250 Straight 6 Auto or Manual

[jlampka]

I have a 1971 C20 Chevy pickup with a 350 V8 and an automatic transmission. I am thinking of pulling the 350 out and putting a 250 straight 6 in for better gas mileage. I have a line on a 250 engine from a 68 Nova that had a manual transmission. Are there any modifications I need to make to this engine or will it bolt on the auto transmission in my 1971 C20 truck?

[CST10]

Your gas milage will depend on the rear gear. I have a truck with a 462ci with turbo 350 tranny and 3.08 rear gear that gets 10 mpg and I have another trick with a 250ci with 3 speed manual and 3.73 rear gear, guess what it get 11-12 mpg. Not much better. And the 3.73 geared truck turns high RPM's on the highway.

I wouldn't change it out thinking I'd be getting a huge change in mpg.

[Wrenchbender Ret]

It will bolt right up to your presant trans. I'm with CST. You would be better off putting a 700R4 trans. in it. Then drive it real slow & sluggish like you already have a 250 in it. You will get as good as or better mileage.

[18436572GMC]

Originally Posted by Wrenchbender Ret

It will bolt right up to your presant trans. I'm with CST. You would be better off putting a 700R4 trans. in it. Then drive it real slow & sluggish like you already have a 250 in it. You will get as good as or better mileage.

Or a 6.2 diesel.
Yes it will get better mileage
Yes it will be gutless above 40mph
But at least there will be something cool about it

[LukeandI]

When I was in my 20s, I had a 65 c10 fleetside with a 250 and single downdraft rochester. I got 12-13mpg in the mountains, valleys ...pretty much everywhere I drove. Last year I picked up a 70k10 short box stepper with 350, 4" lift, 33s, and the same 373 dif and 4spd. I drove that over the Coquihalla (google it) in November and got dead on 16mpg. 250s are cool but no gas savings there...

[cayce_brunswick-]

I'm on board with everyone else. Over drive or a gear swap or even both. I have been really thinking about a 200r swap. But I have a line on one I can pickup almost free.

[nascarguy320]

I am installing a 700r4 with my 250 six.

[jlampka]

So then the bolt pattern is the same as the 350 to my TH350 auto trans?

[nascarguy320]

Yes it is.

[faribran]

I have had both 6 cyl and "for-kid" eights in these trucks. There is no way I would go in this direction. The 6 cyls I had got less gas mileage and not nearly as much power.

[nascarguy320]

Well I had work done to my six so its at 400 horses now.

[R.L.]

I pulled the 700r4 out of my truck, I'm running a 250 and it never was in it's power band. I went to a 3 sp manual trans. Mine is stock, I'm sure if you get the HP up it will be fine but going to a 6 for MPG is not going to happen.