Another fuel economy check with my '65 one ton

[66Submarine]

Originally Posted by factorystock

Rear axle ratio on '62 C 20 was 4.57, there were no other optional ratios in '62. Engine condition is also a factor in MPG. A worn 235 will not get the same mileage as a healthy 235 or a worn 250 will not get the same mileage as a healthy 250. A speedo is not accurate once the original diameter tires are removed. The formula for RPM: RPM= MPH x axle ratio x 336 divide by tire diameter. Example typical C20 at 60 mph ( a top speed which I would be comfortable with: 60 mph x 4.57 axle x 336 constant divided by 32" tire diameter = 2879 rpms . With a 4.11 pumpkin change 2583 rpms.

A 32" tire is also not actually 32" for the purpose of figuring speed (etc.); you need to use the loaded radius/revs per mile.

My 32" tires take 650 revolutions (or so) to go a mile, which would make them 31" if you were using diameter to figure speed. Using 32" gets you 630 revs per mile.

[LuckyLightning]

Yeah, this is a worn 235 for sure. Although I also checked one of my GPS apps on my phone to confirm the 40mph.

Nice video 66sub. How would the clutch slipping affect my top speed when it's not engaged? How can I check on that without tearing the whole thing apart?

[edub981]

I have been getting in the 9ish range with my 63 C30. My top speed is also around 40mph. Ive had my wife pace me to confirm that the speedo was acurate. I did get it up to 43 once but I was going down a hill. Needless to say i dont take her out for long road trips at this point. I assumed the lower speed was due to the smaller 2 barrel I put on the 292. The consistency and reliability is totally worth it until I can upgrade the manifolds to dual carbs. I don't have a tach on her yet but I have plans to put one in the right side tilt reference window. I don't use my first gear either unless I need to pull some out of the ditch, or rip out tree stumps. My clutch is def not slipping.

[LuckyLightning]

Originally Posted by edub981

I did get it up to 43 once but I was going down a hill.

it's funny you mention it, because I did get my truck up to 42mph yesterday - while going down hill. It sounds like you have the exact same issue I do.

I talked to the PO earlier today and he said he would take it up to 55mph, and sometimes (but not too often, I guess) up to 60mph.

I'm taking the truck out again tomorrow, I'll push it past 40mph (slowly) and see what happens... and probably invest in a decent tach too.

[66Submarine]

Originally Posted by LuckyLightning

Yeah, this is a worn 235 for sure. Although I also checked one of my GPS apps on my phone to confirm the 40mph.

Nice video 66sub. How would the clutch slipping affect my top speed when it's not engaged? How can I check on that without tearing the whole thing apart?

If it's slipping it will allow the RPM's to climb as though you are still pushing it in (like a torque converter does with an automatic trans).

Drive up a hill normally in high gear and note the RPM, then hold the throttle to the floor and watch it; it should only slowly climb as the truck increases speed (or doesn't ). Likewise, when you let off it shouldn't immediately drop.

Originally Posted by edub981

I have been getting in the 9ish range with my 63 C30. My top speed is also around 40mph. Ive had my wife pace me to confirm that the speedo was acurate. I did get it up to 43 once but I was going down a hill. Needless to say i dont take her out for long road trips at this point. I assumed the lower speed was due to the smaller 2 barrel I put on the 292. The consistency and reliability is totally worth it until I can upgrade the manifolds to dual carbs. I don't have a tach on her yet but I have plans to put one in the right side tilt reference window. I don't use my first gear either unless I need to pull some out of the ditch, or rip out tree stumps. My clutch is def not slipping.

Not sure about the carb you used (maybe it's not opening up all the way?), but I've had this truck with the little 250 up to about 80MPH before. I'm not sure what the actual top speed would be on flat ground, but it will run 70+ just fine.

[LuckyLightning]

Originally Posted by 66Submarine

If it's slipping it will allow the RPM's to climb as though you are still pushing it in (like a torque converter does with an automatic trans).

Drive up a hill normally in high gear and note the RPM, then hold the throttle to the floor and watch it; it should only slowly climb as the truck increases speed (or doesn't ). Likewise, when you let off it shouldn't immediately drop.

I took the truck out again earlier today and did not notice any sudden or unexpected changes in the RPMs. The truck is responsive when I throttle it at any speed/RPM, and the tach lets off gradually (I think?) when I let off the throttle. The only thing I could re-evaluate is the rise in RPMs while going up a hill because I didn't floor it, but I did accelerate: the tach didn't jump up when throttling, just gradual changes as engine noise changes and speed changes. I also want to test to see if jumps when downshifting unnecessarily.

Here's what I experience:

At idle right after ignition, it's around 1k RPM. After driving for a few minutes, idle RPMs go up to 2k and stays there during any idle while driving for an hour.

I can stay in 4th/high gear from 20mph(2.8kRPM) to 40mph(4kRPM). I can hear the engine revving very high at 40mph. I know what my engine sounds like when revved up high, and this is it. I believe the tach is accurate.

A few notes:

• After about 20-30 minutes of driving around 30-40mph, I let it idle for about 5 minutes. Getting back on the road, the idle overall seemed to reduce by 500RPMs at any speed except idle.
• When I shut off the truck, the tach rested at 1k. However, I pushed in the clutch to put it in gear (1st gear is currently my parking brake), and the tach needle quickly dropped to 0. I thought it was odd, but since the engine wasn't even running, I'm thinking it's the tach?

Will test again later today.

[factorystock]

Originally Posted by LuckyLightning

I installed a 3rd party tach I found at a salvage yard.

I would not trust the accuracy of it of the tach. A 235 is not really designed for high efficiency at high rpms. The simpliest way to squeeze a few extra MPG's is to swap in a 4.11 pumpkin ( about a 2 hr. job with no fabrication whatsoever ). A 4.11 pumpkin is easy to find in a '69-'72 C 20 with V8 auto, hard to find in a '63-'66.

[edub981]

Originally Posted by 66Submarine

Not sure about the carb you used (maybe it's not opening up all the way?), but I've had this truck with the little 250 up to about 80MPH before. I'm not sure what the actual top speed would be on flat ground, but it will run 70+ just fine.

I put on a Weber 32/34 which is smaller than the original single barrer Rochester, but not 40 mph less smaller. When I drove the truck home it had a camper on the frame and gas dripping from the carb but even then it only got up to 45mph. I just assumed these were old torquey workhorses that didn't go too fast.

[LuckyLightning]

I took the truck out again last night and could not replicate any of the issues that would indicate a slipping clutch.

Honestly, it just seems that the engine gets stuck revving 1k higher from "ignition idle" to "driving idle".

[LuckyLightning]

I should have said:

Honestly, it just seems that the engine gets stuck revving 1k higher from "ignition idle" to "driving idle" and while driving/in gear.

[66Submarine]

Originally Posted by LuckyLightning

I talked to the PO earlier today and he said he would take it up to 55mph, and sometimes (but not too often, I guess) up to 60mph...

...I can stay in 4th/high gear from 20mph(2.8kRPM) to 40mph(4kRPM). I can hear the engine revving very high at 40mph. I know what my engine sounds like when revved up high, and this is it. I believe the tach is accurate.

The tach is wrong. 40/20=2, so 2,800RPM @ 20MPH x2 would actually be 5,600RPM. Even if 4,000 @ 40 was somehow right, that would make it 6,000 @ 60...no way.

Originally Posted by edub981

I put on a Weber 32/34 which is smaller than the original single barrer Rochester, but not 40 mph less smaller. When I drove the truck home it had a camper on the frame and gas dripping from the carb but even then it only got up to 45mph. I just assumed these were old torquey workhorses that didn't go too fast.

Yeah, it should do much better than that wide open.

Originally Posted by LuckyLightning

I should have said:

Honestly, it just seems that the engine gets stuck revving 1k higher from "ignition idle" to "driving idle" and while driving/in gear.

You mean after it's warmed up? Idle speed should be around 500RPM. Sounds like you are setting the idle speed with the engine cold and your choke isn't hooked up/working?

[LuckyLightning]

Hmmm... the choke is operating fine, but you bring up a good point about setting idle speed when cold. If I set it when warm, how do I keep it from dying when starting up (cold)? I'll try adjusting it tomorrow. Although, I think the idle screw setting shouldn't effect overall engine RPMS when in gear and driving.

[66Submarine]

Originally Posted by LuckyLightning

Hmmm... the choke is operating fine, but you bring up a good point about setting idle speed when cold. If I set it when warm, how do I keep it from dying when starting up (cold)? I'll try adjusting it tomorrow. Although, I think the idle screw setting shouldn't effect overall engine RPMS when in gear and driving.

The choke has a fast idle cam that increases the idle speed; you need the set the idle speed when hot and work with the choke to make it idle when cold.

Assuming it's a manual choke, you should be able to just leave it pulled out a little bit to bring the idle speed up and keep it from stalling until it's warmed up for a minute or two (or until you get on the road and it doesn't need to idle).

It's not going to affect the engine speed/RPM going down the road, but it's obvious to me that your tach is not reading anything even close to what it should be. Even if the truck was actually running 4,000@40, that would mean that you would need to hit 6,000 to get to 60...like I said before, no way.

You should actually be running about 2,000RPM @ 40MPH (2,250RPM if someone put 5.14's in the rear).

Your 265/75R16 tires = 660 revolutions per mile. 660 x 4.57 = 3,016.2, which would be the RPM @ 60MPH.

[LuckyLightning]

Thanks 66sub. I went ahead and took the truck out today and set the idle when warm. It seemed to help, but I can still hear the higher rev difference in the engine when driving. I did take it up to 50mph (@5.5k RPMs!), but I really doubt that should be a cruising speed for the engine as it seems like I'm pushing it to its limit.

I took video of idling in between driving (a few 30 second spots merged into one longer video) to show what the tach does when idling. I'm at the point where I believe the tach is faulty. However I still think the engine is too loud when going 40-50mph. I'll try to post video of driving the truck.

Also, yesterday I put on 235/85/R16 tires. I don't really notice a difference though.

[66Submarine]

Originally Posted by LuckyLightning

Thanks 66sub. I went ahead and took the truck out today and set the idle when warm. It seemed to help, but I can still hear the higher rev difference in the engine when driving. I did take it up to 50mph (@5.5k RPMs!), but I really doubt that should be a cruising speed for the engine as it seems like I'm pushing it to its limit.

I took video of idling in between driving (a few 30 second spots merged into one longer video) to show what the tach does when idling. I'm at the point where I believe the tach is faulty. However I still think the engine is too loud when going 40-50mph. I'll try to post video of driving the truck.

Also, yesterday I put on 235/85/R16 tires. I don't really notice a difference though.

No way you are actually turning anything near 5,500RPM! Like I said, the actual engine speed will only be about 2,500RPM @ 50MPH...maybe 2,800 if it has 5.14's. That tach is definitely busted!

To put 5,500RPM in perspective, my later 250 will only do about 5,000RPM if I hold it on the floor in neutral!

The 235/85's are the same diameter as 265/75's, so the engine speed/effective gearing doesn't change.

Here's a really lame clip of a quick rev to 4,300RPM for a reference of what that sounds like:

250 Chevy six rev to 4,300RPM - YouTube

[LuckyLightning]

Thanks 66sub for confirming, and for the video, and for all your help. I picked up a new tach tonight and will install it either tomorrow afternoon or Friday. Will post an update then.

[TJ's Chevy]

Here's a clip of me and dad driving my 66 with a 292 for better reference....3.73 rear axle ratio...stock 4 speed.... advertised 30 inch tire...but probably 29 inch after weight of the truck is applied.

4500 rpm in 2nd

4300 in 3rd

almost 3500 in 4th. I've had this truck up to about 80 or 85....speedo is shot. lol. I've done 5100 rpm with this engine.....doing a burnout of course. lol Here's the vid. https://www.youtube.com/watch?v=YoiM...fzpcUXyK_5-uiw

[TJ's Chevy]

Originally Posted by LuckyLightning

Thanks 66sub for confirming, and for the video, and for all your help. I picked up a new tach tonight and will install it either tomorrow afternoon or Friday. Will post an update then.

I'm glad you got a new tach. I could tell in the vid yers was gone. If you do another vid could you do one of the engine bay as well? That 235 is capable of 4000 rpm...though I wouldn't leave it up there forever....the major torque range on that engine is between 1000-1800. The reason they are slow getting up to 50 is probably cause its worn out and waaay past its torque range. I've noticed that when I shift mine causally....shifting from 2nd at 3000 and into 3rd it sits at 1500 and acceleration from 1500 to 3000 is nice, but after 3000 rpm since I've lost alot of torque the acceleration is a bit slower...which...you'll notice in the video I posted above...in 3rd gear as the rpms got higher they really slowed down....I'm outta torque, But..a large 4 barrel carb helped.

[LuckyLightning]

Hey TJ, yeah the new tach fixed it. So far I've just idled in the driveway but I can already tell a difference. I'm taking it out again tonight and will test the 50mph "limit" again.

Is there anything specific you want to see in the engine bay? I'm happy to do the video, so just tell me what you want to see and I'll be sure to focus on it.

[edub981]

66Submarine said "Yeah, it should do much better than that wide open."
Correct!

I adjusted the throttle linkage so now there isn't any binding or kinks and now the carb opens all the way when the pedal is pressed. I (for the first time) passed several cars and went 65 mph this evening. I can't wait to upgrade to a cable style pedal for a smother action. But WOW! Much improved acceleration and top speed.
Thanks!
Now to do another mph test with my unregulated foot.

[TJ's Chevy]

Originally Posted by LuckyLightning

Hey TJ, yeah the new tach fixed it. So far I've just idled in the driveway but I can already tell a difference. I'm taking it out again tonight and will test the 50mph "limit" again.

Is there anything specific you want to see in the engine bay? I'm happy to do the video, so just tell me what you want to see and I'll be sure to focus on it.

Oh that's ok...I was wanting just a general video of the engine bay so I could here the engine idle...I've heard enough of them idle to kinda know what RPM they're at. lol. You should try 65 with that thing...heck...I've had mine up to 85. lol

[66Submarine]

Originally Posted by LuckyLightning

Thanks 66sub for confirming, and for the video, and for all your help. I picked up a new tach tonight and will install it either tomorrow afternoon or Friday. Will post an update then.

Happy to help.

Originally Posted by edub981

66Submarine said "Yeah, it should do much better than that wide open."
Correct!

I adjusted the throttle linkage so now there isn't any binding or kinks and now the carb opens all the way when the pedal is pressed. I (for the first time) passed several cars and went 65 mph this evening. I can't wait to upgrade to a cable style pedal for a smother action. But WOW! Much improved acceleration and top speed.
Thanks!
Now to do another mph test with my unregulated foot.

Glad to hear! I can only imagine that the performance must have left something to be desired before.

Originally Posted by TJ's Chevy

I'm glad you got a new tach. I could tell in the vid yers was gone. If you do another vid could you do one of the engine bay as well? That 235 is capable of 4000 rpm...though I wouldn't leave it up there forever....the major torque range on that engine is between 1000-1800. The reason they are slow getting up to 50 is probably cause its worn out and waaay past its torque range. I've noticed that when I shift mine causally....shifting from 2nd at 3000 and into 3rd it sits at 1500 and acceleration from 1500 to 3000 is nice, but after 3000 rpm since I've lost alot of torque the acceleration is a bit slower...which...you'll notice in the video I posted above...in 3rd gear as the rpms got higher they really slowed down....I'm outta torque, But..a large 4 barrel carb helped.

Just to be a stickler, the torque curve for a 235 is actually pretty flat from 800-2,800 (although there isn't much of it! ).

[LuckyLightning]

Originally Posted by TJ's Chevy

You should try 65 with that thing...

I dunno man, doing 50-55mph seems like I'm pushing it. I don't think my 235 is healthy enough for that kind of strain.

Tomorrow I'll post the video you asked for.

[TJ's Chevy]

Originally Posted by 66Submarine

Happy to help.



Glad to hear! I can only imagine that the performance must have left something to be desired before.



Just to be a stickler, the torque curve for a 235 is actually pretty flat from 800-2,800 (although there isn't much of it! ).

LOL! Ok...I was close enough. Yeah...195 foot pounds is on the lean side fer sure. But one must say they were one heck of a tough engine.

Originally Posted by LuckyLightning

I dunno man, doing 50-55mph seems like I'm pushing it. I don't think my 235 is healthy enough for that kind of strain.

Tomorrow I'll post the video you asked for.

My 292 in my 66 is 47 years old internally...all I've done to it is bolt on an intake, headers and 600 cfm 4 barrel and HEI...I rev the engine over 4000 like its a race engine. lol. Have done 5100 rpm with it before....and unless I saw the tach wrong...for a short second it may have bounced to 5500....but that's probably me dreaming....lol. Thanks on the video....and if you want...I posted a video of driving the truck a few posts above...that's my proof to ya.

[LuckyLightning]

Hey TJ, I saw your video already - thanks for posting it. I probably won't spend a lot of money on this 235, since I have a 327 in another truck that I plan to use within the next year. Right now I'm trying to figure out what to do about the gearing in my rear axle.

Do you guys think it would be easier/cheaper to replace the ring and pinion with something like this, or just replace the axle? How can I tell if that ring and pinion is compatible with my rear axle?