RPM's vs MPH

[Greenleaf]

Guys,

Lets talk truck.....

When I drive 55mph, my engine speed is (±)1,660rpm

when in OD w/LT245/75R16's and the 3. 54:1 (correct?) diff.



What do y'all got?



Greenleaf

 

[Grey Wolf]

Greenleaf,



In 5th at 55mph with LT235/85/R16 and the 3. 54 I'm at 1591 rpm



Edit: In direct it's at 2064 rpm!

 

[JLEONARD]

At 70 mph, I'm at about 2000 rpm in 5th gear.

That's the Getrag and 3. 54 ratio.

Jay

 

[mazdarotary]

at 70 mph I am at 2000 rpm plus or minus a bit in a 92 D250 with a NV4500,and 3. 54:1 gears. stock size tires.



Chris

92 D250 CC LE

74 Mazda REPU

83 Mazda RX-7 LE

 

[dpuckett]

What kind of OD do you have behind that transmission? I may need it for empty driving across KS and No CO. Seriously, though, 70mph is about 2000RPM, a little more in the 5 speed, little less than 2K with the auto. Maybe you meant 1610? That would be more like it.



Daniel

 

[sparks57]

Same as others. 4th, 2k at 55... 5th, 2k at 70.



±, of course.

 

[CDegner]

Originally posted by Greenleaf

When I drive 55mph, my engine speed is (±)1,160rpm

when in OD w/LT245/75R16's and the 3. 54:1 (correct?) diff.

Gotta be a typo, speedo error, or inaccurate tach. Never seen one of these trucks that could do 55 at 350 rpm over idle. Ok, yes I have ... I have done 75 mph at 800 rpm with my pyro reading <100F. I was coasting down a 3 mile long grade with outside temps at about 35F.



Assuming (I know) level ground:

55 mph 1550 rpm

70 mph 1950 rpm

105+ mph Didn't even think about looking at the tach.



Carl

 

[jbolt]

55??? geeze does anybody go that slow?



70mph 1700-1750, 33" tires, 3. 54 (thanks to DTT)

 

[Greenleaf]

You are right. I pushed in the WRONG number. Probably should go back and edit Sorry. I did not realize it until now.



For the record; it's 1660rpm's (±) a few. BTW, it's kinda tough looking at all them little bitty lines while the cab is bouncing around AND watch the road too



Sorry for the misprint. Actually... ... . I was testing you guys You know you're stuff



Scott

 

[DKasper]

My numbers are:

'93 D350 @ 70mph is right at 2200. 3. 54/stock auto, 215/85's

'93 W350 @ 70mph is 2150. 3. 54/Suncoast, 215/85's



Can't wait to get the Toyo 235/85's on this summer!

Both are getting 17mpg running on the highway between 65 - 70 mph.

 

[CAROLINA CUMMINS]

Mine runs about 1900-2000rpm at 70, and 2500 or thereabouts at 90 This is in 5th with 285/75's.



Steve

 

[MMiller]

Hmmm, guess I should get a tach! All I know is it don't hit the governor till about 95mph!



Michael

 

[JLEONARD]

All I know is it don't hit the governor till about 95mph!

Micheal,

I HAD that problem too... . easily solved with a couple of turns on the gov screw

Jay

 

[Bob Beauchaine]

I never use 5th in town at 55 I run an NV4500HD and see about the same numbers as you guys.



Bob

 

[Thierry]

I see 1900 RPM at 70 mph with 3 speed auto, Gear Vendor and 3. 08 rear gears, loaded or unloaded.



Thierry

 

[Daniel John]

If your like me and don't have a tach or you want to check the accuracy of your tach here's an easy formula.

RPM = (MPH x Gear Ratio x OverDrive Ratio x 336) / Tire Diameter



Tire Diameter = 2x (Section Width x Aspect Ratio)/ 2540 + Rim Diameter example 2x(235 x 85)/ 2540 + 16= tire diameter

 

[MRickard]

I have the same ratio and get about 1550 at 55. Remember that tire size is irrelevent if you never corrected the spedo for it. My rpm at crusing speed around 70 is about 1800 (do not know what the spedo read as to be honest I do not really look at it! But I do not know if mine is shot or they are like that but the torque converter will change the 1800 to around 2000 or 2100 to go the same speed up a steep grade, so with these non-lockup autos, it is hard to get readings. My rear tires are 34 inches tall, 3. 54 differentail (the sticker says it is 3. 5 though)







formula for calculators (have it in mine - you can enter it into the SOLVER part) so it is a lot easier, plus you can solve for any of the variables if you know all the others:



((88 · SPEED)/((TIREHT · &pi/12)·dRATIO)(tRATIO)(tcRATIO)=RPM



for longhand, it is:



<u>&nbsp;&nbsp;&nbsp; 88 · SPEED&nbsp;&nbsp;&nbsp;</u>&nbsp;&nbsp;&nbsp;dRATIO·tRATIO·tcRATIO=RPM<br>

^{&nbsp;TIREHT · &pi; / 12}







dRATIO= diff ratio

tRATIO=TransRatio

tcRATIO=TransfercaseRatio

TIREht= tire hight, in inches



REMEMBER-

auto trans w/o lockup have slippage factor

and to calculate the tirehight, the formula is:



TIREHT = HD - 2( HD - VD )



VD= Vertical Diameter

HD = Horizontal Diameter

 

[dpuckett]

55??? geeze does anybody go that slow?



jbolt- Yes, the roads here prohibit you from doing much more without leaving parts of the truck on the road, or jolting you through the roof.



MMiller-

Why werent you looking at the speedo? You had 5mph left. . Like ECJay said, a few turns on the governor screw will alleviate that. I can go 86 with my 4. 10s and having to push through a wall of water called the atmosphere 'round here.

 

[jbolt]

Daniel-



I can't put my finger on exactly what it is but I'm getting the feeling you don't like where you live?



WCJ

 

[MMiller]

I don't know bout you guys sometimes!!



95 mph on 33" mud tires is pushing it in my book!! I know a p-joke can blow by me at that speed but by that time the lesson is already learned. I don't need this truck to go any faster. Now if I had a 2wd, some street treads, I'd be pushing the speed end of the spectrum.



I have fun off the feeling of getting set back in the seat when the charger spools and the smoke starts to clear. I really like lighting the tires in 2nd and chirping them in 3rd, on my way to blowing the doors off the same vintage chivy, or p-joke. The last guy I took for a ride all the way out of town was screaming "this is insane!" I love when pulling the trailer how it leans right into the hills the boost gauge comes up and the dual exhaust starts roaring. Then look over at the co pilot as see then shaking their head, going this ain't possilble!



There is just too many deer and bears(state troopers) around here to be opening up anything that far!!



Two weeks ago I was comming home from a late night a work, running 85 in my new service truck, at midnight. An oncoming car on the other side of the interstate lit up the brake lights and drove through the median. I could see the state patrol stripes. I hit the brakes and slowed to 70. I never saw his headlights or the cherries either. I will take that as my hint to slow down.



Oh by the way the new service truck is a FFFFF, a FFfffoorr, well you know what I mean. It is a 650, and fortunatly it is not stroker powered, but its not cummins powered either. Cat power, and since I hooked up the laptop computer to it, now it will run pretty good. Muffler will be coming off soon hopefully.



Michael