what gears in your y block truck?

[Rusty_S]

Originally Posted by charliemccraney Z spec T5, 2.95, 1.94, 1.34, 1.00, .63. As far as I know, it's the stock 3.70 out back. My engine doesn't seem to like the overdrive ratio; I get the same mileage in 4th as 5th, at the same speed. Gotta find out what's going on there. that overdrive is too much I would say. Thats why your getting same gas miliage or worse in overdrive as you do in 4th. In overdrive you basically have a 2.33:1 overall ratio where as in 4th gear you have a 3.70:1 overall ratio. Honestly you would want to keep it lower than 2.50:1 (lower geared such as 2.50:1, 2.70:1, 2.90:1, 3.08:1, 3.27:1) when in overdrive or your rpm will be dropped too much and your miliage wont be improved or it will decrease. If you have a tach you should shoot for 2500 rpm at highway speeds that is the most effcient a internal combustion engine runs at. My 2.50:1 rear axle 78 is at 2,500 rpm at 80 mph so with your 2.33 in overdrive you would have to be over 100 mph to get up to that ideal 2,500 rpm.

[charliemccraney]

I think you're probably right. But I haven't had a chance to look at it. I think, because of the build of my engine, it should be somewhere between 2500 and 3000 at cruising speed. Currently, 2200rpm is about 80mph in overdrive and 3500rpm is about 80mph in 4th. I just looked at my notes. I got 13.33mpg in 4th and 14.28 in od. So od is slightly better but not what it should be.

[Rusty_S]

I wouldnt want to condone this but I would say run her up to 2500 rpm on the highway regardless of the speed and see if you get better miliage then youll know your rpm`s are too low. But I dont want to recomend speeding like that though.

Odds are your gearing is too low. I mean you only got a 0.95 mpg increase so that indicates to me personally that your drop in engine rpm helped but the drop was too much. 3500 rpm personally I would say is too fast but if thats what the engine likes thats what it likes. 2500 - 3500 though should be ok though. Iknow on mine I don think I would want to put her on the highway cause running at 3,000 rpm at 80 mph with a 4500 redline (what ever the 292`s had as redline stock) I just dont like the sound of that. But thats just me.

[charliemccraney]

What I plan to do is try some smaller diameter tires which will be roughly equivalent to the gear ratios available until I find the ratio which works best. Aerodynamic drag is significant enough at that speed that simply traveling at different rates and comparing fuel efficiency may not be so accurate. Plus the fact that 80mph is already 10 or 15mph over the speed limit.
Another possibility is that the vacuum is opening the power circuit and dumping fuel but the engine wasn't loading up so this probably isn't it. I'm not finished with the carb tuning so it is a possibility. My motor should have redline around 6000rpm. It starts pulling hard around 1500rpm. 2200rpm is barely in the powerband. Around town, it really likes the 2500 to 3000 area.

[Rusty_S]

if you have a vac secondary like a holley 4V you can swap the springs out where they open at specific rpm ratings. You can do that and go with one just over 3000 rpm that way they dont open on their own till 3000 rpm or you go full throttle.