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700r to sm465 swap mpg major drop

MechroMenace

MechroMenace

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East Idaho or Southern Utah
So i recently swapped my 1989 suburbans 700r4/208c for a sm465/205. I was getting 12mpg city before the swap and now I'm getting 7mpg, I'm not sure what I did wrong, I rebuilt the sm465 before putting it in, haven't touched the 205 it doesn't even have the shifter yet. It also doesn't feel sluggish at all, and I hooked my laptop up to it it doesn't seem to be slipping it's at 2500 rpm at 35mph in 3rd and 1400rpm at 35 mph in 4th. The truck is on 31 inch tires and 3.73 gears. I'm at a loss as to what I did. From what I've read is that most people are getting 10-12 with simular set ups. I wasnt going for more mpg doing the swap but at the same time I didn't want to lose anything. Also I'm not driving aggressive I broke my wrist during the swap so I'm just happy to get it in gear not trying to race it. Any ideas?
 
You lost your overdrive by changing to the 465......the 700R4 has a .70:1 overdrive ratio, whereas the 465 is 1:1 in high gear...you're turning more RPM on the motor to get the same speed.
to get the mileage back you'll probably have to swap to higher ratio (3.08) gearing in the differential.
 
true i do not have an overdrive, but in city I never used the overdrive in the 700r4, mostly because it was only running slightly above 1000rpms and whenever i'd give it a little bit of gas it would downshift. plus I can't imagine that getting rid of the overdrive would knock 5 mpg off of a suburban even if it was on the highway. who knows though I'm just a little sad because I like manuals and if it wasn't for the gas mileage tanking I would fine right now. my only other idea is that I live in a super small college town and I'm stopping every block because of a red light or someone cutting me off. and maybe the automatic handled it better, or I'm just not good at efficiently shifting. I was kind of hoping I didn't know as much as I did and my confidence of doing everything right during the swap was false, and someone could be like you did this right? I guess i'll see what happens to it when I drive to Idaho falls today on the freeway if mileage improves over the city considerably or it gives me the middle finger and gets 8mpg because why have the power of a big block if you can just have the gas mileage of one.

The broken wrist isn't really too much of a problem i just cant move my thumb so my hand writing looks like a something a 4 year is capable of, but i can still type, shift a manual somewhat and use the excuse at work "can't lift that, doctors orders"
 
Did you swap to a manual trans PROM (or whole ECM)?

TBI seems to be a lot less critical of transmission changes than other EFI setups, but the PROM is always different manual to auto. Never seen anyone analyze a before/after without swapping PROM's. How did you handle the Park/Neutral switch?

It's also likely there might be some differences related to how you drove with the automatic, and with the manual. If you don't already, anticipate lights, and don't jackrabbit start/stop. In traffic if possible, I just try and pick a gear where I'm avoiding catching the vehicles in front, and don't have to accelerate/decelerate constantly. Make sure to downshift and let compression braking do much of the work, not throw it in neutral or depress the clutch pedal...compression braking essentially shuts off fuel (AFAIK, it does with TPI that vintage), and most likely will result in increased economy.

FWIW, highway I got the same economy going from a 305/700R4/3.08/31" setup as I do/did with a 350/465/3.42/33" setup. Equates to ~1400/2000RPM respectively at 60MPH. In town 10MPG seemed about the norm, with either one. It never varied hugely.
 
so that is one thing I didn't even think of, i thought the ecm was the same between automatic and manual due the 700r4 being controlled by a cable. and honestly I broke my wrist while swapping transmissions so I never was able to get around to removing the automatic shiftier on the column, I've just left it in park and its doing just fine. i'll probably rewire it as a kill switch though when I get the cast removed and I have use of my wrist and thumb. I also don't really use the engine to brake for me, nor does this transmission like to down shift without double clutching it. I guess I should start with changing how I drive it, now that I think of it I was driving like a grandma since the automatic would rev up between 1st and 2nd unless I was all the way on the throttle or I was hardly pressing it, and I get cut off too much to always floor it. It seems as though I can't mercilessly beat on this boat like my i did with my jetta that I use to have, it would get 25mpg no matter what I tried treating that car nice for a month and it was still getting 25mpg so I continued to drive it like a race car.
 
My blazer with 350 TBI and SM465 (but 208) and I get about 13.5 - 14 mpg. With 35's and 3:42 gears.

Yeah, shifting takes some getting used to. I don't have to double clutch, but you certainly can't be in a hurry, everything has to be very deliberate.
 
The SM465 & NP-205 combo probably weighs a lot more than a 700R4 & NP-208 too,and more bulk = less mpg...probably not enough to drop it as much as you've noted,but it wont help either...
 
Well, you've got before MPG numbers, so "I've just left it in park and its doing just fine" may not be true, right? :) You'll see the statement after EFI swaps/mods, "it runs fine"...without datalogging and a wide band O2, or swapping to the correct parts and comparing something authoritative like MPG, there is absolutely zero way to make that statement and be correct.

What you are doing is driving the truck while it's thinking it's in park. I've never tried to work around a manual transmission with an auto trans PROM, but most likely this affects target idle speed, and probably eliminates any MPG benefit gained from decelerating while in gear. Manual/auto PROM also has differences in timing, which could lead to hitting the knock sensor and reducing performance, or just having timing wrong for the engine, and again hurting performance, which will reduce mileage.

No "proper" way to work around those issues without a different PROM, timing is programmed and the truck needs to know park/neutral, but I'd *guess* that the ECM interpreting that it's in drive all the time would work better for MPG numbers than being in park, but that would probably adversely affect idle.
 
I say speedo error. How exactly are you measuring your fuel economy anyways?

Loss of OD isn't gonna drop your fuel economy by 40%...
 
tRustyK5 said:
I say speedo error. How exactly are you measuring your fuel economy anyways?

Loss of OD isn't gonna drop your fuel economy by 40%...
Click to expand...

Very good point...unless done by GPS, or the speedometer gears were changed, pretty unlikely the speedometer/odometer readings are correct. Would easily account for more of an error than any ECM programming differences.
 
I'm using the tripometer on my dash I just take the miles I've gone and divide it by the gallons I fill it with, I try to use the same pump at the gas station too. I also double checked the speedometer with my phones GPS after the swap and it's spot on at 35mph. I was also just going through the ecm with tuner pro on my laptop and it looks like it's idling at 1000rpms which seems high for a v8, and it looks like the ecm recognizes if it's in park or drive via changing the shift lever on the column. I guess I'll try putting it in drive and see if that changes anything, and I'll try driving it nice.
 
Doesn't tunerpro show you the target idle speed?

Yes, 1000RPM is wrong for idle whether in park or in gear, if the engine is up to operating temperature. Again likely based on P/N, since you didn't change anything but the trans.
 
Did the VSS get removed or unplugged going to the 205?
 
I swapped my 1988 V10 from a TH700R4/NP208 to an SM465/NP208. I did not change the PROM, I did nothing with the neutral safety switch.

My fuel economy didn't change enough to notice.

Martin
 
The VSS should be in the instrument cluster all I did was unplug the speedometer cable from the 208c and plug it into the 205 and as long as my GPS on my phone is accurate then that means the speedometer is correct too. I ended up driving from idaho to southern utah yesterday and I got 12 MPG all the way at 65MPH which, leads me to the conclusion that I'm not great at driving manual in an efficient way for city driving. I also found out during the drive that my heater core is shot.
 
I get (got) about 16MPG at 60MPH. Turning a bit less RPM, but not enough to make that much difference IMO.
 
I'm happy with 12 in this tank but i'm gonna have to see what I can do with it. the motor i feel is worn out it was struggling to get up hills at 3000 rpms, it was full of my stuff since I was moving but still I should be able to maintain speed at 65mph. its not like it was super heavy. it also spits out a cloud of smoke when I start it i'm pretty sure it has more than the 160,000 miles the kid that I bought it from claimed it had. I'm gonna see if I can find a vortec motor and put the tbi unit on that, along with some headers and an exhaust it should give it a little more pep.
 
Loss of overdrive is the biggest issue in my mind. Regardless of the fact you didn't use OD that much with the 700. Gear splits on the 465 are so wide you have a huge drop in RPM when going to the next gear. So your engine drops below it's torque curve and you end up flogging it to accelerate. From my experience with my '75 having a carb'd 350, 465, 205 and 3.73 gears and 31" tires it spun close to 3000 RPM at 65 mph. At that speed the q-jet was running on both primaries and secondaries as you could hear the typical bogggg noise they make when the secondaries open. That meant I was dumping fuel at an alarming rate. Carb or TBI, running along at highway speeds at those RPM's the engine is going to consume a lot of fuel.

So you can go to the "poor-man's" overdrive. Meaning get larger diameter tires. 33's will drop the cruise rpms by around 500 or so. Jump to 35's and it will drop more. Caution, going bigger reduces the overall effectiveness of the axle gearing. Meaning the 3.73's are going to behave like a taller gear ratio when you go taller on the tires. This makes it more doggish off the line from a stop, but your cruise rpms will come down. Of course if you haven't lifted it, 33's are going to be the limit for the size. 35's will require lifting or cutting fenders.
 
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