He had a high chance of having a heart attack or falling asleep on the job.
"I was always eating on the run and overloading my body," he says.
"I'd stop for a break and grab a sausage roll, a pie and a soft drink."
"Servos were an easy option when you have no smoko room and are doing odd hours."
Doctors suggested he stop driving.
"They were going to recommend to NZTA to take my licence off me."
This was the fright Mr Rumbles needed to turn his health around.
With the help of Te Awamutu Medical Centre nurse Sally Davies and his supportive family, he transformed his diet habits.
"I'm only 55 years old - halfway through my life. I want to live to see grandchildren."
In four months his blood sugar levels dropped dramatically and he lost more than 10kg.
"I eat less sugar and carbs and more fruit and vegetables. I've also hugely reduced my alcohol intake."
"I feel better and more energised and my clothes fit me well again."
Mr Rumbles now eats smaller portion sizes and prepares healthy food options to take to work.
"It helps to be prepared and organised rather than gorge on bread or takeaways.
"I can buy a cooked chicken from the supermarket, add some veges, and that will last me at least three meals.
"Every now and again I binge but then I remember how bad my body feels afterwards - tired and lethargic.
"You can still have 'bad' food, just less often."
His new habits have improved his finances, too.
"The change in the bank balance is amazing."
Mr Rumbles is keen to help others struggling with diabetes - particularity his workmates.
He urges those concerned about it to seek medical help and support.
"Listen to your doctor and diabetes specialist," he says.
"Don't get offended when they are only being honest. They are there to help you."
Mr Rumbles is one of an estimated 240,000 New Zealanders with type 2 diabetes.
This November is Diabetes Action Month - a campaign aiming to educate the public around the major health issue.
Diabetes NZ chief executive Steve Crew says the awareness month is not exclusively for people with diabetes.
"It affects all New Zealanders, because all of us either live with or know people with the disease, and everyone is at risk of type 2 diabetes."
