Tire size vs actual speed & RPM

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KRKSR1

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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
 
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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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.
 
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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the why not told.(to ask) so guess only curious .?
28:" stock and 35" tires 88 to 110" circum (difference, ) This is grade school math here. not dynamic data.
well the circumferences (Pi x Dia) is 22" off. the car moves 22" inches farther per turn of that actual axle (any axle)
the speedo in dash is matched (ask how) to the actual axle ratio (pinion /ring) in car. (mine has VSS plastic gear sets for axle choices)

is 20 % error in distance and RPM (called ticket get'ter tires) 50 mph is 60mph actual on 35s

the speed does not matter, only the error TIMES speed, in any gear. matters the error is constant (simple math)
the error 20% works for RPM too. (lower)
the prop shaft speed is only measured, at VSS. tail of transfer case.

50 is 60mph, a huge error. SPEEDO error
the RPM on the prop shaft will 20% lower. no matter what axles is there.
the SPEEDO counts the rear prop shaft turns per minute. (RPM prop, and computes MPH) (using head gears or MCU brain in the cluster)

KRKSR1 said:
speed difference vs RPM
Click to expand...
the real speed , GPS or road side mile marker, and stop watch calculated will be 20 mph too fast. at 50 on dash indicated..
or RADAR/LIDAR measured, (cops)
called ticket GITTER,

answer is crude and assumes all else is stock
and we know the 2 tires ,exact brand and model tire, at Say BF Good rich will have super more accurate tire data for Circum differences.
my answer is from math and from real cars both. ( there are no two ,35" tires the same (brands)) the make that clear, 35" ,means ABOUT That.
you do know tires get taller at high speeds right? but sure nobody goes fast in any YJ so is moot. and running low pressures, tire, has smaller circum.

the why asked not told

are you going to correct the error, ,ask. how,
or this ,speedo 88 is gear driven cable cluster gauge.

www.quadratec.com

Jeep Speedometer Gear Tooth Chart

If the ring and pinion and/or tire size has changed on your Jeep vehicle, then you will want to replace the factory speedometer gear with the corresponding gear on the chart below. The chart lists the gear ratio and tire diameter to easily guide your purchase into the correct size application.
www.quadratec.com www.quadratec.com

good luck , all this is gear head answers.

no note book here has this(my work)

The Math for speed at RPM given.
MPH = (((( RPM ÷ total ratio) X circum") ÷ 12) x60) ÷ 5280 )
12 converts to inches to feet.
60 converts minutes to hours.
5280 converts feet to miles.

the gear is here.
xfr.JPG

speedo cable fits left and the gear plastic sold in sizes.(ratios)
 
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