higer ratio diff
 

Hi Guys,I'm currently using my 37 pickup for daily commute of 50 miles round trip,I enjoy using the old truck but the gas bill is horrible.I'm running a 4.11 diff with 5 speed.I have a 3.78 converted to open drive in bits but need to get a driveshaft made up.Will changing to this bring my mpg down enough to consider feasable considering cost of setting it up and the new shaft? The trip is a 50/50 split between town and open road.I'm also using a 390 cfm holley with 50s main jets, should I go down a size or two?Thanks in advance Tony.

Re: higer ratio diff
 

Tony i dont think you will not gain much ,the 5 speed should give you that , gas at $9. a gal your using 25 gal a day , someone else may chime in .

Re: higer ratio diff
 

I am just getting a 3.54 put together now for the reasons above Tony, gas is a bitch out here now.

Re: higer ratio diff
 

I don't think the gear change will make much difference and I don't think the engine will respond favorably to smaller jets. That 390 Holley is '"on the edge" right out of the box. I would suggest getting the CR up as high as possible, getting the intake system as hot as reasonable, tuning ignition for maximum "cruise" efficiency and going easy on the right foot. Try installing a vacuum gage and drive in a manner which results in the highest consistent vacuum readings.

Re: higer ratio diff
 

I don't think that "5 speed" tells as much about his gearing as Tony would think. Those T5 ratios are all over the place, but with the various overdrives and other ratios would suggest that the top end was already well geared, even with the 4.11 rear. Often the first gear is mostly useless, especially around here, where the common donor for the trans is the wide ratio S10. Evaluation of trans applications is all in the numbers and the rear ratio is only a third of the story. Good Luck: Fred A

Re: higer ratio diff
 

There is a guy up your way who fitted a new Stromberg to a 34 and got 25X MPG

Re: higer ratio diff
 

Your T-5 5th gear ratio will dictate if a switch to 3:78s will be enough of a difference to offset your expenditures, and the height of your rear tire. For arguments sake, let's assume your 5th gear is a .8 (not the best one). I have 3:54 gears in my roadster w/.8 T-5 and @ 70 I'm at 2500 rpm with a pretty tall rear tire (approx 30"). Gas mileage is hard to figure since I don't have a speedo (and it is off the road at the moment awaiting it's new engine) but if I had to guess I'm probably say around 17-20 MPG as a conservative estimate, US, and at speed. If I babied it I would guess 20-22 MPG. I'm running a "Y" adapter w/2 94s, progressive, former engine was a stock 40 Ford, the new one is a 59, 4" crank, 030 over, stock cam, probably around a 255 cube. Once it's together I now have a GPS so I can calculate gas mileage but that will not help you now - I will post mileage figures then.

Re: higer ratio diff
 

Thanks guys for all your input,I'll give the diff change a miss at this stage,I'll pull the 4 barrell and run the original single 94 and see what that does.I do the same route each week so will be easy to compare.I have very High compression now because the engine machinest planed hell of my edelbrock heads,I spent two days hand sanding the domes on the heads so they wouldn't hit the pistons. Hot air chamber on manifold is working.I 'll try driving with vac guage hooked up. Point 85 5th gear about 2300 rpm at 60 mph.Cheers Tony.