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