Engine/Transmission (1994 - 1998) 45 mph cruising, which gear?

[TLanier]

in a situation where you must cruise along on a flat road at 45mph (speed limit of course) , would it be better to be in 4th @1800 rpm, or use 5th @ 1400 rpm? Consider the flat road, with no lugging. Which gear would give the best mpg?

 

[mikel]

Better for the transmission would be 4th. Your 5th gear will thank you.



Any money you might save in fuel will be more than offset by the extra work and expense of fixing 5th from running it at that low RPM.



Mike

 

[surfbeetle]

mikel is dead on. I have 4. 10 rear end and never use 5th if the RPMs would be below 2000. For me that's around 60 mph. The basic rule of thumb is if you can use 4th instead of 5th, do it. I have 147,000 on the original trans.

 

[radixr]

At low throttle, you won't have any loosening of the fifth gear nut. It's only under heavy load that there is a risk of the nut eventually coming off. Your mileage will be better in fifth.

 

[pwerwagn]

I agree with the fact that it is better on your fifth gear not to use it, but I have almost 160K miles on mine, with stock 5th nut, and I run 5th as low as 40mph on flat ground.

Reason being: I just filled up my tank about 20 minutes ago, and got a hand calculated 22. 7mpg!! My best yet... ...

I know it may not make up for the cost of a 5th gear fix, but when that time comes, so be it.

--Jeff

 

[TLanier]

i see a few conflicting viewpoints. hmm. i agree that in theory, use fourth when possible. but when im talking about using fifth at 45 mph @1400 rpm, im driving with and egg under the throttle, with NO LOAD at all. that means if there is a hill, i downshift. here in denver, we get alot of flat stretches.



been getting around 20. 4 mpg city/highway mix, shifting under 2000 rpm, using fifth at 45mph, and doing a nice 60mph on the freeway.

 

[biggy238]

Look at a factory dyno curve for your motor. The peak torque should occur at 1600 I think??? That peak is the most efficient point for the enigne. Get it there and run it.

To me, I would rev it a little more. . I think it offers better lubrication and fuel supply.

 

[Signal73]

I kinda like running it in fifth in the 1500-2000 range, there is so much rediculous torque to be had there its hard not to. Not to mention smoke on demand.

Its kinda like smoking cigaretts, you know its bad and one day its gonna catch up with you, but you just have gotta do it anyways :-laf

 

[surfbeetle]

TLanier,

Yes there are two separate issues being expressed here. To answer your original question, YES you can drive that slow in fifth, the engine can handle it and your mileage should be good. The underlying issue for those of us who have the NV4500 five speed is the fact that eventually the fifth gear will get damaged due to harmonic vibrations, the nut getting loose and slipping up the half splined shaft that it is installed on. Basically, you end up with a four speed and neutral where fifth used to be. This is why many of us don't recommend it. If your trans has been repaired with a fully splined shaft, you probably have nothing to worry about. I am still on my factory original trans and therefore won't take the chance.

 

[MHalloran]

I would also agree with mikel. "stay in 4th". Almost all of the time my 96 has run with no load and one day, just cruisin' along in 5th at about 50 mph, (about 150K on odometer) the trans just "fell" into neutral. 5th gear nut ran off. Maybe the mileage had something to do with it. Can't say it was because of runnin' loaded. Got it fixed & now use 5th on highway only. Thought I'd share my story with ya. Good Luck. Mike H.

 

[Kernel]

Did anybody catch the tables in the TDR magazine a couple of issues ago by I think Joe D. ? It listed rpms in relation to hp, torque, and fuel consumption with the sweet spot something like 1600-1800 for fuel use, but I can't remember if it mentioned the 215 pump. Kind of interesting to show that lower and higher rpm used more fuel. Anybody have the issue handy?



Scott

 

[biggy238]

That was what I based my info from, that and some other dyno sheets.

 

[jbrowne]

Got to remember, folks, that old Cummuins has a bundle of low end torque and she has only 6 cylinders. That is only three booms per rev. and at low RPMs it litterly rattles the power train enjough to loosen the tightest fifth gear nut loose. Keep the RPMs up, you can hear the strain in the transmission when it is growlen' at you.

jbrowne

 

[TLanier]

yeah, that makes sense. you know, the frustrating thing is that i WANT to use all that bottom end grunt. ive driven toyotas for so long, with their thristy but weak little revy motors, and man being able do pull along at low rpm is heaven to me! its like we have a motor matched to a drivetrain that wont let it do its thing!



so, is this mainly a fifth gear problem; that is, since i shift at 2000 for mileage sake, 3rd to 4th makes a miserably huge drop. should i really rev 3rd up before the shift to keep her outta the bottom in 4th too?