We get asked a lot about tire size and how it affects HP. Yes you will lose a little HP because of larger tires, but that is not the main reason performance suffers. It has more to do with your RPM band and how changing tires size move where you are at in the rpms at a certain speed.

Check out this video to learn more.

 

I agree that modifying the truck changes its characteristics but putting on larger tires does not affect horsepower. It will change the effective gearing and your truck's performance (in addition to having to move that additional rotating mass) but your horsepower will stay the same.

I understand that the term "horsepower" is being used synonymously with "performance" but they are not interchangeable.

I'll get off my soapbox now.

 

Horsepower is determined by the engine. Nothing you change downstream will alter that. Larger tires will change your gearing and will therefore affect acceleration and top speed, but they do not add Hp.

 

at 70mph in OD you will have you will have 200 less HP available with 37s vs 31s

This picture is in the video but I broke it down to show better what is being explained how rpms/speed affect HP available. This will vary by truck/trans/engine/etc... but this will help understand the data behind it better.

If you change your tire size, then it will change where you are at in the RPMs based on a certain speed. Which great affects how much useable HP you have available at that speed.

This happens to be a 7.3 dyno graph, because that is what is in the video. But that is not important because all diesels will suffer from the same issue.

If you want to take a step further... the red line is a modified stock size turbo and blue is a larger turbo. You can see a similar issue with having less available HP at a certain speed also happens with a larger turbo. Same thing happens again with bigger injectors with larger nozzles. Now add bigger tires + bigger inj + bigger turbos + stock gears and you will have a hot smokey mess.

 

Horsepower measures the power generated by the engine, brake horsepower measures how much of the power produced by the engine is sent to the wheels which makes the car accelerate. So with that said it doesn’t effect the actual HP.

 

I see both sides and it is semantics.

Charlie develops and owns one of the more successful aftermarket companies I have seen in quite a while. He tests and develops the best turbos on the market, and yes I own one.

Sorry boys but at this point your opinions/arguments aren't holding much weight against his.

To each his own.

 

This is semantics but we can start a new thread discussing hp vs rwhp vs bhp vs awhp vs crank hp and help anyone that is confused.

But HP is a general term that does encompasses all of the above. If you want to get into more specifics then I much prefer to use crank hp vs rwhp if someone is confused. But when you dyno a vehicle it is common to use the word Hp... when technically it can be measuring rwhp. In fact when you read the results on a dyno it says (HP), but who is going to hop on a thread and argue about rwhp

If you want to go a step further... technically the 200hp difference/loss in available HP at a certain speed comparing the 2 results could be determined as crank hp and rwhp because it has nothing to do with the drive train and just where you are at in the rpms which will affect both numbers.

 

Yup. The graph clearly shows that larger tires affect gear ratio and not Hp. It's why big trucks have 18 gears to keep the engine in it's max power range. You want more horsepower at a given speed regardless of the tire size? Just shift down a gear.

 

The chart shows at 70mph in OD you will have 200 less hp available with 37s vs 31s with nothing else changed.

That may seem obvious to some, but not to others. And trying to argue against that fact is not helping those who don't understand. Trying to confuse someone by playing semantics with hp vs bhp vs whp vs whatever also does not help.

The point of this thread is to help those who don't understand, not to patronize those who feel they have a grasp on the subject. I deal with 1000s of people who don't understand this subject. That conversation goes something like this:

"I have XXXX setup and I am having issues. I am constantly downshifting and struggling to hold OD going up hills, The truck is running hotter, smokier, and is constantly downshifting and then the rpms scream.. I never used to have this issue, can I send my turbo/inj/whatever in for repair to fix the problem." After some dialog we find out they have recently upgraded to 37,38, or even 40" tires. When explaining the actual problem they don't always understand. They typically don't want to regear and don't understand why that would help. They typically want a bigger turbo, better tune, etc... but those won't help the problem.

It is a "gearing problem" but understanding how that gearing problem affect available HP at a certain speed is the part that is missing. Seeing a graph and understanding your power curve and that you make 200hp less at 1600rpms with 37s vs 200 more hp at 2000rpms 31s really helps those people. That will often convince them to re-gear or drop down a tire size next time.

Alternatively I have seem people drop down a turbo size, injector size, etc... that can also help with the issue.

 

Remember dynos measure torque and calculate horsepower. Horsepower is torque over time.

 

The two forums (three threads) you are getting the same pushback about the presentation. On the org it looks like the threads were combined, and now locked.

Quote - "When explaining the actual problem they (caller) don't always understand. They typically don't want to re-gear and don't understand why that would help."

Sometimes an issue is quite obvious, to some. We all can state things in a manner that others feel is off the mark, it happens. Usually, one steps back and re-evaluates. You keep trying to push the same narrative, which may be why you keep getting thousands of questions on the subject.


Quote - "When explaining the actual problem they (caller) don't always understand. They typically don't want to re-gear and don't understand why that would help."

Yeah.

 

Large tires do not decrease HP. HP is the same, the larger tires are just ABSORBING some more of it than smaller tires.

By the OP's theory, if I took a 200hp Honda Civic and put go-kart tires on the front, is it now 600 hp? Of course NOT! It's still a 200 hp engine.

 

This is a good discussion topic for the importance of proper tuning to match the engine components (injectors and fuel delivery, turbo, exhaust, and even tires).

 

You can see this phenomenon when a dyno operator doesn’t use the correct gear. Runs must be made in direct drive 1:1. You will get inflated readings from the mechanical advantage of gearing if done in a lower gear.

 

so it’s a marketing post fair enough