I own a 2004 Mazda RX8 with 130,000 miles. I recently replaced the motor and flywheel at 128,000 miles. How do I now calculate the value as I might want to trade it in. I put over 5 grand into her. It’s a rotary engine, which is maintenance prone
No matter what you calculate, the true value is whatever someone is willing to pay for it. If you go to eBay or other markets you will get an idea what similar cars are selling for.
We get frequent questions like yours, and the answer is always the same; supply and DEMAND determine what you get. Whether you sunk $5000 into the car or $10,000 does not matter a great deal; someone must still want to buy that car. And it does not qualify as a collector’s car.
I think we can assume that the value of an RX8 with a busted motor is pretty low, but there’s no formula that says that the car is worth $5000 more today than it was worth before the repair.
The only way to “get your money back” on the repair is to keep driving the car. Let’s say the car now has a trade-in value of $5000, and let’s imagine you could have gotten $1000 for it with a busted engine. Then you’re only down $1000, and if a new car would cost $500/month, then you make your money back if you keep the car 2 more months.
Try Edmund’s True Market Value calculator:
http://www.edmunds.com/popupinfo/abouttmvprices.html
Twotone
No matter what you calculate, the true value is whatever someone is willing to pay for it.
That is worth repeating. There are a number of services that look at sales and publish results. However, remember that YOU really decide what it is worth, not the prior sales.
Even the Edmunds’ TMV calculator is extremely rough.
I just sold my 14 year old Ford for nearly twice what Edmunds says I should have been able to get for it, in under 24 hours, with multiple offers. I never even bought a “For Sale” sign for the car.
On the other hand, their estimate for the trade-in value was spot-on what the stealership offered me. 
“Even the Edmunds’ TMV calculator is extremely rough.”
I think they are accurate for late model used cars. The RX-8 is half the age of your car. I would expect that the $5000 spent recently would upgrade the car from “rough” to “clean” condition.
Edmunds suggests that this car with a manual transmission in clean condition and lots of accessories could be traded for about $6200. talan7, you did not tell us what options are on the car or whether it is an automatic or manual transmission. The big detractor is high mileage. That knocks off $2500. You might get some of that back with the new engine. Take it to a used car dealer or a Mazda dealer and see what they will offer you to buy it.
What you put into it doesn’t neceesarily translate into an equal increase in value. The car is 7 years old with well over a 100k miles and the condition would be “good” at best. Even that may be a stretch.
It’s worth what somebody is willing to pay for it and not a penny more…To find out that number, check eBays “completed auctions” for similar cars that actually sold…You will notice that many of the cars on eBay never bring the “reserve price”. This means the owners think they are worth more than they actually are…