Inliners International Forums General Discussion Engines best gas mileage

What is the best mileage I could get with my 59 Suburban. It's roughly 3800 pounds. I have a 235 with a mono carb. Let's say I installed higher gears so I could run 70 on the interstate.
Let's say I was willing to change carb, intake, etc.

Somebody talk me into keeping this 6 cylinder.....

There are always methods on older vehicles to get better mileage, but there are always trade-offs. Putting a V8 in it will not guarantee you better mileage. An old Suburban is like pushing a very heavy brick down the road and you will never see Civic like mileage.

Define exactly what you want it to do when you are done with it and the configuration of what you have now and there is a good possiblity you will receive several reasonable ideas.

I think it depends on how much you want to tweak it. Keeping the stock engine well tuned up is half the battle. A slippage of a degree of timing or a few degrees of dwell (point gap) can result in several MPG lost. I would start with a recurve of the stock distributor and electronic ignition to optimize things. A good hot spark will use all the fuel in the cylinders and make the engine start easier.

Second, one barrel carburators are tradeoffs between milage and performance. The Rochester is especially poor. A well tuned Carter will help, or even a more modern 2 barrel carb or fuel injection. Higher gears will really help, too. The 235 is a fuel waster when spun at higher RPM's. There is a limit to this, though. When the gear ratio gets to the point that the engine is lugged while cruising, the vacuum the carburator sees drops quite a bit, which causes the power enrichment circuit to operate in the carb and the vacuum advance to drop back, even on flat ground, which will definately keep the gas milage low. This can be even worse with a performance cam. An adjustment to the vacuum can and lowering the spring tension on the power valve will help.

I would try 3.55 gears with an overdrive transmission of some kind (T-5 or Saginaw 3 spd OD, or something) or 3.07/3.08 gears with a 4 speed direct type box, like a 2 or 3 groove Saginaw. Tires should also be a consideration. Tall, narrow profile (skinny) tires will get you better gas milage because of lower rolling resistance. They should afford plenty of traction. You don't need 70 or 60 series fat tires for traction with a 235.

Another enemy is compression ratio. The stock 235 head is pretty low compression for modern gasoline. Raising the compression a few points by shaving the head while doing nothing else should help fuel economy and torque.

Finally, the engine needs to breathe well. The air cleaner should not provide a significant restriction to flow as it will act just like a choke and cause a richer mixture. The exhaust should flow well, a low restriction muffler and cast iron headers would be a good addition.

A PCV system would be a good idea to keep the crankcase clean. Whether or not it will help with fuel economy is up to debate, but it will definately prolong the life of your oil and keep the air cleaner.

Be prepared to do a lot of tuning to get good fuel economy. My truck ('50 with a 261 and 4.11 gears) weighs about what your suburban does, and I have gotten anywhere from 11 to 19 mpg depending on the state of tune it has been in and driving conditions. I suspect you should be able to do 15-20mpg on a regular basis if you tune things well. All the mods you do will also increase your top speed, but resist the urge to drive faster if you want fuel economy.

I have a 3.07 rearend and a 4 speed saginaw that I am going to put in, and plans for a head shave and mild cam for the 261.

Thanks for the info.
My plan for this truck is to drive it. It see's a good deal of Interstate travel. I live outside of Nashville but work inside Nashville. When it's more driveable we will probably put 500 miles on it a week.
I know I will never see 20+ miles to the gallon but I know I can do better than 12. The stock carb is giving me fits. It's a 1 barrel...probably rochester. I rebuilt it months ago but it has never been set up properly. I think the air horn is warped. tonight it was running terrible. I took the breather off and the idle jumped waaaaay up. The breather is not dirty. It didn't do this a week ago. This is new. Some of the trouble, maybe all, could be the trash I found in the carb a few days ago. working on that fix from another post.
I think the timing and points are set right. The engine starts instantly. I am looking for another rear axle with higher gears.
Would I be better off getting a 2 barrel and would it fit my intake? How much$$$???

A good start to better mileage is to get a Holley Webber progressive 2 brl. carb. They are available from Stovebolt supply for about 65 bucks. You will need to get an adapter also. Just my 2 cents. Hope this helps.

I have a 51 Chevy truck,stock 261 with a Holley 2300 series carb , 3.40 gears and overdrive.
The trucks gets about 20 mpg at 65-70 mph, goes up to 22 mpg if driven at 55. Around town it get 16-18.In OD the overall ratio is 2.48, the engine turns 2100 rpm at 70.This still give reasonable engine vacuum at cruising,14 inches which helps with fuel atomization as does a heated intake.In a hilly area, overall ratio of 3.08 might be better as mentioned above.
The frontal area creates a lot of drag on these old trucks, the weight is not a big factor on the hiway unless you're ramming up hills all the time.

Tony...i'm in awe of those numbers. lol. once the 250 hits the road..we'll see. so you really reccommend the holley? i was thinking multi-progressive. prolly 3x1's. for effect and performance. but..until the cash comes around. is the holley a good choice? i know a OD would help immensely..do you run a T5? or one of the other ones?

just a few Q's. trying to get it right the first time...thanx.

SamWise.
.for every 1 mile of road theres 2 miles of ditches.