Why is my top speed 70 mph?

Keep in mind when you are working on a forum such as this, we can't see and don't know exactly what you did or didn't do. It's important to relay the info as precisely as possible so everyone can see what you see, if that makes sense.

I know you mentioned brakes, but I can't see that being the main cause if you can run the car up to 70.

My guess would be fuel related (starvation), poor tuning, and it could very well be a trans or converter issue. The key is to work through each step, and respond back precisely with your findings, and then move to the next issue.

I didn't see if you answered the question about catalytic converter. If it has one and it's plugged, that would cause the symptoms you describe.


I'm also going to throw this out there as stupid as it sounds. Have you inflated the tires to proper air pressure?

I bought a mid 80's Grand Prix about 20 years ago, and got it real cheap, it was a really nice car. The guy I bought it from had taken it to a local dealer and they told him transmission troubles, this and that blah blah blah. So he sold it to me for a song because he was having the same problem, couldn't get it over 50 or 60 mph.

I handed him the cash, grabbed the title and took off for home, he was right, it wouldn't run. I stopped at one of those quarter air machines and inflated the tires, jumped in and took off up the road toward home, promptly hitting about 90. They didn't look low at all, but that was all that was wrong with that car, My wife drove it for 5 or 6 years.

True story!