A big thanks to everyone who gave this ago, I have concluded to stick with this formula and will some time next month or two create a new formula for damping, thanks,
RR
P.S. Hope everyone found it worth their while!
Where is the formula? I see your inputs, but what is getting divided or multiplied by what and in what order?
I know what I have below is wrong, but I need a formula to actually try for you.
ex: Car_Weight / (Car_Tire_Front_Width / (Car_Tire_Front_Radius / 100) / Car_Tire_Front_Height)
Can you post the formula or are you still tweaking it? Seems easier to do it ourselves than post back and forth in here
If you read what I posted in the OP, I’m testing it by use giving me values for your cars which you want tyre pressures for, then the tyre pressure values which I get I will post on here and use, hopefully, will test the values to see whether they are better or not and give me feedback.
The formula is very long unfortunately so posting it would only make people complain, as usual! To get the pressures you need to calculate like 7 other things in order to get the final outcome, that’s why I have created the formula on a program in Visual Basic.
Hopefully that answers your question,
RR
I have all my car values in excel, as I was using your last tire formula. I won’t complain about a 7 value parenthesized formula 
I always appreciate your contributions to the community.
Drive Type RWD
Power HP 515
Torque LB-FT 418
Displacement L 3.7
Weight Distribution % Front 40%
Rear 60%
Weight of Car LBS 2212
Redline RPM 8310
Peak Torque RPM 5637
Top Speed (Calculated) MPH 179.4
Front Aero Downforce LB 90
Rear Aero Downforce LB 182
Upper-Spring-Rate lb/in 916.8
Lower-Spring-Rate lb/in 114.6
Front Ride Height in 5.2
Rear Ride Height in 5.2
Front Tire Width in 245
Front Tire Radius in 20
Front Tire Height in 17
Rear Tire Width in 295
Rear Tire Radius in 25
Rear Tire Height in 18
Are you sure you have titled that info correctly?
Just to check, your:
Front Tyre: 245/20R17
Rear Tyre: 295/25R18
If they’re correct, using the font tyre, the 245 is your tyre width, 20 is your tyre wall percentage and the 17 is the Rim Diameter! That’s what it should be titled.
Going by this info and what I suspect you mean, the:
Front Tyre Pressure: 26.9 psi
Rear Tyre Pressure: 24.8 psi
You may not mind a “7 value parenthesized formula” but my new one would be like 20! It’s more complicated hence why I had to create it on a program!
But thanks for telling me but unfortunately, it would be too long and too many people would complain =/
Anyway, have a go with those values, if the information I used was correct, see what you can do. Please give me feedback as it’ll help me know whether the forumla works,
thanks,
RR
WOW, alot better, but my car is still wonky, 
=D Nice to know!! The “wonky” would most likely be your springs or damping…
By a lot better what do you mean? The feel? Is it faster?
If it’s faster please tell me where and how much.
Thanks,
RR
I don’t know if this helps…
Cold
Temperatures
Front Left
- Inner: 117 °F
- Center: 117 °F
- Outer: 117 °F
- Front Right*
- Inner: 117 °F
- Center: 117 °F
- Outer: 117 °F
- Rear Left*
- Inner: 117 °F
- Center: 117 °F
- Outer: 117 °F
- Rear Right*
- Inner: 117 °F
- Center: 117 °F
- Outer: 117 °F
PSI - Front Left PSI 26.9
- Front Right PSI 26.9
- Rear Left PSI 24.8
- Rear Right PSI 24.8
3 laps on Indy Grand Prix track (not oval):
- Faster, yes. No specifics because I can’t drive.
- More control, yes.
Temperatures - Front Left*
- Inner: 231.1 °F
- Center: 231.1 °F
- Outer: 231.1 °F
- Front Right*
- Inner: 231.2 °F
- Center: 232.0 °F
- Outer: 232.2 °F
- Rear Left*
- Inner: 233.7 °F
- Center: 233.7 °F
- Outer: 233.8 °F
- Rear Right*
- Inner: 233.7 °F
- Center: 233.8 °F
- Outer: 233.8 °F
3 Lap Heated PSI - Front Left PSI 32.3
- Front Right PSI 32.3
- Rear Left PSI 30.1
- Rear Right PSI 30.1
The information you provided is really for Camber but thanks for showing me!
You have it correct 245/20R17 and 295/25R18.
I thought the 20 and 25 was the radius of the rim; and the 17 and 18 were the side wall heights.
Shows how much I know about cars hah! 
And yeah, I think 245 and 295 are in mm, not inches, I have no clue.
Thanks,
I’ll give your psi numbers a try, they are quite different from your previous fornula. I like it at a glance.
I would like my pressures in: psi
Front Tyre-width/Side-wall Percentage/Rim Size: 295/35R18
Rear Tyre-width/Side-wall Percentage/Rim Size: 355/35R18
Vehicle weight (in pounds): 3178lbs
Front Tyre Pressure: 27.1 psi
Rear Tyre Pressure: 25.4 psi
First of all thank you for the time taken to calculate these PSI’s. They were about 4 PSI’s lower than what I had them at originally.
Feedback: Because I was tuning mostly from feel I don’t have any telemetry based info really(temp etc)but what I will say is that my braking distances improved on the stats screen after adding in these PSI’s instantly.
This seemed to put the car into a range where I could make tiny adjustments to other parts of the car namely, diff, springs, aero, and gearing and was able to get braking distances and accel figures lower than what I have had them set at before, collectively. When running the car on track I was getting more grip everywhere as well, the car was in a happier place to push. I didn’t test very much with the PSI change on it’s own (maybe 1-2) laps because I really want to get this car released soon so had to plough on with the rest of the tune but I really feel that as a result of these Tyre pressures I have a better all round base to fine tune around. It’s put the car in the tuning zone so to speak!
Key point about my build: My build runs only Front Aero, the car is naturally good at handling but this extra grip from the Tyre Pressure’s has destroyed any doubts that having Rear Aero as well in exchange for power would be faster.
You’ll enjoy this next bit…
Tune before your Tyre Pressure changes plus minor adjustments: 1.25.5 on Road Atlanta
Tune after your Tyre Pressure changes plus minor adjustments: 1.24.2 on Road Atlanta
Now all I need is 1.3 second and this thing is touching on top 100. Will be an incredible achievement to get this particular non LB car there if I can.
Thanks a heap dude, very positive results!
1 Like
Really great to know!! Just wow! Thanks for giving them ago, it really helps to get feedback like this.
I noticed that if you tune a car around a certain tyre pressure and then you change them, you don’t get much improvement, sometimes you go slower.
If you put the tyre pressure I give then tune around them, I find that it gains it’s time back and is faster, which is what you did, so thank you for giving it a proper go!
RR
P.S. If there is anymore cars you want to test, which are for circuit, just post the info about the car on here.
Yeah it would certainly make sense to tweak after the tyre pressure changes. This edition of Forza, as I understand it, has more focus on Tyres and the physics around them so it’s good to see some actual method going into this. Somebody should call Calspan up and ask them what they think is the best way to determine Tyre Pressures for Racing haha.
Anyways I’m still searching for the 1.3 seconds so here’s a couple more requests if you don’t mind…
I would like my pressures in: PSI
Front Tyre-width/Side-wall Percentage/Rim Size: 295/35R18
Rear Tyre-width/Side-wall Percentage/Rim Size: 345/35R18
&
Front Tyre-width/Side-wall Percentage/Rim Size: 295/35R18
Rear Tyre-width/Side-wall Percentage/Rim Size: 335/35R18
Vehicle weight (in pounds): 3143lbs
Thanks
Assuming that both are the same weight, I guess…
Front Tyre Pressure: 27.0 psi
Rear Tyre Pressure: 25.6 psi
Front Tyre Pressure: 27.0 psi
Rear Tyre Pressure: 25.8 psi
Thanks and yeah I forgot to put the weight on this one it was actually…
Front Tyre-width/Side-wall Percentage/Rim Size: 295/35R18
Rear Tyre-width/Side-wall Percentage/Rim Size: 345/35R18
Weight: 3128lbs
Hope you can do that one for me please and sorry again for the mistake.
Front Tyre Pressure: 27.0 psi
Rear Tyre Pressure: 25.5 psi
Great job buddy! Man, you keep racking up the charts with new stats everywhere. I am glad the psi calc for you. Mine felt good, yet, no PB…yet, I know my driving is inconsistent! I need to work on that then re-evaluate the calcs.