Attention: Gearhead Mathematicians, I need a little help...
 

I'm trying to bring the down the highway cruising speed rpm's on my Isuzu NPR. I'm trying to do this by increasing the tire size, but I'm not sure if it will make enough of a difference. It's currently running 17.5" wheels but later models had 19.5" wheels. If I get a set of 19.5" wheels I can run tires with a diameter of 33.6", stock is 30.2". This will decrease revolutions per mile by ~9.7%.

My question is, will a 9.7% reduction in revolutions per mile at the wheel equal a 9.7% reduction in engine rpm's or are there other factors I need to account for?

Thanks!
https://www.superhawkforum.com/forum...1&d=1264556875

Yes the reduction will be the same % at the engine, and the only other factor is your new speedometer error.

(edit: some reduced mechanical advantage.. your low gear will be higher so starting from a stop with a big load might be harder, but I doubt you were pushing that limit before and probably won't now either)

that's a very big change...

You might consider slowing down to the cruising speed the truck was designed for.

It seems the trucks cruising speed is 55mph.... too slow for the 800 mile round trip to California and back and definetly too slow for the 2400 miles trip to Dallas and back.

(edited your post by mistake, sorry)

Your moderator is showing.. tuck that back in. :D

If it has ABS you need to think about that, since it's operation would be affected, as well as transmission shift points if it's an automatic.

I am a little confused. I didn't think that they made 17.5 or 19.5" rims. Are you talking width. And, the issue that you are going to run into here is rolling resistance and increased rotating mass, with a decrease in the handling characteristics (if that vehicle even had any to begin width). I would really resist the urge to chase something that doesn't exist, an over-the-road mudding vehicle. I could see where (if it had a direct lockup and 1:1 diff) where the decrease in revolutions of the tire would have a direct correlation to the engine rpm. Maybe I am wrong.

Ooh, I forgot about ABS.

As for being Automatic.. I haven't seen that issue when upsizing wheels on other trucks. But I don't know if I ever pushed 10%, and they were all dumb autos not electronically controlled ones so I don't know if newer trucks will have issues or not.

It doesn't have ABS... just a simple air over hydralic system. It does have a auto trans, but it's a heavy duty Allison AT542, plus, we won't be hauling that much weight... just 6-10 motorcycles (4k-5k lbs). It also has a 6 cyl 6.5L, intercooled turbo diesel, so I'd imagine that it will pull just fine.

So what part of Ca are you going to?

Here's a handy little tire size calculator I have used in the past.

You it gives you circumference and rotations/mile for different tire sizes.

Think Semi or Motorhome etc.

Moreno Valley, L.A. and San Diego.

http://www.oceantrucksales.com/2003%20ISUZU%20NPR.JPG

One of these? Okay, I will talk to you again. I myself am a bus owner, and know how it is to be moving a little slow. My bus will do 68mph comfortably, while getting 11mpg. I would suggest doing the bigger tires or a differential change. Are you moving yourself, or other people? (cough, drugs, cough) j/k That should have the power to move it, you will just need some running room.

He is moving motorcycles.. Either from or to his shop. :D (his place of business)

don't forget about the extra weight you will be adding.

Yeah, not a problem. The extra weight is well below the weight capacity of the vehicle. The buss hauls 4 street bikes, dudes, gear, couches, tv, fridge, etc. and has less power. No problem.

unles its been said already, one thing to consider is that while running a larger tire the rpms will go down but the added rotational mass while eat up more fuel. so you need to figure out if the added weight and wear on drivetrain parts is gonna leave you with better overall fuel economy. common problem with guys who upgrade to larger offroad tires. example on my explorer if i throw on a set of 32 inch mud tires, my rpms will drop a good bit but ill also loose about 3 mpg. so it really isnt efficient for me to do that for everyday driving

Once you're up to cruising speed, I think the flywheel effect of the extra weight would offset any loss under load during acceleration or on hills.

Do you have a good read on your gas mileage with the stock wheels? Let us know how it comes out, if you make the change.

I see your point and it makes sense for light duty vehicles, pickups, etc... but I highly doubt the larger diameter tires will be noticeable on this truck. It sure as heck didn't care about the 5 bikes in back vs. being unloaded. You'll definitely lose mpgs on the Explorer if you threw larger "mud" tires on it for 3 reasons, 1. it's a gas motor, 2. the mud tire will weigh a lot more as they are much wider, 3. larger contact patch = more rolling resistance. The tires I'm looking at are 3.4" taller, but are actually .10" narrower.

the other thing about truck tires is that they are not soft sidewalls and contact patch. those mudd tires have a ton of flex and rolling resistance. Heavy truck tires do not. They are hwy ribs too, so that should pretty much seal the deal. Put them on and haul.

i get what your saying, i used the mud tires as an example. whats the difference in weight between the 2? best way to find out is to do a trip with the larger tires and then without. also i dont know if you can get this for your truck but its a pickup truck and van toy, its a doubler that goes behind the tranny and basically the best way i can descirbe it is it doubles the gears in your tranny. so say you have a 3 speed. youd go through first, and then before hitting second it would engage a first gear over drive so to speak, and then second, and so on , im sure its not the best option but something like that would significantly drop the cruising rpms down and put less strain on part while towing heavy loads. plus you could also lookinto regearing the rear end. see what a set of used gears is worth with labour vs what the new rims and tires are

last thing, i dont think its been answered, if you havent figure out the engine rpm thing yet find an online gearing calculator and plug in the tire size with gear ratios and it should tell you the engines rpm. something like one of these might be helpful

http://wildcatent.freeyellow.com/zookmods/calc.htm