right and unless it was free would buy before making. I had a guy work for me was about compulsive about loss, v drop, pressure loss, recovered materials. All of it. He was an early mentor and it took me a while to get past that and copy other old timers in regards to acceptable performance.
I got a bud, started his career threading steel, 50 years later in plumbing still has the same 1/2 air line and another master still uses a 10 cable to his 225 40 yrs ago when he copied another basic installation. These are masters, could have any of it for free but they size near common code,,, which while it is the minimum in regards to wire it doesnt mean its inadequate, wire and pipe sizes are good and peak and some parasitic like lights gone down. There is more equipment but less 28A motors, a few more 22, welding peak has gone down which is so intermittent its irrelevant.
Another thing to the nature of the op question is the nature of the question and what should have been solid advice.
1,, how far is it. He fundamentally included number 2 is what type a comp and another hugely relevant for the truly genius is how long is the circuit extension its on. 3rd,,,,,,,,,,,,,,,,,, Cost is important, it really is despite some big spenders. One big reason is you can afford more new or replacement if the cost is reasonable vs a fukkin gut wrenching saving and shopping project. We got some here so smart they earn enough it really doesnt matter, their work numbs them to real cost and return on investment. The fuggin world has changed and there is a whole level of adequate equipment to do almost anything that in a real economic sense,,,, is it worth it??????????? NOOOOOOO its not.
Doesnt mean its bad to want better or more but the standard has risen to the point the math is obvious. Its a different question than how someone feels about it. Could take a Stanley pro wrench today and stamp it Snapon and the user couldnt tell, hand them to critical people that knew nothing about the brand and they the same,,, I can put 10 adjustables on a table, have them judged on fit, feel all of it, the generic China now comes out on top. Despite the test, despite the history there are always some can reason a way around it.
How ever it took me a couple tries to get to look under the skin till I new what to look for and could get the same 14$ toaster that the rest of the line was based on all the way up to 89$.
Sure you can buy a better cord even in same gage and a large cord is worth a repair end at some point unlike the compulsive saver using up all my hi dollar replacements on damaged 16 cords and setting up simply using one as a whip where exposed to damage. One of my cord regrets was in a drug store, had a whole sale box of 10 ft orange 3 conductor, 2$, I bought 2 or 3 and should have spent 25 on them.
In the end one piece of 12 that hangs on the wall and 1 I carry on the truck with welder is all I need for 120 and a couple pieces of 10, got a short and a long does it all. Use 100 ft 16 once in a while.
Whats that got to do with anything or point 4? The value added advice