10 mpg

[raidmagic]

That's not good. It would be better if it was some high horsepower small block but I'm not rolling around with a 6 banger getting 10 mpg. I guess in the short term I'll throw an extra 350 I have laying around in there until I can gather all the rest of my stuff for the LS swap.

[Captainfab]

Which 6cyl, a 292?

[Palf70Step]

Normally it's not the engine that kills ur mileage on these trucks, but the rear end gears. So many had 4.11s or lower that kills any chance of the mileage we think of today.

[raidmagic]

A 250. It has headers, 4 barrel and a cam. PO did it. I have no idea what cam. It does have 4.11's and a 3 speed on the column so that doesn't help but like I said if it's going to get bad mileage it will me fun to drive. lol

[Palf70Step]

The 4.11s are what is killing you. Depending on the tune, I would say 13-15 would be normal driving, maybe 17 on highway. That would be with a 3.08 rear gear and still with the 3 speed.

BTW that is about what my 64 with a 350 did before and after the gear changes. I was getting around 10-11 in everyday and highway. changed from 3:73 to 3:08 rear and 13-14 in town and 16-17 @ 75 on interstate. With the stock 3 speed manual.

[curvedglass]

I am running my original 230 3-spd in my daily driver 63. PO changed the rear to 3.08's. I run a weber 2 bbl with an adapter and an hei ignition. Set the timing and I am averaging high 17's with best avg of 18.2. the 18.2 was an average after 5 tanks of gas with back road & short highway trip driving just before winter storage. - I don't trust my speedo so I use my speedometer app on the phone which uses GPS to track speed and odometer.

you might want to consider an OD trans. I picked up a 3spd OD off E-Pay and I plan to go back to original 3.73's - only bad part of the 3.08's is not enough offline power. With those gears and the OD it will spin better than the 3.08's at high speeds. Should help with your 4.11's.

[raidmagic]

Originally Posted by Palf70Step

The 4.11s are what is killing you. Depending on the tune, I would say 13-15 would be normal driving, maybe 17 on highway. That would be with a 3.08 rear gear and still with the 3 speed.

BTW that is about what my 64 with a 350 did before and after the gear changes. I was getting around 10-11 in everyday and highway. changed from 3:73 to 3:08 rear and 13-14 in town and 16-17 @ 75 on interstate. With the stock 3 speed manual.

How bad does that 3.08 hurt?

Originally Posted by curvedglass

I am running my original 230 3-spd in my daily driver 63. PO changed the rear to 3.08's. I run a weber 2 bbl with an adapter and an hei ignition. Set the timing and I am averaging high 17's with best avg of 18.2. the 18.2 was an average after 5 tanks of gas with back road & short highway trip driving just before winter storage. - I don't trust my speedo so I use my speedometer app on the phone which uses GPS to track speed and odometer.

you might want to consider an OD trans. I picked up a 3spd OD off E-Pay and I plan to go back to original 3.73's - only bad part of the 3.08's is not enough offline power. With those gears and the OD it will spin better than the 3.08's at high speeds. Should help with your 4.11's.

That's what I was wondering . Seems that 3.08 would be a killer. I really like the idea of going back to a 2 barrel too. I like this set up but this truck is going to be my daily and I need to depend on it. My current truck gets 13.5 so I could live with that. I can't live with 10 mpg and no power though.

[Palf70Step]

You will not have a drag truck with a 3.08, but you should do fine as a daily driver. My 81 GMC has a 250 and T350 automatic with 2.76 gears and 31x10.50-15 tires. No there is probably no burn out power, but I don't seem to have any issues for highway at 75 ish and around town driving. It gets out of the way. Not nearly as fast as the 350.

I don't think going to a 2bbl carb will help your MPG. You might be able to play with the 4bbl carb jetting and get things to tweak out more MPG.

[andyh1956]

My son's classmate has a 67 c10 250 / 3 spd. It was 3.73 & I changed it to 3.08s. He loves it now!
Be advised I set those gears up six times before they suited me, the smaller the gear the harder to set right. But it has stood the test of time, about 20 yrs now.

[vince1]

I can remember way back when our 65 Merc with 352 208 HP 3 on the tree used to get 16 MPG imperial, so almost 13 US. I don't know what ratio it had.

I have yet to check my 66 250 with 3.07 gears but don't expect it to be much better and it has a lot less oomph. As soon as it gets warm I'll find out. No use skewing the numbers at -20C with snow on the ground. Someone I know who has a lot of these trucks claims the 350 will get similar mileage to the 250.

[65chevysub]

Sorry to jump in to this thread, but would 3.08's be good for a high mileage 283, with a 4 bbl carb, and a TH350? I was hoping to get a 350 this month, but funds won't allow it for at least 6 months. I currently have 3.73's

Thanks

[vince1]

That's my plan, I have 3.07's and would like to change the 4sp to a TH350. The 3.73's were too slow. An overdrive transmission would be OK with the 3.73 though.

[LostMy65]

Originally Posted by 65chevysub

Sorry to jump in to this thread, but would 3.08's be good for a high mileage 283, with a 4 bbl carb, and a TH350? I was hoping to get a 350 this month, but funds won't allow it for at least 6 months. I currently have 3.73's

Thanks

Did you ever find an answer?