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Tire size / Gear Ratio / RPM


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Rather than continually look up engine RPMs for various tire sizes and gear ratios, I'll let y'all do it yourselves. A friend on another (non-Jeep) forum just clued me in about Google Docs. I just uploaded my famous tire size and gear ration spreadsheet so everyone can look at it.

 

IMHO this in the definitive reference. Most on-line calculators use the mathematical rolling circumference of a tire to calculate RPMs. In compiling this spreadsheet, I consulted tire manufacturers' printed data sheets and web sites, and for most tire sizes I was able to find a published figure for actual (real world) revolutions per mile. This takes into account the amount of "squat" in the sidewalls, which results in an actual rolling radius that's less than the mathematical value.

 

https://spreadsheets.google.com/pub?hl= ... utput=html

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Cool. Said I had to request access though. Did you set that up intentionally?

 

FYI to others, you'll need a google account to access this.

No, I did NOT set it up that way. And I don't know how to change it.

 

If anyone uses Google Docs, can you coach me on what setting I need to adjust so everyone can see it? It should not require that you have a Google account to view it.

 

See if this link works: https://spreadsheets.google.com/pub?hl= ... utput=html

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Link seems to work :thumbsup:

 

It's just a "static" spreadsheet though, but I'm unfamiliar with the sheet you uploaded so I'm not sure if it's working "right" (as in, the way you wanted it to). Seems to be consistent with an HTML spreadsheet though.

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It **SHOULD** not require any special access permissions. I'm not philosophically opposed to granting permissions, I just don't care to have to deal with it on a continuing basis. The intent of posting was that it would be public. If y'all can't view the link, I'd rather figure out why and fix it.

 

Does everyone get a message saying you need permission, or is it working for some people and not working for others?

 

DANG! Google is fubar. I just tried to access the file behind the link, and I'm getting Google in either Chinese or Japanese. Don't know how to do anything if I can't read the stinkin' menus. I'll try again later.

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The second one comes up for me too. But Don, I don't know about you but I am a little disappointed. I thought Eagle stored all this stuff in his head. Didnt you? :rotf:

 

He probably does Gary. He just posted it up for poor slobs like you and me. :cheers:

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Works fine for me, and very close. (my tire is 235/75/15, chart says 720 rev/mile, the mfg (firestone) says 723 rev/mile).

Just not sure why it stopped at 70mph though. Should have at least ran it up to 85 where the speedometer quits.

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I used whatever data I could get, which for the most part was from Cooper. Even within one brand, different tires of the same nominal size will have different static radii and different revolutions per mile. That's why I went to the trouble of creating the spreadsheet -- because any calculations derived mathematically from the nominal width and profile ratio would be moderately inaccurate.

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  • 6 months later...
  • 1 month later...

Here's one that has been up and running for years, and being updated regularly. The difference in numbers compared to the spreadsheet and this Calculator was 3% difference in one particular case. It has most transmissions, most transfer cases, most tire sizes, and if it doesn't have yours, you can enter your own numbers.

 

http://www.grimmjeeper.com/gears.html

 

jamminz.gif

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  • 8 months later...
NICE! - Here's some more - fast & dirty - http://www.onlineconversion.com/automotive.htm

It was the "fast and dirty" part that I wanted to avoid, because most people use numbers like "30," or "31" or "33" for tire diameter, and they very rarely come close. For example, most 30" tires actually measure 29-1/2" in diameter off the vehicle, and the effective diameter with the vehicle's weight is closer to 29". So I went to tire company spec sheets and based the spreadsheet calculations on the most accurate number available -- the actual revolutions-per-mile provided by a tire company.

 

But ... even within one brand, different models will be slightly different, so the spreadsheet can't be dead nuts on the mark for everyone. But it should be closer than just about anything else you can find.

 

Note the disclaimer on the linked site:

 

Ever wonder how far off your speedometer is with your new bigger tires? I'm using a simple ratio calculation, so this info will only approximate your corrected speed and depends on the accuracy of tire size diameters (assumimg nothing but tire size has changed).
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  • 2 months later...

That depends. Cruising at constant speeds, you will get better mpg with larger diameter tires (i.e. if you spend a lot of time on the highway). If you're doing a lot of city or other driving when you're stopping and accelerating almost more often than you're traveling at a constant speed, then the better wheel torque you get from smaller tires will reduce your fuel economy, not needing to burn so much fuel to get up to speed. The trade off is higher rpm when you get to speed, which then equates to worse mpg.

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  • 1 year later...

Ok guys, as an engineer, I can give you a simple "plug n play" formula for gearing. This is the best formula I have found as there are various gear ratios offered in our vehicles between models.

OTD / NTD x CGR = EGR

OTD=Old Tire Diameter

NTD=New Tire Diameter

CGR=Current Gear Ratio

EGR=Effective Gear Ratio

 

Now Effective Gear Ratio is what you vehicle "acts" like it has in the axle.

 

IE (I'll use my Mj) 27/31 x 3.55=3.09 So by going to 31 inch tires, my Mj acts like if it had 3.09:1 gears in it, hence the slower acceleration and feeling that it "lacks power".

 

Now the goal in the math is to plug different gear ratios into the CGR slot untill the EGR number comes as close to stock gearing as possible.

 

IE: 27/31 x 4.10 = 3.57:1

 

So by regearing my axles to 4.10, my Mj will act and perform with 31's just like it did with the stock 27 inch tires and 3.55 gearing.

 

Now if you use metric tires you must convert the size into inches for the formula to work. If your math cones out to a decimal, round up to the nearest number IE: 32.6 = 33".

 

Now seeing as this thread was about RPMs with tire sizing and gearing, if you know what your current RPMs are at a certain speed, this formula will help you get an idea for what the RPMs will be with larger tires seeing as you are regearing for stock like performance. Yes, I know that tire sizes vary depending on manufacturers but by using a rough measurement of it's diameter you will as stated earlier, have atleast a good idea of the general RPM range.

 

Hope this helps.

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  • 3 years later...

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