Muddytazz
1 ton status
Only running D44/14FF combo, D44 has a truetrac and the 14FF has a Detroit. I'm not into rock crawling or anything like that, this is just my hunting camping back and forth to work rig. So it see mild to moderate trails.
CK5
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How many RPM's at 70 mph?
- Thread starter cbbr
- Start date
Screw all these conflicting calculators:
RPM divided by 1 (3rd gear in a TH400) divided by 4.56 gives you axle revolutions per minute. Multiply that by 37*pi (the diameter of the tire in inches) gives you inches per minute. Divide that by 12 (to get to feet per minute) and then by 5280 (to get to miles per minute) and multiply it by 60 (to get miles per hour) and set that equal to 70 mph.
Solving for RPM you get 2899.93. Your tires probably aren't actually 37" tall and your transmission will slip some, so I stick by my guess of 3100 RPM.
I had that exact setup (except I have a 465 speed instead of a TH400). I felt it was too low for the highway. 4.10 would work better for a street driven truck with 37's, IMO.
RPM divided by 1 (3rd gear in a TH400) divided by 4.56 gives you axle revolutions per minute. Multiply that by 37*pi (the diameter of the tire in inches) gives you inches per minute. Divide that by 12 (to get to feet per minute) and then by 5280 (to get to miles per minute) and multiply it by 60 (to get miles per hour) and set that equal to 70 mph.
Solving for RPM you get 2899.93. Your tires probably aren't actually 37" tall and your transmission will slip some, so I stick by my guess of 3100 RPM.
I had that exact setup (except I have a 465 speed instead of a TH400). I felt it was too low for the highway. 4.10 would work better for a street driven truck with 37's, IMO.
Last edited by a moderator:
blazerbones
1/2 ton status
I've got close to the same setup as cbbr (4.10's w/ 35") numerically, and I've been thinking about this situation for a while. Basically decided that I can:
1. spend big bucks on new tranny/drivelines (like 4l80e or nv4500) to get an overdrive... I'm not interested in a 700r4
2. regear the th400 to have a lower 1st (2.75:1) and 2nd (?)... go huge on tires... and have a big gap between 2nd and 3rd gear... but still be okay off the line and better on the freeway (due to bigger tire)
3. aftermarket overdrive (after the transfer case) and a rear driveshaft (expensive based on my research)
4. drive slower
Does anybody know if there's a gearbox that can be put on the back of the transfer case (np241c) that is CHEAP and would work as an overdrive (1:1.2 or so) when in 2wd? Then I could just shorten the rear driveshaft and tilt the rear axle to fix the angles... that would be kick ass. Oh, yeah... and I have the fixed output kit (no slipyoke in t-case)...
I'm probably just dreaming/hoping on this one.
1. spend big bucks on new tranny/drivelines (like 4l80e or nv4500) to get an overdrive... I'm not interested in a 700r4
2. regear the th400 to have a lower 1st (2.75:1) and 2nd (?)... go huge on tires... and have a big gap between 2nd and 3rd gear... but still be okay off the line and better on the freeway (due to bigger tire)
3. aftermarket overdrive (after the transfer case) and a rear driveshaft (expensive based on my research)
4. drive slower
Does anybody know if there's a gearbox that can be put on the back of the transfer case (np241c) that is CHEAP and would work as an overdrive (1:1.2 or so) when in 2wd? Then I could just shorten the rear driveshaft and tilt the rear axle to fix the angles... that would be kick ass. Oh, yeah... and I have the fixed output kit (no slipyoke in t-case)...
I'm probably just dreaming/hoping on this one.
There is no cheap overdrive option, unless you feel like continually finding used 700R4's.
You are better off driving slower and paying for gas than trying to deal with OD stuff.
Formula i use is : S x AR x FD x 336 (355 for non lock up auto) / TD
S= speed in mph
AR=axle ratio
FD=final drive in tranny
TD=tire diameter.
70 mph, TH400, 4.56's and 37's (assuming they are 37" tall) comes out to 3062 rpm.
Same combo with a manual tranny would turn 2898 rpm.
I found this formula in some old 4x4 magazine and remembered it. I use it often, and it's accurate.
Rene
S= speed in mph
AR=axle ratio
FD=final drive in tranny
TD=tire diameter.
70 mph, TH400, 4.56's and 37's (assuming they are 37" tall) comes out to 3062 rpm.
Same combo with a manual tranny would turn 2898 rpm.
I found this formula in some old 4x4 magazine and remembered it. I use it often, and it's accurate.
Rene
I checked the forumla once using my tach and my GPS. It is probably more accurate than my tach, although the tach was damn close too.
Chad...
70 mph, NV4500, 4.56's and 37's = 2116 rpm.
Rene
Chad...
70 mph, NV4500, 4.56's and 37's = 2116 rpm.
Rene
Cajun4x4
1/2 ton status
zero cool
1/2 ton status
40's would drop the rpm's into the 2700 range. Still higher than I want. 42's on the other hand might work.
Cajun4x4
1/2 ton status
Stock 3/4 tons came with 4:10's and 1:1 tranny, so turning 3000 was nothing. A 350 will handle that all day long. Even at those rpms, i noticed mileage around 13 with 33's (highway) that was with my lt1.
I wouldnt worry about 3000 with a gasser. It would be nice to have it lower, but its certainly not going to hurt the motor.
I wouldnt worry about 3000 with a gasser. It would be nice to have it lower, but its certainly not going to hurt the motor.
vortec
1/2 ton status
on my last wheeling trip, my friend and i were in a hurry to get there, so we mashed our skinny pedals (actually, mine is a chrome foot
) pretty hard and i was cruising at 2500-3000 for a lot of the way. got about 12mpg. on the way back, it was dark and we were tired, so we took it slower and i kept it around 2100rpms. my mpg went up to 16.3. that's roughly a 36% improvement and it didn't add all that much time to the trip. that's worth driving 5-10mph slower, IMO. man, i'm getting old.
Gaffed
1/2 ton status
I still have your 4.10'ed 3/4 ton set-up sitting in my driveway... I can arrange to get them to you... LOL!
But I like the idea of going with 42's.
But I like the idea of going with 42's.
You turn them into a 60/ff and its a deal.
Slapperbar
Retired Navy NDT Examiner
Dude! I've been trying to find that link for weeks, I'de lost it.
drop the hammer
1/2 ton status
Not to highjack but Does anyone one else here have a 456/205 with 35's. I swapped it in replacing a blown 700-R4, and it seems that the engine is just screaming on the highway, as well as rowing through the gears. I'm not sure what gears I have in the 12blt and 44. but I am 90% positive that the were never changed from stock. and as far as I knew stock ranges were around the 3.08 range. With that in mind even though I have lost the overdrive, It just seams real high. I gotta invest in a tach
Not to highjack but Does anyone one else here have a 456/205 with 35's. I swapped it in replacing a blown 700-R4, and it seems that the engine is just screaming on the highway, as well as rowing through the gears. I'm not sure what gears I have in the 12blt and 44. but I am 90% positive that the were never changed from stock. and as far as I knew stock ranges were around the 3.08 range. With that in mind even though I have lost the overdrive, It just seams real high. I gotta invest in a tach
I've had a 465/205 with:
31's/3.73 stock, worked fine but probably could've had a little better mileage with higher gears
33's/3.73 Probably better IMO. You always have the super low first gear if you need it.
37's/4.56 Felt like it was revving really high at 70 (3000 RPM). This was after I built the motor and power was not at issue. Still got good mileage (13-15) but I felt the motor was spinning faster than it needed to.
42's/4.56 Hasn't seen the road too much but I think this is more like the right ratio.
