Engine rpm

[At38bc]

I’ve got a 2005 3.0 auto 3.73 255-55x18 wheels/tires. Is 2100 rpm at 60 unadjusted, is that normal? 2300 at 65 which I believe is 70. Went by the solar powered speedo on the highway.
Thanks for your time,
Jp

 

[tw205]

Those work out to. 29” tire. About as close as you can get to factory. 4x4 had 235x75 15 which is 28.9. 2x4 had a little smaller but not a lot. 225x70 15 @ 27.4. There are a few different options with 16” rims but overall your is still close to factory. I would say your RPMs are normal.

 

[pjtoledo]

it's a slow night, plenty of time to waste crunching worthless numbers, soooo,
mathematically a 26.8" tire would be 2105 engine rpms @ 60 mph with 3.73 gears and .75 overdrive transmission.
in reality tires and tachometers vary a bit, sometimes a lot.

if you too have time to waste it's easy to determine the data. 60 mph is an ideal speed for calculating since 60 mph equals 1 mile per minute.
these numbers are based on a approx 29" tire, yours may be smaller/larger.
mark a rear tire and roll it exactly 1 revolution, it will be in the vicinity of 90 inches. (you can count driveshaft revolutions to verify gear ratio)
divide by 12 to find feet per tire revolution, around 7-1/2 feet.
divide that into 5280 (feet per mile) and you get about 700 tire revolutions per mile at 60 mph so tires are going 700 rpm.
multiply that by rear gear ratio, 3.73 which is driveshaft rpm of around 2600.
multiply that by the overdrive ratio of .75 to get 1950 engine rpms.

that's about 8% less than your observed readings.

and it's still a slow night.

 

[At38bc]

Thank ya’all so much. Just missed a step in the math. It when ya are as old as i am, ya sent ta forget some things, lol.
Thanks again,
have a good evening,
JP

 

[At38bc]

Well spell ck did it again!

 

[pjtoledo]

I'm 68, if I'm remembering correctly

 

[Stumblefoot]

a cheap or borrowed GPS will give you accurate speed.. even my scangauge in my 04 Ranger shows a different speed than my speedometer....but to the OP yeah those RPMs sound about right.

 

[tw205]

There are phone apps that are GPS speedometers. I use one to tune my truck speedometer when changing gears or tire sizes.

 

[At38bc]

Appreciate all the answers. Going back to stock size tires and rims. The Saleen rims and tires will be sold.

 

[rumblecloud]

it's a slow night, plenty of time to waste crunching worthless numbers, soooo,
mathematically a 26.8" tire would be 2105 engine rpms @ 60 mph with 3.73 gears and .75 overdrive transmission.
in reality tires and tachometers vary a bit, sometimes a lot.

if you too have time to waste it's easy to determine the data. 60 mph is an ideal speed for calculating since 60 mph equals 1 mile per minute.
these numbers are based on a approx 29" tire, yours may be smaller/larger.
mark a rear tire and roll it exactly 1 revolution, it will be in the vicinity of 90 inches. (you can count driveshaft revolutions to verify gear ratio)
divide by 12 to find feet per tire revolution, around 7-1/2 feet.
divide that into 5280 (feet per mile) and you get about 700 tire revolutions per mile at 60 mph so tires are going 700 rpm.
multiply that by rear gear ratio, 3.73 which is driveshaft rpm of around 2600.
multiply that by the overdrive ratio of .75 to get 1950 engine rpms.

that's about 8% less than your observed readings.

and it's still a slow night.

You are messed up...
But thanks for the info. I enjoyed it immensely.