There is a calculator out now for FM4 that is pretty nice. I use it to get a baseline tune and then do a little tweaking from there when I have time. Unfortunately the forza forums are down right now. This is the link to the thread on the official forums.
http://forums.forzamotorsport.net/forums/thread/4859908.aspx
There's a long-winded tutorial on youtube that explains how to use it.
http://www.youtube.com/watch?v=by7JDh-5Y1g
But basically the steps are:
build the car to your spec.
select a part to upgrade and get the "buy this?" popup screen. it will show the current lateral g and lateral g after purchase. use that current lateral g number in the spreadsheet
go to the "benchmark" page and input the top speed from there into the spreadsheet
then go to the car in my cars and hit the blue button and hit rb to scroll through the needed info, hp, torque, weight, weight distribution, power rpm, torque rpm, and redline rpm, tire size
make sure you select the correct engine position (front mid rear) and driveline (rwd fwd awd) in the spreadsheet
*you can put in whatever you top speed you want. I think it's tailoring the ratios to squeeze the max amount of acceleration out of the desired top speed redline point
It will spit out all of the parameters at that point, including individual gear ratios etc.
After I get a baseline I then go into test drive and tweak it to my liking. Usually this involves getting the back of the car doing what I want through and out of corners. I usually tighten mine up and end up softening the rear roll bar by about 5%, adjusting the diff accel down to decrease throttle-on oversteer coming out of a corner, and adjusting downforce.
Based on the feel of the car and the times I'm getting I'll go back and mix up parts. The biggest thing you're switching up and balancing out is weight, power, and tire compound.
It looks like a lot when written out but it's quick once you do it a few times.
Because the forza forums are down I uploaded the spreadsheet to a host, which I'm not used to doing so hopefullly it works. It's a 275kb spreadsheet. They recommend using excel but I run it in open office (free) and haven't had any issues yet.
http://www.mediafire.com/?bvppunygy226fyq
I think I'll be there tonight. There was a chance I was going out of town but I'm staying now.
I had taken my cars out of the club garage because of the clutter we talked about. I'm going to share maybe 6 A-class cars that I've been tweaking this week. Feel free to try them out and use them tonight whatever.
It's easy to get a car setup to drive like a dream. It's tougher to get it to also be fast. I've been racing in the C class and now A class hoppers this week. With some of these cars I can dip into 1:30.xxx on road atlanta full which is where I tune the most. That's a top 1% time but then the guys online are getting 1:28s!
Sorry for the rambling nature of the post. There are many little tips and tricks and concepts to tuning.
Such as - I was racing mostly C class online before this week. I like good handling cars but you have to give up some handling to get speed and good times. I always used to keep the wheels as small as possible with the lightest wheels to decrease rotational mass. However, in c class most guys are running big wheels with crappier tire compounds, and it's quicker. So I do too. I think what's going on is with the tire physics running the huge wheels and super low profile tires limits the crappy rubber's sidewall flex and overall wishy-washiness (technical term) which provides better feel than tall tires and allows you to allocate the PI elsewhere (power)