RPM Question

[rcvanness]

Im not sure if this is the right section or not. But i have just done a tranny swap in my 1987 ranger i have a 2.9L V6 engine and since the swap running on the highway the rpms are running up over 3k. I swapped the stock A4LD auto i had for a fm145 i got in a parts truck. now my ratios are 4.10 were the parts truck i took the tranny out of has 3.45. if i change the ratio's will it allow my rpms to drop?

 

[Beanmachine7000]

Leave it... Trust me, you don't want a 2.9 with 3.45's... I wouldn't want a 4.0L with 3.45's... 3.73 is about perfect for the 4.0L... 4.10 is great for the 2.9L... I'm looking for a set of 4.10's myself for my new Ex...

 

[rcvanness]

I want to find a way to drop my rpm's to increase my mpg im down to 13 and when i got the truck i was getting 19-22. i like the 4.10 gears for the power and all but i dont use the truck for hauling or pulling weight. and see the tranny i put in the truck come from a 1986 ranger 2.9L with those 3.45 aslo i dont think it would be that bad cause im running oversized tires cause with 3.45 it calls for 195/75/15 and im running 235/75/15. my motor has 177k on it and i really dont wana have to rebuild it sooner than necessary and i do run the interstate alot.

 

[rusty70f100]

Here's my experience.

I run a '87 Bronco 2, 2.9L, M5OD, 3.45 gears, and 235/75R15 tires. Driving around town is fine. On the highway it's usually fine, depending on the direction of the wind and the grade of the hill. On the interstate it is rather tricky, as the 2.9L just doesn't have the torque to keep the speed up to about 65 to 70mph. It just doesn't. At the slightest breath of a headwind, it just knocks the speed right back down to about 55-60mph. Forget about pulling hills. Of course, I just put it back in 4th gear and put my foot down, and that immediately corrects the problem. But then the mileage goes down to maybe 20mpg. Running 4th gear with 3.45 gears is about like running 5th with 4.10's.

But on the highway, I can get up to 26mpg in summer, so the lower (numerically) gearing does help with mileage. Like I said, if I'm on the interstate, I can just run in 4th.

So yeah, swap it back to 3.45 if you're not pulling or hauling anything.

 

[rcvanness]

Thanks thats what i wanted to know. cause i know the 2.9L is no powerhouse but for me the truck is perfect. cause i really dont fit in cars well.

 

[Beanmachine7000]

I beg to differ taller gearing is only going to make your engine work more... If where you drive is perfectly flat then yeah 3.45 would be fine but if there are any hills whatsoever and especially if you are running 235/75-15 you will more than likely get your best mpg with a 3.73:1 ratio (though, I would use 4.10 on a 2.9)... Trust me it's alot easier on your motor to spin 3-4000 rpm all day long than stop and go traffic... My 4.0L's all have 3.73:1 and 235/75-15 and run over 3k rpm for hours at a time on the interstate at 70 mph... One has 170k, one 210k, one 110k, and one has 164k miles... If you are really that worried about it drop to 3.73... Not 3.45... Ford was stupid to even consider putting those in these trucks... If you dropped from 22-13 mpg there is something else seriously wrong... Gearing might make up 2, possibly 3mpg... not 10

 

[rcvanness]

i was getting 19-22 then the a4ld automatic went out so i put in this fm145 5speed and thats when the mpg dropped. only reason i was asking is cause i have these 3.45. as far as shifting i dont have a problem with that. im just trying to find the best route to go.

 

[Beanmachine7000]

I'm not saying shifting is a problem... But if all you did was switch trans and saw a TEN mpg drop you screwed something up in the install or something else is broken... It shouldn't change... You should actually see a 1ish mpg increase... 3.45:1 ratio isnt going to get you any better fuel mileage especially with the tires you are running... If it really bothers you THAT much drop to a 3.73:1 which probably won't gain you anything but less power... I'd fix whats wrong first then see if you still want to swap... 4.10's are perfect for the 2.9L...

 

[rcvanness]

ok, any ideas wat could be wrong? and could it lead to my warm idle fluctuating between 500 and 1200rpms?