3rd member swap, too low?

Just installed my 3rd member from ecgs yesterday. Went with the 4.56 set for my 31's. hit the turnpike this morning and im taching 3000rpm at 70mph. Before the swap i was lugging in 5th at 1900rpm and 75mph Isnt this a little high? On the turnpike one needs to go at least 80 but i dont want to turn even higher rpm's regularly. After i run a full tank ill have an idea of fuel mileage. If i keep it at 60 it turns 27-2800 rpm and seems more comfortable there. Then im getting flipped off or looked at funny. Are these gears too low for me?

 

Yea that is pretty high man. I would not have gone over 4:1 with small tires like that. You should be cruising around 2k at 70. Your fuel mileage will suck.

 

I got the 4.56 based on what the guys at ecgs recommended. Ill be SUPER pissed if i have to change them out

 

I ran a gear calculator and found that in fact I should be running a 4.11 to give me 2898 rpm at 65mph.

And even the 3.91 set is rated at 2757 rpm at 65mph and the 3.73 is 2630rpm. However, based on my original 3.31 gears and these 4.56's the calculator is running a little higher than actual.

So the trick question is..... which ones? I need just enough to be able to run 80 on the turnpike and still maintain some kind of fuel mileage, but also be able to pull a utility trailer occasionally. I would be happy with anything over 20mpg.

 

i thought 4.56's were for tires around the size of 33's. a few of my buddies are tellin me 4.10/ 4.11's would be good for 31's.

 

My 2009 5speed turns about 2500 at 70MPH in 5th. 3000 is a bit high, but other than MPG, I doubt you're hurting anything. This motor doesn't even sound strained at 4000 RPM, just noisy.

Hell, my V6 6-speed turns at 3K RPM at 75MPH. I have run that all day in 100 degree heat with no problems.

Had a 71 Datsun truck that cruised at 4,000 RPM at 70 MPH. It would do that all day, just wear ear plugs.

 

Yeah im concerned anout the mileage. I was getting 16-17 mpg with the stock gears and these tires. I havent run a full tank since the swap yet to check, but with these rpm levels im not expecting any improvement and that was the whole point of doing the swap. If it turns out that im 20+mpg ill be happy and will leave them in.

Amyone want to guess on the mileage til I find out? Im guessing if it improves at all it will be 17-18.

What say you?

 

I'd leave em in, especially if you ever plan in getting bigger tires.

 

Buy bigger tires.

 

I don't think you'll see an improvement. Make sure to account for the gear ratios and tire size.

time for 33s.

 

I know the bigger tires will adjust the rpm's better but will they actually improve the mileage?
These 31's are a month old so it will be a long time until im ready to go bigger

 

isnt the stock ratio on a 5 lug 4.10? to keep the the new 3rd member and have the rpms settle down you need to get 33s

 

Stock ratio is 3.31

 

So I got to check the mileage today. I babied it for the whole tank and never went over 65mph or 2750rpm.

Drum roll......

16.4mpg!
This is completely unacceptable! These gears are comming out! I would be better off going back to lugging with the 3.31's at 18mpg.

 

Did you account for the mismatch between the rear end gearing and your speedometer gear? Rear end spinning at 4.10 speedometer counting at 3.31.

 

Speed based on gps. Mpg's dont change either way right?

 

what year is your truck? what size is your differential? I assume you are a 5 speed so yes you are overgeared for your 31s. you need 32s or 33s.