Physics Engineer needed -- HP loss when increasing tire size and gear ratios?

[NCJL]

I think I need a Physics Engineer.
Problem.
Trying to figure out the HP loss when increasing tire size and gear ratios together. Lots of calculators out there when increasing tire size without changing gears.

For example. I re-geared my JT to match a factory JT Sport with EcoDiesel. 3.73 gears with 31” tires.
My JT has 5.13 gears with 42” tires. The RPM’s basically match each other at speed.

How much HP did I lose? I had to lose something.
With all the rolling resistance talk going on. I found calculators showing HP loss when going from all-terrain to mud terrain, up to 5hp using the same size tire.
Can’t find anything addressing the diameter, width and weight change of a tire within the same RPM band.

Thanks.

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[ShadowsPapa]

I think I need a Physics Engineer.
Problem.
Trying to figure out the HP loss when increasing tire size and gear ratios together. Lots of calculators out there when increasing tire size without changing gears.

For example. I re-geared my JT to match a factory JT Sport with EcoDiesel. 3.73 gears with 31” tires.
My JT has 5.13 gears with 42” tires. The RPM’s basically match each other at speed.

How much HP did I lose? I had to lose something.
With all the rolling resistance talk going on. I found calculators showing HP loss when going from all-terrain to mud terrain, up to 5hp using the same size tire.
Can’t find anything addressing the diameter, width and weight change of a tire within the same RPM band.

Thanks.

You can't figure that because you don't lose HP. You take more HP with more WEIGHT, wind resistance (which varies with tires! as well as the Jeep height)
The HP is still there - it goes to increases in rolling resistance, height, frictional losses of various gear sizes and other factors.

You would have to know the rolling resistance of that TIRE BRAND, tread pattern, rubber compound and more. You can't figure that stuff.

We can take two "identical tires" in size/diameter, width and so on. both of them being A/T types - and tire A may have a very different rolling resistance vs. tire B. Even the state of CA is looking into this in proposed laws. Tire brands and rubber compounds and so on matter- not just sizes.

Unless you are racing, you are going down a deep rabbit hole and even then unless you know the exact rolling resistance of the MODEL of tire you are buying - it's all a guess. You can't plug in a tire size and know what you want.

What is the goal, seriously - why the need to know that number?

 

[Gvsukids]

Your engine produces the exact same horsepower at the crank, but what reaches the pavement during acceleration or shows up on a chassis dyno tells a very different story.
Because your effective final drive ratio matches almost identically (\frac{3.73}{31} \approx 0.120 vs. \frac{5.13}{42} \approx 0.122), your engine runs at the exact same RPM and power band at any given road speed. The losses you feel come entirely from physics downstream of the engine:

 

[Vtur]

No Physics Engineer would own a Jeep so you probably won't be getting an answer from one .

We are doing nearly everything against physics here.

 

[IRAH]

I'm certainly no engineer. Ex army truck driver/examiner here, but one thing I learned is there's a lot more people out there that know things I dont.

But I found this!

https://gearsmagazine.com/magazine/...ain-concerns-to-address-with-oversized-tires/

This seems to make sense to me. What do you think?

(TLDR, It was found that the old jeep 3.8 lost about 3.5% of torque per inch of tire size increase)

 

[Master_Cylinder]

I had physics in high school.

 

[RougeShot12er]

I stayed at a holiday inn

 

[Cadguy]

Your torque at the pavement should be approximately the same, but the problem is, you have a LOT more rotating mass as well as diameter you have to spin up, even though your gearing is approximately equal. If you lifted the truck onto jackstands and slammed the throttle, there would be a significant difference between the two setups. There would be very little difference in crawling where you are not accelerating. (other than much better traction)
I ride, and used to race mountain bikes, and I would look for a few GRAMS in advantage on my tires, as the are the furthest from the axis of rotation... and I only had two tires

 

[Cadguy]

I stayed at a holiday inn

Ahhh, that's the problem, it has to be a Holiday Inn Express

 

[BlackRuby23]

Problem.
Trying to figure out the HP loss when increasing tire size and gear ratios together. Lots of calculators out there when increasing tire size without changing gears.

For example. I re-geared my JT to match a factory JT Sport with EcoDiesel. 3.73 gears with 31” tires.
My JT has 5.13 gears with 42” tires. The RPM’s basically match each other at speed.

Holy smokes. The original Bigfoot monster truck had 48" tires, and he was running a 600 HP big block that was underpowered. I think he ended up at 1,500 hp in that first truck. You have a massive tire:hp/tq ratio. That poor thing must be struggling.

 

[Jrgunn5150]

Lol, you don't engineer physics, they just are.

Anywho, drivetrain losses, which is what you're getting at, are a thing, only way to really know without an educated guess, is dyno a stock one and dyno yours and compare.

 

[NCJL]

I could have done a better job of wording my original Post.
Do I really need to know? No. I thought it was an interesting way to look at tire/gear ratio changes. Have an idea of the amount of HP/TQ that is used.
Is it a rabbit hole? Yes. However so is the MPG shown on every window sticker.
The EU requires “rolling resistance” for every tyre sold. This is where I found the RR difference between AT and MT tires. Equalling 3-5HP. This was at 0% grade, 55mph. This is just for a tire tread change, same size tire.
I have an IDash in my jeep )diesel). I can monitor real time HP/TQ numbers. Guessing, not exact. 55mph at 0% grade would be around 25HP needed. Change in tire tread 30HP needed. To me, that’s a big difference for just tread, RR.
What I can’t verify is how to include tire change size. I have found rolling resistance for a 31” tire not a 42” tire. Maybe they don’t sell a 42” in the EU. Maybe it’s excluded from RR. IDK.
I have read many articles and discussions. Some say the extra mass can just be included in the vehicle weight. Some say it must be included as a separate line item. Same goes for the increase in diameter.
I have found calculators addressing these items, however there based on EV’s. These calculators say about 30HP extra is needed at 55mph. Going from 31” tire to 42”. EV’s only have one gear (on these calculators). So maybe it’s an acceptable number to start with. I changed gear ratio to match tire size. Still the same RPM’s between sizes.

Does Gear ratio changes help slide the HP number used for the change? More weight is still more.

 

[Jteakus]

Are you happy with the results? We usually gear a little deeper than a “stock match” to cover the added weight. Not so much with the diesel though.

 

[NCJL]

Are you happy with the results? We usually gear a little deeper than a “stock match” to cover the added weight. Not so much with the diesel though.

Yes. My JT with Eco Diesel is great. Heat Soak has been an issue. Lots of posts/info shared in the Diesel section on that subject. It’s a daily driver. I also tow a 4k off road trailer. Power has never been an issue. I think I made the right choice matching gear/RPM’s with a factory Sport. The Sport did have the highest MPG and Tow ratings with the ED. JT weighs about 7k too. Staying in the torque curve is easier than a gasser.

My TJ and LJ on 40’s not so much. Gears helped but couldn’t overcome the difference. The 4.0 was better than the current 3.6. Lower torque curve. HP without TQ isn’t the same.

 

[Jteakus]

Agreed.
I like my torque off idle not at 4,500 rpm.

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