For all you math majors out there.....
 

I am wondering if one of you self-proclaimed math wiz's can help me with an equation?

I currently have 22.5LP tires on the back of my rig. At 63mph I am at 1,500rpm

Now instead of changing the gears, and seeing I'm due for new tires, I was debating on going to a 11R24.5 rim/tire..... any idea what the RPM would be (or speed if it's easier) at 1,500rpm????

The overall diameter on the tires:

22.5LP= 40.9
11R24.5= 44.3

Not a math major, not a self proclaimed anything, though sometimes I can be a dick. Is this a trick question!?

Given the diameters you provided, the ratio between them will be the same as the ratio between the circumferences, and that's 40.9/44.3 which is .923. That's like magic, isn't it!? LOL

63/.923 = 68.25 mph Seems like it should be harder but it's not. If your engine runs 1500 rpm and your gearing doesn't change, the tires with the larger circumference will travel 5 1/4 miles farther in one hour. Now, that's just math. Don't ask me about slippage or some other thing specific to trucks I might not be thinking about.

I use this sometimes to help with various things http://www.roadranger.com/ecm/groups.../ct_062746.swf

What gear ratio do you have? I may have a chart that would satisfy all your needs.

The gear ratio is a constant so it wouldn't make a difference in the above scenario. The variable is the distance you travel when the axle makes one rotation.

It's like pi in C = pi D, it's a constant, so you can take the ratio of the diameters and the ratio of the circumferences and they're the same.

I will say, this is a fun little brain-teaser, but it's really just first year algebra.

Yeah I know but it's still fun to visualize everything.

All right ... let's see... The diameter of the tires isn't as useful as the REVOLUTIONS PER MILE of each particular tire. Usually 22.5 LP tires do about 510, while11R 24.5 spin at 475 revs/mile. So, assuming you have a 3.73 axle ratio and 0.73 top gear

510 x 3.73 x 0.73 = 1388 RPM at 60 MPH

1388 / 60 * 63 = 1458 RPM at 63 MPH

If you switch to tall rubber (appr. 475 revs/mile) the formula above will now look like this:

475 x 3.73 x 0.73 = 1293 RPM at 60 MPH

or 1293 / 60 * 63 = 1358 RPM

You can multiply the result of 1293/60 by any speed number and this will give you your RPM at that speed. 1293 / 60 = 21.55

So, if you want to know your speed at 65 MPH, you multiply 21.55 by 65 and you get 1,400 RPM. Or: 21.55 x 70 gives you 1508 RPM at 70 MPH.

All this looks pretty good on paper but with taller rubber you will get:

- less torque at the wheels (you change your axle ratio!); and
- worse fuel mileage due to higher resistance and heavier weight of the tires.

When I was faced with a similar choice I went for axle ratio change (from 3.73 to 3.42) and I"m pretty happy cruising at 1,325 RPM at 62 MPH (I got a Cat).

Tracer, the loss in torque shoul be the same whether you reduce the ratio by tire size or changing the ring and pinion. Remember the rolling resistance is affected by tire revs per mile. Meaning the less they turn, the lower the resistance per mile. You are right about the tire weight affecting how much power it takes to acelerate and decelerate the wheels. The same could be said for the wheels wanting to keep rotating at the same speed when you encounter a hill though.

Well, I like that with smaller tires (295 LP) my truck sits lower above the road. I think it's better for fuel consumption, plus even with my Conestoga tarping thing, the usable height inside on the main deck is 120". I wouldn't be able to achieve that with taller tires. So, it depends upon personal circumstances and preferences.

Well, I run a dedicated lane. My trailer weighs 16,000 loaded and 10,900 empty. I do half n half. I have 3.90 rears and a little 430HP S-60 in this rig.

Torque and weight me nothing to me, I have the BIG truck for the heavy stuff if I ever get back into that.

I can't justify spending $4,000 for a gear change and $3,500 on tires when i can just spend $3,500 on tires and get the same results. My truck already has a 3.5" drop axle and the DS404's don't help much either. It is really a regional/local truck but I'm running OTR with it........

Well, i'm not that, but i'd guess that you could run about 5 miles faster with the same RPM, or drop about a 100 RPM at the same speed....

Ok so, the diameter gives you the circumference and that gives you revolutions per mile. But, you made asumptions about the gear ratios that may or may not be correct and are unnecessary anyway given the situation he describes. And, your speed at 65 mph is...65 mph! ;-)

This is what he said: Now instead of changing the gears, and seeing I'm due for new tires, I was debating on going to a 11R24.5 rim/tire..... any idea what the RPM would be (or speed if it's easier) at 1,500rpm????

We don't even need to guess what his gear ratios are. If he says the number of wheel rotations in an hour is constant (1500 rpms with the transmission and differential that he has) and the low pros will take him 63 miles in one hour, just knowing the ratio of the diameters of the two different tires will tell us the ratio of the distance the truck will travel in an hour.

I mean, the stuff you said was fine, and the advice you gave was fine, but he said "this truck" and "this rpm" and that just makes it all easier.

Thanks for the info guys........ I think I'm going to go for it, and I will post the changes in MPG. I don't plan on driving any faster than now, and just figured that if I could drop the RPM's down about 200 then I could do a little better.

I just want to make one obvious point, and maybe it's already been said, Kevin Rutherford says...

...I'm sorry if there's anyone sick of people repeating Kevin Rutherford, it's just I have no other knowledge of the situation...

...anyway, he says the higher MPG always comes from the reduced rolling resistance of the smaller tire even when considering the lower RPM...

I believe he only recommends lowering the RPMs by changing your differential.

Anyway, I just felt the need to repeat the obvious at this point. Tell us what you find, Bon Jovi fan. LOL ;-)

Will do...... and I don't believe most of what Rutherford says.

I called his show 3yrs ago and told him I had a falt-top Pete and was thinking about buying a Turbo Wing for the roof since i pulled van trailers. He told me that it would be a waste of money as it would take too long and make such little difference to make it worth the $1200 i was gonna pay.

Well i bought it anyways and it netted me another .6mpg on my truck. Paid for itself in the first 4 months and since then, I don't believe everything he says.
Sure, by going slower you save fuel, and by having lower resistance tires you could save maybe .2mpg, but at the sametime, driving into a 20mph wind will lose me .3mpg, so really I think the less wear on my engine at 60mph is better and saving $5,000 by NOT changing the gears and just swapping the tires will be more helpfull....... In this case, buy going to a 3.55 gearset and then replacing my tires, will put me more in the hole than I will make up in MPG i believe.

See, my truck with the loaded trailer and 300gals of fuel weighs in at just under 32,000lbs........

Will you reset your speedo for the new tire size or just drive a little slower?

I will reset the speedo..... its quite easy to do on this truck.

My truck originally came with 11R22.5 and when I sold my KW I just took the brand new 22.5LP's off it and put them on the FLD. My speedo was off so I removed it from the dash and just flipped the tabs from 390 to 395 and it's dead on the money. I'm guessing with the 11R24.5's i will have to go to 385 and work it from there.

Does that mean it leaks?


He doesn't have a marketing deal with any wing manufacturers.

BTW I have a Taylor wing for sale in great shape in mid TN. Size for a stand up Pete or F-liner or IH.... taller than stand up - forget it. Would work on flat top too.

Anyone who actually listens to Rutherford has more serious issues than fuel mileage, the guy is a shill for anyone who "sponsors" him. Look at the "TURDBLOW3000"

I enjoy listening to him just for the STUPIDITY lol