Forget Yahoo answers....
If you want info about RC you should ask your questions on this forum.
I can tell you now that 3:1 won't work.
The car will stand still while you slowly burn your clutch to crumbs.
For a big car like that I'd go for at least 8:1.
Another thing to consider is at what RPM the clutch engages.
I've never had any experience with factory RC cars, but I know that a lot of them clutches engage at 6000 or 8000 RPM depending on what strength spring they use.
The problem with using chainsaw/weedeater motors is that the clutch engages at way lower RPM.
Depending on the clutch setup you may be able to
A) Get stiffer springs
B) Drill holes in the clutch shoes to make them lighter and in turn get the clutch to engage at higher RPM.
I say this to you now to avoid disappointment - don't worry about making calculations on paper about how fast the car will go.
Build it and be prepared to rebuild and redesign as you go.
Also, if you're worrying about the cost of adding an extra set of sprockets and chain in, you're on the wrong track.
Setting a budget before you've even started building is a bad idea.
Are you that same guy posting on RcGroups by any chance?
[EDIT]
Also, I just realised that you said 30-50
MPH. [X(]
Unless you've got access to a private Nascar track or something, I don't think driving a large scale RC car at anywhere near those speeds is advisable.