In the late 60s or early 70s I read a series of books (3 at least) about a young race car driver (probably 18-20 in the first book). They were hardback books from a library, so could be 5-10 years older. (So could have been published in the 50s or 60s.)
Specifics I remember from the series:
In the first book at least, the races take place on short tracks (might have been dirt tracks) of less than a mile. Probably 10, 20, or 30 lap races.
The driver has a habit of hanging back for most of the race not pushing for the lead, and observing the other drivers and taking it easy on his car. So during the last lap or two he floors it and takes the lead and wins (because he knows the other drivers' habits and how to pass them). Thus he gets the nickname "Last Lap (name)" [i.e. Last Lap Stevenson or Last Lap Hamilton.]
I'm pretty sure he rebuilt his own car to race in. Eventually he gets a mechanic to assist him/be his pit crew. The mechanic is with him the rest of the series. I'm pretty sure the mechanic was German, (or Dutch, or Bulgarian) and takes over most of the engine modifications and tuning.
At one point the mechanic takes out a stethoscope to listen to the engine, causing the other drivers/pit crews to laugh at him, but he is able to hear small problems in the engine to tune it to run much better.
The second book: they are winning so much they are becoming well known. A very fancy expensive car (Rolls-Royce? Mercedes Benz?) drives up to their garage with a terrible sounding engine. The owner wants them to do an engine disassembly and inspection with photographs. He tells them he has put 50k(?) miles on the car and never changed the oil, only adding oil when it was low. The mechanic is very upset at what the driver has done to destroy the engine on this very expensive car. But the owner tells them he owns an Oil company and wants to show people that they need to change the oil every few thousand miles and not just add oil when its low. In exchange his company will be a sponsor their race car, paying for all parts and repairs for future races.
The driver and mechanic design and build their own race car from scratch. (Frame, engine, body, etc.) They do so well in races, they get requests from others to build cars for them.
The 3rd (or later) book, which I'm pretty sure was the last book: By this time they are running a small company building several race cars at a time and selling them. The driver is now married to his girlfriend (she had been with him through all or most of the series) and they have bought and are living in a very nice house.
At the beginning (first chapter?) the driver and his wife have dressed up (guests coming over for dinner. Either the mechanic and his wife, or an investor and his wife). While waiting for them to arrive, the driver heads out to the garage to do a little adjustment on his car engine. The wife tells him to not wipe his fingers off on his pants when they get dirty, to use a rag to clean them. But when he finishes his adjustments, he does remember what she said, but not seeing a rag, decides to just use his pant leg anyway. Later when the guests notice the grease, they all laugh and say something like clearly he is the man of the house, or something like that.
The driver now rarely gets to race any more, as he is spending most of his time doing car designs/building. But their cars are becoming very popular with other racers, and they are doing very well.
The diver/mechanic decide to enter one of their cars in a major car show, since winning will greatly increase their sales of the cars they are building. But are having trouble deciding color schemes, and how much to chrome. [at the time in car shows having lots of chrome was popular] when the driver's wife says "Pink and Copper" with (white?) interior. When they scoff at how that would look, she pulls out a plastic model she had built and painted to show them. Pink paint with copper finish instead of chrome on the exposed metal parts. (i.e. the bumper, grille, hub caps, gear shift, etc.) And they agreed it looked very good and was unique. So that's how they styled their entry.
They win first place in the show and the driver and his wife drive the trip back home.
As they are driving the long trip home the wife falls asleep and the driver realizes the car is so quiet and smooth, he has been driving like 20+mph over the speed limit and never noticed. [Someone, who either lost at the car show, or is annoyed this pink car is speeding through his area, indicates they want to race him] So he gets on a long empty stretch of road and decides to see what the car will do, and speeds up. It gets up to like 130mph, leaving the other car way behind, when he hits a bump at the start of a bridge which cause the car to go flying. It clips the guardrail, or a bridge support beam, and spins off the bridge. This causes the couple to slam their heads together killing them. And the car lands in the river and sinks. The second car then tops the hill before the bridge, but he does not see the pink car anywhere and can't believe it's that fast. He floors it trying to catch up. That's how the book ends
Now it's possible that the "car show/ending" is NOT from the series I am describing but from another book, but everything else IS from the series.