My intention is to the same. Vs an e car my mog is actually carbon efficient (i.e. uses less carbon) to 45k miles. But then if I change it surely that carbon is scrapped and we start again so mine has zero emissions from the build & better for longer?
Not sure about the breakeven point, but yes, any carbon emissions in the construction and use of your car up to now are gone so it's only the output from burning fuel that needs concern you.
Each gallon of petrol produces 11Kg of CO2, so say 8,000 miles per year at 30mpg means you chuck out just under 3 tonnes of CO2 annually.
If you want to continue to hoon about the place guilt-free and feeling virtuous you can invest in a carbon offsetting scheme like this one:
https://carbonneutralbritain.org/pages/become-carbon-neutralFor the cost of just one gallon of gas, £7 a month you can offset 15 tonnes of CO2 per year, so you'll be making positive steps to fight global warming too!