Say I'm getting about 12 or so miles to the gallon... If I put in 3:80's..where's that going to drop me to? 347 with a C4. Trying to figure out not if I can afford the swap, but rather the cost of driving the car!
12 mpg . When I went from 3.25s to 3.89s my city mileage increased 'cause I could cruise arond town in 4th gear. But the hiway rpms went up and mileage down. That's when I went to an overdrive trans, best of both worlds.
yeah I dont speed..but I next to NEVER waste time getting up to the speed limit..I have to fill up usually once every 6 days.. but thats interesting..I might be outta luck unless i got a 4 spd
My Sprints mileage went up with a swap to 3.80 gears. Of course it has an AOD in it. If your engine is tuned well. Timeing/carb size etc... You should be seing 12 to 14mpg. Dont forget tire diameter has an effect on gear ratio too.
Depends if you can keep your foot out of it with the new fun factor of a gear swap. I'm guessing NO. Not accounting for that it may actually improve. Depends where your engine is most efficient when cruising. My car with the 4:11's at 60 mph is surprisingly efficient and hardly works at all to keep speed. It loves that 3000 rpm area. Kind of a sweet spot.
Mine improved from 2.89 to 3.80 gears. As long as I kept my foot out of it. I still get 20-22 mpg with 3.80 gears, if I drive like a granny. Otherwise, I have driven it in ways that get me one gallon to the mile!
Running a high ratio like a 2.79 is bad for mpg because your carburetor is still running rich in the idle circuit. Once you get the rpm up then you’re in the cruise circuit and it takes less fuel because the engine is running lean.
I think rthomas is correct...my 302 and 5.0 liked to cruise in the 2800-3600 rpm range. Otherwise they would suck the gas. I am now cruising 60 mph at right around 3000-3200 and it just purrs along at 20 mpg, with a 750 cfm holley double pumper. Of course, if I stomp it just a little...the numbers change dramatically.
I have no experience with low geared Fords other than my truck but my GTO would average between 4-6 with 4.10's!
I get 15/16MPG city and 21/23 hwy with the Sprint. With the 3.80s' and 25" tire/AOD it turns 2400RPM at 65MPH and 2200RPM at 40MPH w/o OD in city driving. MInd you now this is with a 351 with a fat cam with LOTS of overlap (havent bothered to try and spec the cam yet) Being out of the Idle circuit is definitely a must if you want any kind of mileage,as well as where the engine starts to make power and where in the rpm range your cam starts to do its job. The Sprint comes to life around 2400RPM where the cam starts to really do its job...Soo...Lots to consider Matt...Type of converter you are running has to figure into it as well. I am still running the lame stocker in my Sprint (stalls around 1800RPM)
Matt, I get 16 hwy and close to the same in town with mine and 4:00 gears and our engines are real close. I cruise about 3,400 at 65mph. Put the 3:89 in your car will like you for it. If your worried about mileage get yourself a cheap small car or a bus ticket
Your mileage shouldn't change dramatically in the same car no mater what you use for gears unless it is all stop and go traffic. The one thing that affects mileage more than anything else is the driver. I get 21 MPG from my Maverick (all stock 302 & C4) averaged ove a year. That includes the in-town and highway miles. If I had to guess I would say that I get better mileage on the road than in town just because there are fewer stops and starts. Fuel consumption is a product of the weight and wind resistance of your vehicle. The quicker you accelerate the more fuel is used to get to the speed. The Mean Specific Fuel Consumption of a vehicle doesn't change because of gear ratio unless you are spending a lot of time accelerating hard. The carburetor has an idle circuit, accelerator circuit, transfer circuit, cruise circuit and a power circuit. Accelerating hard uses at least three of those at the same time. That is where your mileage goes - rich mixtures that waste fuel. Retarded timing also wastes fuel but so can timing that is advanced too far. Over-advanced timing can also burn pistons and valves. If you aren't getting 20 mpg with your Maverick or Comet then you are wasting fuel.
