larger tires vs. rpm drop

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So i've got an HDJ81 5spd and quite frequently notice my revs are a little high at highway speeds and really kill my mileage for long distances..
i've thought about re-gearing but thats a little $$.
I'm currently running 31" tires and am wondering if anyone has the answer to running a set of 35" tires and not killing the mileage by over working the engine by pushing these tires, but dropping the rpm..
if so, how much would it drop the RPM down by?
at 110kms, my revs are at 3000...if i could do the same speed and cruise at 2500rpm, i would be a happy guy.
Any input helps.
Cheers,
 

Ron R

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If you want to drop down in RPM from 3000 to 2500 that's a difference of about 15%.
So if you find yourself tires that have a circumference that is 15% taller than you have now, your problem sould be solved....

Just switching from 31 to whatever (33, 35?) is not enough as other factors play a key role as well.
Just ask the tireshop to compare what you have now against alternatives.
There are tables out there on the internet, but I cannot provide a link at this moment.
 
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here is a calculator I found with just a quick google.....not saying its good or anything....

here is a chart I found. Not sure it applies though.

Tim
 
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Tapage

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Auto or manual tranny .. ?

Mine .. 315 with my OME level stance kit and A442F tranny .. flat surface ..

main.php


Notice after my post that your 81 it's manual .. so less OD ratio.
 

sandcruiser

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if your goal is better mileage, stay away from wide tires. Look at a 33x10.50 or a 255/85r16 tire. Either of which should drop your rpms by somewhere between 10 and 15%
 
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getting my rmps lower on the highway and better fuel consumption is also something I would like to accomplish. The PO put 30 9.5 15's on right before I got it. My 3B with 4 speed really doesn't like the highway. I would love to drop a 5 speed in some time, but until then.....I have been thinking about 33 9.5 15's. Will I see much difference in anything or will the old girl have to work that much harder pushing the larger tires around?

I have stock suspension, not drooping all that bad really, will 33's look really out of place. I do zero off roading, forest service roads don't count as off roading;p

Thanks,
Tim
 

RufusTheDufus

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I put 33 9.5 15s on my BJ42. (I removed 31 10.5 15s.) I thought it looked OK even before I put the OME 2.5" lift on it. The back tires didn't rub on flat pavement. They looked like a tight fit and might have rubbed I did any off road riding.

The thing I noticed immediately was that the vehicle was much easier to steer. I know 10.5 to a 9.5 doesn't sound like much but it certainly felt that way in the steering wheel.

My engine is very tired but it has a turbo. It moves the larger tires along just like the 31s.
 
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i do have a 2.5" lift on the truck, so accomodating to larger tires shouldnt be an issue; my big issue and concern is, am i going to drop fuel economy by having lower revs, but gain it back in pushing larger tires. One idea that someone brought up is to put larger but narrower tires on, and that might help off-set the large tire issue...
 

brownbear

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You want the tires 10.5 wide.

255/85/r16 is a good recommendation.

The 80 series section can give you tire info for your 80. Not the diesel section.

3k rpm is pretty high rpm in my opinion for 110. My H55f 5 speed standard on my 60 puts me at 2200 or so at 100-105. I think 2500 is max you want on the highway. Lower rpm will save some fuel.

Slowing down will also help. I don't think pushing 120 is good for fuel savings. Try to stay at 110 max, 100 when you can. I know the highways like #2 are 110, with most doing 130. But this is a cruiser. They were not designed to go fast.
 
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I'd go for it. With the 1hdt you may even see your economy get better. Cookie cutters will help balance out the extra rolling and air resistance of more rubber. Your RPM does seem a bit high for 110 kph. In mine, I am about 2600 at 110 kph.
 

Ron R

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i do have a 2.5" lift on the truck, so accomodating to larger tires shouldnt be an issue; my big issue and concern is, am i going to drop fuel economy by having lower revs, but gain it back in pushing larger tires. One idea that someone brought up is to put larger but narrower tires on, and that might help off-set the large tire issue...

There is no such thing as a need for the engine to work harder with bigger tires to maintain a certain speed.
To drive with a certain speed (everything else being equal) the only thing needed is a certain amount of POWER.
So as long as the engine is operating within the needed powerband, there will be no need for extra power thus no extra fuel. Because the engine will be revving at lower RMP other losses that are engine and powertrain related will be lower, therefore giving better economy.
What you will loose is a certain amount of torque. Acceleration will be slower, when towing and/or driving in mountainious areas you will find you must downshift sooner than you were used to, not because of loss in power but in torque. Because the torque is lower you will get out of the power-band sooner (because the engine rev's are lower) and that may feel as if the engine is working harder.

Instead of choosing larger tires ( don't go to high, from 31 going to 33 may be sufficient) you can also re-gear.......but that will be a bit more expensive and if you don't like it after all....putting on smaller tires is easier than re-gearing again...
 

sandcruiser

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larger tires weigh more, it requires more fuel to get them going, and you convert more energy into heat when you slow them down. So all in all, larger tires are less fuel efficient than smaller tires when you take going and stopping into account. Add to that the fact that larger tires also raise the vehicle which increases wind resistance, lowering mileage.

Having said that-- if you are in top gear (5th) and rolling along on the highway at a "high" speed of 120kph with your engine operating well above peak torque, you should gain some economy by adding taller tires and thus getting the revs down to a more ideal level. If you spend a lot more time at 120kph than stop & go, taller tires might save you some fuel. At the very least, they might quiet your engine on the highway.

FWIW: 31x10.50, 4.11 gears, in 5th w/ 1hd-t I spin above 2,600 at speeds above 110kph. Not sure of the exact numbers because on our lousy roads I don't want to be staring at the tach when I'm going 110 :) But looks like near 2,700rpm
 

Ron R

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larger tires weigh more, it requires more fuel to get them going, and you convert more energy into heat when you slow them down. So all in all, larger tires are less fuel efficient than smaller tires when you take going and stopping into account. Add to that the fact that larger tires also raise the vehicle which increases wind resistance, lowering mileage.
hmm, I don't agree.
Why more fuel needed to get them going? The weight difference is that small you will hardly be able to measure the difference.
Same with braking. You won't notice the difference. Your way of driving will have much more influence than this.
What I didn't mention and nor did you is that larger tyres roll with less resistance just because they are larger. Compare it to a small tyre like the one you will find on a small childrens bike. Try to imagine the difference if you want to drive up to a sidewalk. The larger tyre will get up much more easy than the bicycle.
That's why modern cars tend to have larger tyres, not just because they look nice.

When the vehicle is raised, because of the larger tyres, this won't increase the frontal area as whole which stipulates resistance greatly.
The opposite can happen. Because the body is raised the airflow can 'escape' easier underneath the vehicle thus lowering the over all restistance.
 

Tapage

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When the vehicle is raised, because of the larger tyres, this won't increase the frontal area as whole which stipulates resistance greatly.
The opposite can happen. Because the body is raised the airflow can 'escape' easier underneath the vehicle thus lowering the over all restistance.

Little concern about this issue ..

Coz when you have the air over and under the truck it can make more " bag " ( I'm not sure if it's the correct term in english ) behind the truck due tu the wind over and under it ..

Even you are more close to the road, you only have wind over the truck and less " bag " efect behind ..
 

sandcruiser

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Take a look at cars specifically made for good fuel economy. Find one with wheels larger than 13" or 14" (production vehicles only, not solar "cars" that run on bicycle tires). Go. Find it. Post a picture. Till then, I'm sticking to my belief that smaller tires go hand-in-hand with better fuel economy.

Tapage covered the lift effect. I don't know the physics of it, but they must be out there somewhere and I'm sure it has something to do with laminar flow vs. turbulent flow. If nothing else- the larger tire is more exposed to the oncoming airflow than the smaller tire because more of the 4 tires extends beyond the fender opening. It may not account for much of a mileage hit, but it will increase wind resistance.

As for weight: an inflated 33x10.50 is a bit heavier than an inflated 31x10.50. An inflated 33x12.50 is LOT heavier than a 235/75r15. Either way, that extra weight may not seem like much, but when you are talking about accelerating the weight fairly rapidly (which is needed to get going or to stop) .... a little extra weight adds a lot of energy need. That's one of the reasons that sports cars, almost without exception, have alloy rims. They are lighter. The car can accelerate faster or slower much more easily with the same motor. Its all about rotational inertia.

but I could be wrong about everything I've just said. Fair enough. Regardless of the reasoning behind it--- if you fit larger tires to a cruiser without changing gearing, you **will** lose mileage around town and you **might** gain mileage on the highway.
 

sandcruiser

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some more "hard" info taken from TireRack.com ( Tire Tech Information - Rolling Resistance )

During stop-and-go city driving, it's estimated that overcoming inertia is responsible for about 35% of the vehicle's resistance. Driveline friction is about 45%; air drag is about 5% and tire rolling resistance is about 15%.

Overcoming inertia no longer plays an appreciable role in the vehicle's resistance during steady speed highway driving. For those conditions it is estimated that driveline friction is about 15%; air drag is about 60% and tire rolling resistance represent about 25%.
 

sandcruiser

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but supporting Ron R's position (I think) is this explanation

The rotary inertia of huge tires can reduce a vehicle's acceleration,
but that's only a factor when the wheels and tires are a substantial
fraction of the vehicle's mass.
Other than monster trucks or beginner-designed RC or robot vehicles, it
usually does not matter much.

from Car Wheel Sizes
 

Ron R

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In certain aspects you are right Sandcruiser, but don't take out of the equasion that we're are not dealing with high performance sportscars here, we're talking cruisers. And they have the same high quality of streamline like a brick ;)
If you have a sportscar, highly streamlined, optimized for low drag, you are right.
Why do 'average' cars, and I mean cars, do have small wheels? Simply because it's difficult to accomodate large ones, let alone cars build for optimum streamline which have the smallest frontal area possible as to reduce the drag.

Why do solar cars have large (diameter) wheels? because of the low rolling resistance. They run their tyres at high pressure, reducing internal deformation and thus drag, offering comfort, and very narrow tyres as to reduce the frontal area, but this is on very sleak very streamlined rigs.

But how much will the frontal area increase when you have 33's instead of 31's?
This adds 2 inches in diameter to the frontal area of which only one inch is open to the airflow (the other one is in the wheelwell) How wide is the tyre? 10 inches? So that's 10 square inches per wheel, is fourty square inches total. How much is this related to the total frontal area?

And where you are saying "During stop-and-go city driving, it's estimated that overcoming inertia is responsible for about 35% of the vehicle's resistance. Driveline friction is about 45%; air drag is about 5% and tire rolling resistance is about 15% ", then this inertia is not related to the wheels and tyres but to the vehicle as a whole. The few kilograms (twenty - fourty?) in total because of the larger tyres account for only 1 (one) or 2 (two) percent of the total vehicles weight and therefore will add about 0,5 % (35% of max 2 percent) to the equasion.

You certainly have a point where you are comparing narrow tyres to wide tyres. But that was not the question here. The OP wanted to know or regearing or larger wheels (not wider) would have a positive influence on highway driving in regard to fuel economy.
To my opinion, yes larger wheels (tyres) can have a positive effect on fuel economy, as long as other aspects are taken into account. (Loss of torque at the wheels - stay in the max engine torque RPM-range during cruise).

And don't forget, the best fuelsaver is the driver....drive at a steady speed, don't try to burn rubber when accelerating or decelerating. Driving 80 to 90 max speed instead of 110 will save about 15 % fuel, maybe even more.

In the States it may be quite different but over here in Europe, there will not be a lot of difference in **average** speed when driving at max speeds of 80-90 km/h or 110-120. I know of a guy (businessman) driving a BMW Sportscar. He says he tries to obey the speed limits but otherwise drives his car freely. He found that his average speed during two years of use was.....slightly over 60 km/h
 

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