This app was mentioned in 7 comments, with an average of 3.00 upvotes
I spend most of my time in iracing and I like the DashMeterPro (DPM) app for my android tablet. It cost ~~$550~~ $5.50 for one game (looks like the AC and pCARS version work together if you play those two games)
if you look around there a couple of free apps out there but not having used them I cannot pass comments on them.
best of luck!
https://play.google.com/store/apps/details?id=com.sensadigit.dashmeterforiracing&hl=en
You can move the black box & other UI elements around by pressing alt-K to unlock them, drag them around with the mouse, then press alt-K again when they're in a position you like.
The only program I know that can overlay info directly over the iRacing screen is the custom HUD part of the SimCommander software which is part of SimVibe (the software used for sending sim data to bass shakers), but the HUD elements are only a secondary use for the software (which isn't cheap at $89), and I'm not sure if the items you want are available for display anyway. You'd be better off with a tablet or phone and suitable app (e.g. DashMeterPro, although I'm not sure if even that shows the specific info you want).
I use this one on android:
https://play.google.com/store/apps/details?id=com.sensadigit.dashmeterforiracing&hl=en
Should be available on iOs as well I think. You can create your own custom dashboards with the data you want (fastest lap, fuel remaining and much much mroe). Really cool software for 5 bucks.
On Android there is Dash Meter Pro
https://play.google.com/store/apps/details?id=com.sensadigit.dashmeterforiracing&hl=en_GB
And on IOS there is Sin:Speed
http://www.subsonicpulse.de/htm/en/main.html
Both I am afraid are paid for apps but I use both and they get updated regular and both work across many sim racing titles.
I suggest dash meter pro.
https://play.google.com/store/apps/details?id=com.sensadigit.dashmeterforiracing
It works great if you have an extra phone or tablet laying around.
I use iRduino (long thread but totally worth it). It is basically a little LCD screen with shift lights connected to an Arduino board that is connected to your PC. You can set up whatever information you want to be displayed on the LCD screen. I have my Water/Oil temps displayed for plate racing and MPH/RPMs displayed on other tracks.
You can also use iBuzzer G which does a million things and is probably the most useful iRacing app I've found. One of the things is does is it will tell you when your gauges are flashing AND it will send a pm to your drafting partner or any teammates you also want to know about your temps.
Before I set that up I just used DashMeterPro on my android phone or tablet.