06/10/2024
Practical Tech info Saturday!!
This will be long, but hopefully helpful!
Lots of customers ask about this topic, so we are going to share some tech info.
Topic: Tire Size and Gearing
The two most common items here are "Cruise RPM" and Race gearing.
Cruise RPM.
It actually matters where/how you drive.
Cruising at 65, is different than cruising at 80.
So... if you have a 3 speed auto vs a 6 speed manual, this is going to change a lot.
Lets say you have a TH400. High gear is 1:1. but the GM T-56 high gear is .63:1 4th is 1:1, and 5th is .80:1.
If you are selecting gears, or thinking of making a tire size change on your truck, race car, hot rod, etc. this is for you.
If you have a say 30" Tire, and th400, and are trying to decide on your gear ratio, where do you want to cruise at RPM wise?
Say you decide around 2500 RPM is where you want to be, and you expect your average cruise speed to be 70.
Here is the math: (It works the same way for all of the calculations)
RPM ÷ Gear or gears = Axle RPM **
Axle RPM × Tire circumference = Inches/min.
Inches/min. ÷ 12 = Feet/min
Feet/min ÷ 5280 = Miles/min
Miles/min × 60 = MPH
** You can multiply your gear ratios ahead of time, and only have a single overall ratio for a specific gear configuration. (Say first gear and rear gear, but this won't work for second gear without recalculating)
Tire diameter x 3.1415926 will give you the circumference. For our example 30" it is 94.25" (rounded. Get this diameter from the tire manufacturer not just say "Its a 30" tire, because they round too on the sidewall)
So, take your 2500 rpm and divide by trans gear:
In the case of high gear on the TH400, you don't need to, as 1:1 (the T56 would be 2500 ÷ .63)
This calculation will give you driveshaft speed. (We will talk about convertor slip/clutch slip etc later)
Driveshaft speed is then divided by the rear gear ratio. Lets say 3.55.
2500 ÷ 3.55 for the TH400 which provides Axle RPM. In this case 704.225 Axle RPM.
Axle RPM x Tire circumference of the 30" tire we calculated earlier will give you inches of travel per minute.
704.225 x 94.25 = 66,373.21 Inches of travel per minute (rounded)
Now we need to turn inches per minute into MPH.
To do this, we take our Inches of travel ÷ 12 (turns it into feet of travel per minute)
Then ÷ by 5280 (Turns into miles per minute)
Them x 60 (Turns miles per minute into MPH)
This tells you that at 2500 RPM, you would actual only be at 62 MPH. So in this case a 3.23 gear ratio would better deliver your desired cruise RPM. giving you 69.79 mph.
Racing??
Well... if the car is lazy leaving despite having power and convertor, or you need to sort out your crossing RPM here is the math. (this can get complex with converyor, and maybe we will dive into this in another section)
Launch Ratio is your Launch gear ratio multiplied by your Rear gear ratio. (Some folks are leaving in second gear for various reasons that we don't need to get into right now)
So... TH400 2.48 first gear 4.30 rear gear your launch ratio is 2.48 x 4.30 this is a 10.664:1(Rounded) Launch Ratio.
Why is it 10.664? My friend Ben is from Missouri, so... He says "Show me"
1000 RPM ÷ 2.48= 403.226 Driveshaft RPM
403.226 ÷ 4.30= 93.7735 Axle RPM.
1000 RPM divided by 93.7735 = 10.664 (Rounded) So... that is your total gear.
Tire Circumference: 28x3.1415926 = 87.96 inches (yes there will be tire growth, and this varies depending on tire design, size, inflation, and speed. A 20 psi 28" Radial vs a 7 psi 36" Bias ply will be extremely different, but this is to get you closer, and if you are already running, you can calculate your errir factor based on using this math, and knowing your current information.)
So... if you run a 28×10.5, and a 8000 RPM shift point your top speed at the end of launch gear would be: 62.487 MPH.
The same car with a 27" Tire would be: 60.256 MPH. So, the engine would effectivlely experience a slightly lower gear (higher numerical) with the shorter tire.
So... You currently trap 126, on a 28×10.5 and want to increase crossing RPM? Let say you are interested in crossing at around 7200.
You will need a lower gear (Higher numerical) than you currently have.
Tire Circumference: 28x3.1415926 = 87.96 inches (yes there will be tire growth, and this varies depending on tire design, size, inflation, and speed. A 20 psi 28" Radial vs a 7 psi 36" Bias ply will be extremely different, but this is to get you closer, and if you are already running, you can calculate your error factor based on using this math, and knowing your current information.)
So... 7200 RPM and a 28 x 10.5 tire.
For ease we will do the TH400 again. with a 1:1 High gear.
A 4.30 gear would achieve 139.47 MPH so the 126 MPH would be down at around 6525 RPM.
As to where a 4.56 gear would have a top speed of 131.52 and the 126 MPH would now be at 6900 RPM.
But... what would the 70 MPH Cruise RPM be?
Well... you would be running through Traffic at 3850 RPM. Trust that this is great for a racecar, and terrible for your daily.
It is critically important that you are honest with yourself as to how you intend to use the vehicle, or you will wind up unhappy with the vehicle, and have spent money to make it unenjoyable. Regardless or who you have do your vehicle modifications, it being the way you want it is important, and if it is built based on an unrealistic estimation of how you use the vehicle, you will end up with exactly what yiu asked for, but not what you actually want. Both you and the builder will be disappointed.
