I have 66 c10, had a muncie 318 3 speed transmission (w/out overdrive) in it. Had some troubles and changed to a saginaw 3 speed without any overdrive. the old muncie was getting me about 15-17 mpg. with this saginaw it seems to have dropped to around 10-12 mpg. does that sound right? is the gearing in that saginaw that much different?
Per the attached site, your Muncie 318 had either a 3.03 or 2.42 low gear. http://www.drivetrain.com/parts_cat...haul_kits/muncie_3_speed_ld_overhaul_kit.html Per the second link, your Saginaw can have anything from a 2.54 to a 3.50 low gear. http://www.musclemotorparts.com/trannys.htm I find it easier to explain gear ratios by the 1st gear launch ratio and final drive ratio. The launch ratio is the transmission 1st gear multiplied by the rear axle ratio. For arguments sake, let’s assume you have a 3.70 rear axle ratio since this is fairly common in older stick shift trucks. High gear is generally 1-1 (direct drive) , unless you have overdrive which can drop to .74. .80, etc. Final drive with no overdrive is just the rear axle ratio. Overdrive lowers the final drive ratio. For example if the transmission has .74 overdrive high gear ratio x 3.70 rear axle, this lowers the final drive ratio to 2.74. Depending on what combination you had before compared to what you have now, yes – it could make a significant difference in overall mileage. High gear in either transmission will be the same, direct drive – so once in third gear there should be no change in mileage. Getting to 3rd is another issue. Again, depending on what engine you have and where it makes torque, here are a couple of examples: 1. If you had a 2.42 low gear before x 3.70 rear axle ratio you launch ratio will be approximately 8.95. 2. If you now have a 3.50 low gear x 3.70 axle, your launch ratio will be more like 12.95. Obviously this has the capacity to spin the engine much faster going through the gears until you get to 3rd. Depending on how long you stay in this gear, you may use more fuel – and will also create more torque so pulling will be easier. The extreme example of this is the granny 4 speed with a 1st gear ratio of something like 7 to 1. Launch ratio would be over 25-1 which is why you can pull trees out, etc. 3. On the other hand, if you previously had a 3.03 low gear x 3.70 axle the launch ratio would have been 11.2. 4. And if you now have a 2.54x3.70 the launch would be at 9.398. If you’re running a heavy rig with low horsepower or the engine makes better torque at a higher rpm range, this may actually lug the engine producing worse mileage. An example of this was my last 54 Chevy sedan. With the 235 six I got about 10 mpg because I drove around with my foot on the floor all the time trying to get it up to speed. After I swapped in a 283 V8, my mileage actually improved to over 16 mpg because I didn’t need to use very much throttle to get it moving easily. Obviously variances in the 2nd gear ratios will have the same effect. It appears whatever combination you had before was more favorable to the rpm range where your engine makes its best torque. Perhaps you can determine what gear ratios each transmission has by data on the linked sites, or you can determine the ratios by pulling the side covers and counting the gear teeth. Hopefully this makes sense to you.
saginaw's are heavier = worse gas mileage just kidding................. sounds like a pretty large drop in mileage when final drive ratio remains at 1:1. markeb01 is right about the gearing thoughts however be certain you are comparing the same type of mileage, if you are driving "in town" vs "highway" as previously used to determine your mileage, more aggressive burst in getting up to speed, etc. is the truck runing properly? need a tune up?? plugs, filters, ignition, carbeuration?
Mark, Leyland thanks for all the info. i am talking about hwy miles and the engine is a 250 6cyl. i do have some minor issues with getting the truck to idle properly it idles at about 800 rmp. i know this is causing some excess fuel burning, however i had the same conditions with the other transmission. sounds like i may need to add a tach and see how much cranking the engine is doing at different speeds. it does seem like i have to hold the accelerator down a bit more to get 55 mph on the highway. does that indicate that i may need to re-gear the rear end maybe go higher gears or lower?
It's hard to know what to consider next until you have some baseline data. An inexpensive tach can help determine if a gearing change will help, and which way to go. A vacuum gauge is also an worthy investment to help with driving technique and engine tune. With both gauges available, it's easy to see if you're lugging the engine or over speeding the gears. The vacuum gauge is also very helpful in diagnosing engine problems. It should read a smooth 16+ inches at idle, and the higher you can keep the reading while driving, the better the mileage. If it reads low at idle or jumps up and down, a quick Google search will explain what the various readings indicate.
in my experience 3:73 gearing in the differential was a pretty common ratio, if you have checked everything else then you could always change to a higher gearset like a 3:42 or even 3:20's Get a tach first and see what your engine is doing at idle, speeding up and at cruising (60mph), this will tell you a lot about your gearset.
