Calculating Gear Using Rev/Speed Ratio

Hi All

I’m a driving instructor using a 2019 Vauxhall Corsa 1.4 petrol. I want a way to display the currently selected gear to help pupils. I’ve got myself an OBD2 reader and have just installed RealDash. I’ll set it up tomorrow.


As a novice user I assumed there would be a way to enter the ratios like this:

Gear 1: 4mph @ 800 revs
Gear 2: 7mph @ 800 revs

From the posts I’ve read it seems that lots of users struggle setting the gear ratios. Wouldn’t having this input option too be easier?



Yes, its true. We get a lot of questions to help with gear ratios and many times that info is somewhat difficult to find.

Also, front wheel drive vehicles use the final gear differently so sometimes you need to multiply the gear ratio with final ratio, sometimes divide.

I think we will make a new option for next release where you can set the gears by driving the vehicle.

That would be awesome. So now I guess I better get my calculator out and try to figure it out. :rofl:

Before I sit down tonight after work and try to figure out the ratios, is there a simple maths trick to work out the ratios based off speed and RPM (assuming FDR set to 1)?

This can be a nice math exercise :slight_smile:

Ratio of each gear is:

Tire RPM / Engine RPM.

To calculate Tire RPM, you first need the tire circumference. Lets say that you have 245/40x18 tire.

Circumference of the tire in meters is calculated as:
(tireWidth * tireAspect / 100 + wheelDiameter * 25.4 * 0.5) * 2 * PI / 1000;

So 245/40x18 tire circumference would be:
(245 * 40 / 100 + 18 * 25.4 * 0.5) * 2 * PI / 1000 = 2.05 meters

Now we have the circumference, we can calculate the tire RPM based on circumference and speed.

Speed in meters per minute is:
V (in km/h) * 1000 (meters in a km) / 60 (minutes in an hour)

So Tire RPM is:
TireRPM = CurrentSpeedKPH * 1000 / (60 * TireCircumference);

The current gear ratio is then simply Engine RPM / TireRPM

Thanks for this. I’ve worked each ratio out so fingers crossed for tomorrow. If it works I’ll buy this app. :crossed_fingers:

Out of interest, any way of just displaying the current gear on the screen? I couldn’t see anything like that in the gallery.

Cheers, Nick

There is no ready made dashboard that shows Gear only, but its quite easy to make one:

  • Open top menu and tap ‘Edit’.
  • Top menu ‘File->New’. Then select the asset set (any of the will do)
  • Top menu ‘Add Gauge->Text Gauge’
  • Drag the new text gauge so position and size of your liking
  • With text gauge selected, go to Top menu ‘Input & Values’
  • Press Select data source button and select ‘Calculate Values->Gear’
  • Press ‘Done’ to go back to the dashboard and then top menu ‘Done’ to finish editing.

Thanks for your instructions. It works!

Please can you tell me if there’s a neat way of rotating the gear number so my phone can remain portrait? I did find one option to do this but the font was stretched.

Also, I’ve now bought the app because it’s great. :+1:

Yes, rotating the text by 90 degrees will introduce some narrowing for the text. There is a workaround for this though.

You have to use some other font than the default font. Then go to Look’n Feel->Text & Font->Choose Font and use the small gear icon on the font selection box.

On the popup, change the ‘Font Load Point Size’ to a bigger value until the text appears sharp. Then try doubling the ‘Font Load Narrowing Factor’ to 2.0.

You may need to tweak these font load settings a bit to get the result you are after.

Thanks for the assist. Can’t find any fonts. It asks me to select an asset. :man_shrugging:t3:

Yes, you would need to download a font file, or etc. has quite many.

Well, easiest way would be the following:

  • Go to the Gallery and load default dashboard.
  • Edit, select all gauges and delete all gauges.
  • Now you can start adding your text and the fonts that were in use on default dashboard are available for you.