Hello and thanks for reading my post. I'm looking for suggestions and I know here is the best place to get them.

I have had my 1968 Ford F100 shortbed for about 7 months now. It came with power steering, power disc brakes and a 351 windsor and a C4 tranny. I love the truck.

I recently replaced the tired 351w 2 barrel carb engine with a rebuilt 351w and a 4 barrel carb, RV cam, and the heads ported. It definitely has more power but still feels a little sluggish and gets terrible gas. Before I was getting about 9 mpg and now about 8 mpg. I drive mostly city but some highway. I do plan on taking some longer trips once I get some paint and interior work.

I'm looking for better mpg's. I looked into my gears and I have 2.75 and it looks like from a 1974 Torino. So at some point I believe the 351w, c4, and rear end were switched out.

Ok so I think my gears are lugging the engine down and causing less mpg, at least that's what I've read so far.
At 25 mph the rpm is 1100
At 40 mph the rpm is 1700
And at 70 the rpm is 2800
So I've been reading that going higher like 3.50 gears would give some more lower power and increase mpg. Any suggestions?

Also I have came across an AOD just rebuilt from a 90 mustang 5.0 at a fair trade. So what would be a good gear ratio for my truck? And what gear ratio would be good for an AOD?

I have 235/70/15 tires.

Thanks for suggestions
Kenny

Also I have been looking for write ups on AOD conversions for F100 but haven't found much. I've read that it's pretty easy but wondering about the cross member?

 

Read this article on the AOD: Ford AOD Overdrive Transmission | Hemmings Motor News

What you're looking for is a final drive ratio (overdrive ratio times rear axle ratio) that will provide a highway-friendly RPMs at 70 MPH... typically between 1900 and 2100 is considered desirable for overdrive. For example, an AOD's overdrive is 0.67 so a rear gear of 3.50 will result in a final drive ratio of 2.36.

Ya have 2.75 rear gears? There is not much more to go lower without being a real dog... a final drive of 2.40 is about the lowest I've heard. That engine better be making lots of torque down low like an FE or is in pursuit of top speed.

8 MPG? There is tuning ought to be done or changing your driving style. But that is I assume stop-and-go driving which can vary widely.

I suggest doing a highway MPG test run at 60 MPH... about a 50-mile loop of flat highway to get an accurate gauge of efficiency. Do the same test loop with every modification (or combination of modifications) for an apples to apples comparison. Sunday mornings are a good time to do one since there is typically little traffic.

 

I just did the calculations for a project with the same engine and tranny. With 3:50 gears it would turn 1846 rpm@70mph, 1967 rpm@70mph with 3:73 gears. If you wish to play with the combo's try Gear Ratio Calculator. I didn't read HIO's link but, I think 88 was the last year for a straight AOD. After that electronics were added becoming the AODE. Something to think about.

 

Hi Kenny, Before you throw more rocks on the pile, maybe a little more work with what you have is in order. "So at some point I believe the 351w, c4, and 2.75 rear end were switched IN." CORRECT none of those were offered in a 68 F100 that I am aware of.

You may raise the mpg a little with city driving but our trucks are not candidates for high mpgs in town driving, but if you have any highway driving factored in you should be higher then 8mpgs.

I suggest that you take a closer looker at the whole package. A 351W with rv cam should not be over sluggish if it is specked right. The smaller 302 was adequate small engine. What carb are you running, what cam? what ign? what is the timing set at?

I see arguments on jacking up one wheel vr two wheels and count the revs, but I believe getting the truck on a "garage floor" level ground have a friend with you mark your left rear tire and your driveshaft at the chunk. Roll the tire one revolution while counting the revs of the ds. This will give you the actual ratio as it is as real time as you can get as this is the way it runs.

You speedo gear can be off a tooth and help give you a bad reading. There are some good smartphone apps like DigiHub that you can use while you have your friend with you to check just how close it and the truck are.

Please spend all of your money if it makes you happy or work on what you already have to see if you can make it better first. A 2.75 rear is a cruiser gear and will suck life from the engine and may be too low for the W. I tried that and went back to 3.25 for better all round performance.



John

 

2.75:1 is a high gear ratio. A 4.11:1 would be an example of a low gear ratio.

A (numerically) low gear ratio is a high gear set. A (numerically) high ratio is a low gear set.

 

The 3.73 ratio wasn't available in a 9-inch rear end from the factory. It would be a 3.70.

The AOD was produced from 1980-1993. Pre-'88 AODs should be avoided. '88-'93 are the better AOD models.

The AODE (Electronically shifted) came out in 1994.

The 4R70W came out in the 1993 Lincoln Mk VIII. It's also electronically-shifted.

The AOD, AODE and the 4R70W transmissions are all related. The 4R70W is at the top of the evolutionary changes made to the AOD and AODE.

At minimum, the AODE and 4R70W will require a stand-alone shift controller.

 

Thanks for the replies guys. That's what's great about this forum as you can get suggestions from guys that have, been there done that.

I have been doing more research and looking at my current set up. First off my Holley 600 is used and I know this didn't help. I had another Holley 600 rebuilt from The Carb Shop in Santa Ana which I got back this weekend and installed. It already feels better and I'm sure it will raise my mpg.

Also I have the Duraspark electronic ignition which works great. My timing is set at 10 BTDC.

So anyways what I was thinking was this Torino rearend probably had a 25" tire and I'm running a 27" tire so that would through the gear off. This is what I understand from what I have read. After looking at the gear charts to get back to the Good Daily Driver and Performance I would go to 3.50 gears.

Going to a 3.50 gear with my set up would be best. With or without the AOD. the C4 would be similar in the city but would benefit on the highway.

My search will continue for the best set up. Any more suggestions would help while I try to understand this all. Also I would like to hear whatever combos you run and like.

Thanks again
Kenny

 

Kenny, your in the same boat I am. I've been looking into the gear ratio situation for some time now.

I have a mild 351C with AOD and 31" tires. The current 3.25 gears are just way too tall. I can't drive fast enough to get the rpms up.

SO, I need to go with 4.11s, because with the overdrive, that will provide a final ratio of 2.75. Just like you have now. For cruising, that is a very good ratio. But not for all-out performance.

Before you go all out swapping gears and trannys, just make sure what you got is set up proper and tuned correctly. A C4 and 2.75s will be a good economy combination if everything else works.