Tire size vs actual speed & RPM


KRKSR1

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Mar 21, 2021
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Bloomingdale Ga
1988 Jeep YJ... what was the speed difference vs RPM ( actual not speed odometer) after increasing from stocks to 35's ? I'm not concerned with power nor difference in speed odometer.
Example: stock tires 65mph/2200 rpm
35's 65 mph/ ??? RPM
I know there are calculations, I'm just trying to see actual testimony
Thanks in advance
 

smgager

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Dec 16, 2020
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Wilmington, NC
can't go wrong with this.


I have 4.56 putting my jeep at factory RPMs with 31s. I can run 70MPH with out a problem but I wish I would have gone with 4.88 to make up for the added weight of the larger than factory tires. 35s will likely eat up the factory Dana 30 and 35.
 
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jeepjoe43

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Hope this helps.
If you're interested the math is as follows, this will initially be calculating for 60MPH, but will be further extrapolated.
Determine the circumference of the tire, Diameter x Pi (3.14), not going be exact but will get you in the ballpark.
That's how many inches it has to roll for one revolution.
There are 63,360" in a mile (5280' x 12).
Divide that by your tire circumference to get wheel revolutions per mile.
Since 60MPH is one mile per minute, it is conveniently also going be the wheel RPM.
Multiply that number by your gear ratio and that's your engine RPM (assuming 1:1 4th gear, no overdrive).
Vary the gear ratio to get into the RPM range your looking for.
Divide the engine RPM x 60 and you'll get a factor you can multiply by what ever MPH you want to determine engine RPM at that speed.
I have 2.5 on 32" tires w/4.88s, my multiplication factor is 51, a stock YJ on 27s w/4.10s is the same, perfect match (on paper).
So even the speedometer is correct, or pretty close to it.
 

machoheadgames

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May 19, 2020
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Dallas, TX
We can't exactly answer that for you without more info. Between three different gear ratios from the factory, as well as two different stock tire sizes, we need more info. Stock tires were 27's (205) or 28's (215), which do you have? 35's usually measure around 33-34" actual so RPM will be 21% lower on a Jeep that had 205s and 18% lower on a Jeep that had 215s stock. So do the math using those percentages to figure it out. Or, a better way is to figure out your stock gears, your stock tire size, enter them into Grimmjeeper's calculator and then do comparisons leaving everything the same except for the tire size.

I would highly suggest NOT just slapping 35's on if you are considering it. They will suck the life straight out of the motor on stock gears, brakes are typically awful on 35's, steering linkage is not really strong enough for 35's, and you will definitely need to address suspension and some body lift for best results. It is expensive to do it right, and can be plain dangerous if half-assed. I'd recommend getting some nice 31's and a small lift and leave it alone. Although I still regeared for 31's because I did not like stock gears, but most don't care enough to go that far for 31's.
 
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