Better gas mileage and performance.

Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.

95Noobie

Newbie
Joined
Mar 23, 2024
Messages
12
Reaction score
14
Location
Grand Rapids, MI
That write up is superb! Thank you! I was taking a test drive (to and from the gas station of course) and noted that at 60mph my rpms at least according to the display was around 1700 (it is a guess on the display as not enough tick marks between the big numbers) I have 33" tires but maybe when pumped up they are 34" per your equation. Either way I see that getting the engine to turn faster at the same vehicle speed (or same thing is making the drive shaft ratio lower) is getting the engine to higher rpms and in its torque band.

Of course the other solution is to downsize the tires to get the rpms up and of course smaller/lighter tires will give less load for the engine to push.

It is clear that when the road goes up a bit of a grade even at 60 that the truck loses speed and engine load really has to pick up to keep it at speed. On the other hand on level highway at 60mph the rpms seem to be a comfortable range given that it only has 4 gears to work with.

Is 4.1 the only other stock option 'above' 3.73? I assume the rating for the front axle uses the same number system so a 3.73 would apply to front and rear on our truck.
 

95Noobie

Newbie
Joined
Mar 23, 2024
Messages
12
Reaction score
14
Location
Grand Rapids, MI
as to the lack of O2 sensor, I purchased a used Innova 5512 reader with adapters that claim to do OBD1 work. Maybe I'm still on the learning curve but I'm disappointed. It will read engine codes but won't reset them (it says refer to your manual or jumper settings to clear codes?) It won't read my ABS or SRS system or do any live data which the Innova web site says it will do. I had no engine light on yesterday when the device arrived in the mail so I had nothing to test but tonight after a highway trip to work and back there was a stored code for low O2 sensor voltage (I think P0131) which makes sense since there is no sensor connected to the harness.

I can't tell more about fuel trim or anything about rich/lean until I can find a way to make this scanner work or buy a different one that will do what we need. I was hoping to read my SRS sensors since my air bag light is blinking red and the one on the passenger framerail looks pretty swollen.

The post above suggests the factory settings are pretty close even without O2 sensor input but I do have dual exhaust currently no cats so thinking we are far from 'stock' engine settings. I hope to have a repair or muffler shop weld in two high flow cats and locate one close enough to the existing O2 sensor wire that we can plug that in pre-cat on one side. The other high flow cat will have a plug in the sensor hole. Maybe that will help the engine a little on the 'too rich' theme. It certainly is smelly exhaust when starting but on the other hand there is no cat to treat the exhaust smell either.
 

boy&hisdogs

I'm Awesome
Joined
Mar 2, 2012
Messages
569
Reaction score
630
Location
Eastern WA
That write up is superb! Thank you! I was taking a test drive (to and from the gas station of course) and noted that at 60mph my rpms at least according to the display was around 1700 (it is a guess on the display as not enough tick marks between the big numbers) I have 33" tires but maybe when pumped up they are 34" per your equation. Either way I see that getting the engine to turn faster at the same vehicle speed (or same thing is making the drive shaft ratio lower) is getting the engine to higher rpms and in its torque band.

Of course the other solution is to downsize the tires to get the rpms up and of course smaller/lighter tires will give less load for the engine to push.

It is clear that when the road goes up a bit of a grade even at 60 that the truck loses speed and engine load really has to pick up to keep it at speed. On the other hand on level highway at 60mph the rpms seem to be a comfortable range given that it only has 4 gears to work with.

Is 4.1 the only other stock option 'above' 3.73? I assume the rating for the front axle uses the same number system so a 3.73 would apply to front and rear on our truck.

Yeah 4.10 is the highest stock option. I think there were some factory special 4.56 but that would likely have been government/miltary trucks and most likely 8 lug. The vast majority of 1500s were 3.73 and 3.42. I looked for years trying to find a 4.10 six lug axle but never did. They mostly came in the 8 lug trucks. And remember whatever you get for the back you need for the front also.

This is what I use to run numbers for stuff like this. Much easier than doing it all by hand and they have all kinds of stuff already built in.


Your best bet is to go down a tire size. Its the easiet, cheapest, and least permanent if you dont like it down the line. At least go skinnier if you must have 33s. Like some of these other guys said you want to shoot for around 2000 rpm, give or take a hundred or two depending on how you want to use or build up the truck. You want just enough rpms so that the truck isn't hunting for gears on the highway. Constantly kicking in and out of 4th when you have oversized tires and too tall gears kills transmissions. I learned the hard way lol.

Higher rpms are actually better than they seem on paper, as the engine is making more power to push the heavier wheels and tires and wind resistance on a taller truck. It's all a balancing act.
 

95Noobie

Newbie
Joined
Mar 23, 2024
Messages
12
Reaction score
14
Location
Grand Rapids, MI
I think in time I can convince my son that we can still come up with some really nice rims/tires. We can lose the excessive offset and get most of the rim back under the wheelwells, we can then goa bit skinner and maybe get diameter down an inch and it will still run nice and look nice. That will get rpms up into the sweet spot. I don't think we have time or energy to swap out front and rear axles for a 1mpg or 2 when the more obvious thing relates to how you drive it and the tires/rims and resistance.

I was hoping full tune up would help a bit. Maybe we get lucky and adding the O2 sensor will help a bit. I guess most aim for about 13mpg as a good thing with mix of highway and city given the situation.
 

boy&hisdogs

I'm Awesome
Joined
Mar 2, 2012
Messages
569
Reaction score
630
Location
Eastern WA
I think in time I can convince my son that we can still come up with some really nice rims/tires. We can lose the excessive offset and get most of the rim back under the wheelwells, we can then goa bit skinner and maybe get diameter down an inch and it will still run nice and look nice. That will get rpms up into the sweet spot. I don't think we have time or energy to swap out front and rear axles for a 1mpg or 2 when the more obvious thing relates to how you drive it and the tires/rims and resistance.

I was hoping full tune up would help a bit. Maybe we get lucky and adding the O2 sensor will help a bit. I guess most aim for about 13mpg as a good thing with mix of highway and city given the situation.

The narrower stance will save your front end too, you'd be surprised how (not) long balljoints and alignments last with super wide setups. And the smaller tires will be easier on your trans, you are getting close to that time. 160k+ is when you start seeing "rebuilt trans" in the for sale ads lol. My transmission was almost two thousand dollars to rebuild and that was several years ago, before covid and all this other stuff. Its probably way more these days depending on where you live.

Considering its a TBI 305 I think your goal of 13 is realistic. Small tires may not be as cool but neither is going down the $$$$ rabit hole, at least until he's got a career and a reliable backup car.
 

GrimsterGMC

Supporting Member
Supporting Member
Joined
Nov 3, 2021
Messages
1,149
Reaction score
3,729
Location
New Zealand
That write up is superb! Thank you! I was taking a test drive (to and from the gas station of course) and noted that at 60mph my rpms at least according to the display was around 1700 (it is a guess on the display as not enough tick marks between the big numbers) I have 33" tires but maybe when pumped up they are 34" per your equation. Either way I see that getting the engine to turn faster at the same vehicle speed (or same thing is making the drive shaft ratio lower) is getting the engine to higher rpms and in its torque band.

Of course the other solution is to downsize the tires to get the rpms up and of course smaller/lighter tires will give less load for the engine to push.

It is clear that when the road goes up a bit of a grade even at 60 that the truck loses speed and engine load really has to pick up to keep it at speed. On the other hand on level highway at 60mph the rpms seem to be a comfortable range given that it only has 4 gears to work with.

Is 4.1 the only other stock option 'above' 3.73? I assume the rating for the front axle uses the same number system so a 3.73 would apply to front and rear on our truck.
To give you a reference, my 1988 K1500 350 TBI/ 700R4 has 4.1 diffs and 33x12.5/15 tires and looking at a recent datalog I can accurately say that on level ground, sitting on 60 mph, I was doing 1800 RPMs with the TCC locked. Hope that helps.
 

KansasOBS

I'm Awesome
Joined
Apr 29, 2023
Messages
135
Reaction score
272
Location
Kansas
Small tires may not be as cool but neither is going down the $$$$ rabit hole, at least until he's got a career and a reliable backup car.

What a bummer, dad. That's no fun!!!

Random babble from me, but I thought it was kinda funny. 5:38 was the closest I could get to a factory ratio with the math on my build. Turns out the speedo is spot on with the basically 40's according to GPS. Better than what my pretty much stock daily driver was, being about 3mph off highway cruising.
 

95Noobie

Newbie
Joined
Mar 23, 2024
Messages
12
Reaction score
14
Location
Grand Rapids, MI
I also found it interesting that I figured my speedometer would be way off with 20" rims and 33" tires compared to the stock setting. But for reasons i can't explain it is very close. using gps app on my phone the speedometer currently reads about 1 mph faster than actual speed, sometimes 2mph. I guess that adds a safety factor so dad can hope that when son is seeing 75 on the speedo he is doing 73 hopefully..

another newbie question. How can I tell when TCC is locked? It is a 'feel' thing?
 

95Noobie

Newbie
Joined
Mar 23, 2024
Messages
12
Reaction score
14
Location
Grand Rapids, MI
yes, agreed about the tranny being on borrowed time. I dropped the pan to change fluids and outside of some liquid black on the magnet it looked good. I changed the accumulator to a pistonless set up. There were 3 springs in the accumulator, 2 larger springs about the same size and a smaller stiff one inside that. In my ignorance I just put them back in and put it all back together after swapping the 1,2 solenoids and a filter change. I also had a new pan ready in case I needed it and I added 4 larger rectangular rare earth magnets. Drove it around a few days. Then read some more about why I had hard shift from 1st to 2nd. When doing any accelerating or going up even a slight grade it would hang in first till rpms got quite high and then a firm shift into 2nd. I wanted to go back to 'stock springs' and see if that helped. So took it all back down, swapped the 3 springs out and put the factory large spring (purple?) and the factory small spring (yellow) in and put it back together. There was a little bit of dark 'dust' on the new magnets but all looked good in there.

The 1-2 shift is maybe a bit better. It is crazy that if I'm going a low speeds or rolling without calling for extra engine work it shifts sometimes like butter. Any load on engine or rising rpms will hold that shift off and will be a little more uncomfortable shift (no slamming) 2nd to 3rd is always very smooth. I'm assuming this is just how it is as the TPS is new and everything else seems OK.

My reverse worries me. It goes into reverse smoothly but if I give a little gas while in reverse or try to back up a slight grade there is a feeling of slipping and then it goes. Not sure if that truly is transmission slip (once it is in Reverse tranny shouldn't be to blame anymore?) or if it is in the gear box or in the transfer case) but if you ease on the gas in reverse no slip. If you give it gas and it slips it is only for a second and then it seems better.

Nothing else I can do as all fluids are changed. I guess we don't use reverse much and slow speeds are fine. I don't plan to back trailers up a grade often. Yes, you can get a used transmission for fairly cheap and some are advertised to be 'rebuilt' but reputable tranny shops here will only rebuild the one in your vehicle not install a 'rebuilt' one from another rebuilder.
 
Top