tire diameter

[dcb]

Aren't you worried about the ground clearance?

[guitarterry]

Thats the first smile ive had today. I was just wondering what would happen if u tested say a 235/75/15 to a 145/82/12 ( i am giving the actual tire for the people who need this) using gears in the transmission that would make the speeds equal at the same rpms. The advantage of weight, width, and even the lowering of the car, should to me be maybe a big difference.. instead i got lessons on how u figure diameters, speedo errors, etc, etc.
PS please dont say the make a special 18 inch tire that is only 2 inches wide so that the width would be narrower for a 18. I really just wanted common tires in standard sizes. I know, but it seems if u dont put every little thing in, somebody will go down the weird road.

[dentprone]

Hey, does anyone know where I can receive free information while I hurl pompous insults at those attempting to understand my poorly written and ambiguous question? Cuz I'm in a really bad mood...........

[Blue Bomber Man]

Two years ago there was a congressional sponsered research about rolling resistance, the study came to be about 180 pages, it found that the best RRC was found in tires that were 15-16".

My suggestion is to find the best RRC tire, than worry about your gearing ratio, so you can differentiate between the two different variables.

Look for my reply on the "success stories" board, regarding tire effieciency and tire inflation, it also covers this topic.

[Frank Lee]

[guitarterry]

Originally Posted by BlackDeuceCoupe

As the tire size goes up, you can actually end up with a smaller diameter tire, e.g. higher RPM at the same speed - not less!

hey as the tire size goes up, the diameter goes down, Yes it says that. heavyier cars get better gas milage, as long as they are going downhill, Hey a very unaerodynamic car can get better gas milage than others, if u have a very strong tailwind. i know blogs are impersonal and its easy to seem rude and pompas. when somebody asks u what size tires u run on ur car, do u say 13,s or 22.75. Cant somebody use some common sense. I realize that my first post did state that rpms and speed are equal, that would maybe implie that diameter is equal, the third post stated that we are talking standard sizes and changing gears to make rpms equal as the diameter changes. I didnt really need to know how to figure tire diameter. Hey dave does point out the a 12 inch rim is 12 inches. I was really glad of that.

[cfg83]

guitarterry -

I don't know where it is, but I think there's a neat 4WD gearing calculator on the net that takes tires size into account (I think Big Dave posted it in a different thread?!?!?!). I don't think it computes MPG, but it relates the parameters you are interested in, tire size and gearing.

CarloSW2

[Red]

I think I'll join the bad mood band wagon just cause

Anyhoo, you are saying you are slapping on tire and you are changing the gearing so that it spins at 3k @ 55. Kewl. You do this for each tire. Congrats, you have just equalized the tires in regards to size to the power band. Everyone running a lifted truck does it all the time. You have negated the tire size.

Now width. If all of the tires have the same width, they'll get the same MPG because as far as the engine is concerned its spinning the same tire. The wider you go, the less MPG you get. In a perfect world, we'd all drive on mag rails

Now rubber composition. If each tire is made out of the same stuff, same MPG. If one is a LRR tire and the other isn't, the LRR tire wins in the MPG

Kewl

Now rims. Assuming the same alloy of materials and density, a smaller rim wins. Reason for, a bigger rim needs more mass to be bigger. Hence forth its heavier, and harder to spin, makes the engine work harder, drops your FE. As an experiment, pick up a steel 13" rim, then pick up a steel 16" rim. Noticeable difference.

Now Tire diameter, no idea why you'd want to run a 5" rim, you'd have brake problems and stuff. Bad mojo, but your call. The only benefit you'd have is that the tire would be super narrow compared to say a 14" rim and tire, and much less weight so its easier to turn. Depending on the aspect ratio of the tire, you'd still get sidewall flex. But if you are running at max load, you'll negate a fair amount of it.

Forgive me, I was bored.....

[guitarterry]

Originally Posted by Red

I think I'll join the bad mood band wagon just cause

Anyhoo, you are saying you are slapping on tire and you are changing the gearing so that it spins at 3k @ 55. Kewl. You do this for each tire. Congrats, you have just equalized the tires in regards to size to the power band. Everyone running a lifted truck does it all the time. You have negated the tire size.

Now width. If all of the tires have the same width, they'll get the same MPG because as far as the engine is concerned its spinning the same tire. The wider you go, the less MPG you get. In a perfect world, we'd all drive on mag rails

Now rubber composition. If each tire is made out of the same stuff, same MPG. If one is a LRR tire and the other isn't, the LRR tire wins in the MPG

Kewl

Now rims. Assuming the same alloy of materials and density, a smaller rim wins. Reason for, a bigger rim needs more mass to be bigger. Hence forth its heavier, and harder to spin, makes the engine work harder, drops your FE. As an experiment, pick up a steel 13" rim, then pick up a steel 16" rim. Noticeable difference.

Now Tire diameter, no idea why you'd want to run a 5" rim, you'd have brake problems and stuff. Bad mojo, but your call. The only benefit you'd have is that the tire would be super narrow compared to say a 14" rim and tire, and much less weight so its easier to turn. Depending on the aspect ratio of the tire, you'd still get sidewall flex. But if you are running at max load, you'll negate a fair amount of it.

Forgive me, I was bored.....

see dcb, common sense is not in play, we dont have ground clearence problems, we have braking problems on my 5 inch diameter tires.

NOW red u seem to be getting the question. yes bigger 4 wheel drive trucks pick their tires first. then use the gearing they need to get the rpm of the engine where they want. again what i was wanting to know from metrompg was why he used 155/80/13 instead of 145/82/12 tires and even taller gears.

[guitarterry]

Originally Posted by 1.5Ldave

The rim size wont make a significant difference at all. Its the overall diameter of the tire that needs to be considered. A larger overall diameter will give you lower rpm by changing the final ratio. This will also effect acceleration.


For those who dont know how to decipher the tire coding a 155/80/13 tire would be 155 millimeters wide, 80 is the aspect ratio of the sidewall, and 13 is the size of the rim.

To calculate total diameter you multiply the width times the aspect ratio
155 x 0.80 = 124 mm.
Divide by 25.4 (#of mm per inch)
124 / 25.4 = 4.88 inches of sidewall.
Multiply by 2 (sidewall above and below rim)
4.88 x 2 = 9.76 inches
Add the size of the rim (13) for a diameter of 22.76 inches for the 155/80/13

Now well run the 145/82/12
145 x 0.82 = 118.9 mm
118.9 / 25.4 = 4.68 in
4.68 x 2 = 9.36 in
12 + 9.36 = 21.36 inch diameter

This explains the better mileage since the 155/80/13, even though almost a 1/2 inch wider, is also almost 1.5 inches taller. Lower rpm at cruising speed = less gas used. A taller tire is the easiest way to change the final drive of your car, though not the most effective.

ok dave you are close.
we agree lower rpm at cruising speed = less gas used
My original question states the rpms are equal. (i know it does I wrote it)
so we take the 12 in 21.36 diameter tire and gear the tranny to be going 55 mph at 2000 rpms in the engine, then the 13 in 22.76 diameter tire going 55 mph at 2000 rpms in the engine. wow that is what i wrote in my original question. we then say what effect will the lowering of height and mass have using the 12,s and why arent we using the 12's if the apparent FE advantages are so apparent.