How much overdrive is too much?

[retrorocket]

My 66 Ford truck is getting a new 2.50 rear gear and an overdrive trans with a .74 overdrive.That yields an 1.85 finial drive ratio.

It will have around a 30 inch tall tire so it should be running around 1100 rpm at 60mph.

I wish it was lower but a 6 speed for a mustang with a .5 overdrive is $3800
so I guess I will live with this gearing.

Maybe I can find a 31 inch tall tire that is not to wide.

I have not figured it yet but what I am interested in is the engine displacement per foot traveled.

What do you guys think?

[SVOboy]

Sounds like a pretty good set of ratios to me!

[atomicradish]

As long as you don't need any acceleration I don't see the problem. There are some more mechanically savvy people here who might know better though.

[usedgeo]

I think you are just going to have to try it. It will depend on your cam and how fast you drive. I recall many years ago ratios approaching this were tried with a 302 in a Mustang. If memory serves me correctly mileage peaked at around 1400 RPM.

[Fourthbean]

I am VERY curious to what your results will be. Do you have a baseline of what your fuel economy was before changing out the transmission, or is this more of a build up and make it for FE in the first place. Either way that truck stock couldn't have gotten more than 15-18 on the highway at 60. If you can pull more than 25 without any special techniques I will consider getting myself some taller gears and new rear end, or just get a different classic to cobble up.

[omgwtfbyobbq]

Imo, it goes like 3L@1500rpm, 6L@750rpm, 1.5L@3000rpm, all at 55mph, etc... so 1100rpm@55mph instead of 1000rpm should be np, especially since it's an older motor. It's a 5L, right? No matter what you should be o.k. since you can always downshift for hauling something really heavy... Unless it's an auto, then changes can be a PITA if you can't control kickdown.

[Bill in Houston]

I don't think that there is any reason to try to get below 1100 rpm with taller tires.

[Telco]

You may find yourself getting worse mileage than you would with lower gearing. I had a 79 GMC that had a 350, turbo350, 275/60R15 tires and 3.08 rear gears. It got 14-15MPG at 65MPH. I installed a 700R4 with no other changes, and mileage dropped about 2MPG at 65MPH. But, it would get the 14-15MPG if I went 80MPH. As it turns out, with the 700R4, it pushed my engine RPM out of the torque band on the low side, and the engine had to work harder (and use more fuel) with the overdrive.

Also had an 89 Camaro RS (before the truck) with an ADS superchip and no other changes. On 93 octane the car would net 30MPG at 80MPH highway. At the time I was in the military, and was travelling between Ft Detrick MD and Anderson SC every weekend to see my wife and kids, who were staying with her parents while I was arranging housing. I'd fill up at Frederick, then not have to stop again until I reached the SC border. Don't know what the gearing on that car was, but I was running about 2000 to 2200RPM at 80MPH. If I went much faster, I had to stop a whole lot sooner for fuel. Never did try going slower because I was on a tight time budget, and gas was cheap at about 1.20 a gallon for 93. When not traveling, in-town mileage was about 25-27MPG.

Do you know what RPM you currently get the best mileage at now? Ideally, your final setup should run at about that RPM at the speed you want to cruise at. If you are making these other swaps, then contact Comp Cams and tell them what you are doing and they can recommend a camshaft swap to help get better fuel economy at the RPM range you want.

[retrorocket]

It comes out to 1075 rpm with a 30 inch tire.

I will try that and see how it works.


Here is a handy link for tire and gearing stuff.

http://www.csgnetwork.com/multirpmcalc.html

I does not have a 302 but a very nice 390.

It has a small solid cam in it but not small enough to cruise at 1075 and even less at 55mph. (986 rpm)

So I will think about cams and have a new solid cam ground for it.

Maybe around 190@.050 with 116 lobe separation?

I have a Carter 500 four barrel and that should work pretty well but I can always put a quadrajet on it. (quadrajets have a very very sensitive main circuit in them)

I have several manual controllers for electronic carbs so I can run one without a computer.

The truck got 11.5 mpg with a 352 and the original crusimatic three speed auto trans and a 3.50 gear with 30 tall tires. It had a 500 holley two barrel then and they are very poor carbs for economy. Holley does not even recommend them for street use.

It weighs 4300 pounds.

I think I can get in the 20's with it but how high into the 20's I am not sure.

retrorocket

[MnFocus]

The 390 should have enough grunt with that ratio - you can always drop down a gear to hit the power band. What compression ratio ? Be careful of a high VE cam and high compression - may cause some detonation issues. Use more exhaust timing to scavenge as much as possible .Big fan of the Q-jet myself -small primaries , Big secondaries and the metering rods are so tunable , just set the secondaries a bit tight (slower opening) . I think low to mid 20's are very achievable .