I just thought of this...roughly how much of an increase in gas mileage would I get from changing my toploader out for a T-5. I have a '66 with a 289 4-V, 3.25 gears, and I get a lot of highway miles.

 

The quick answer to your question is: ALOT!

The more complicated answer is: IT DEPENDS...

Gas mileage depends on a number of factors. And alot of "highway miles" doesn't mean anything different to me from street miles, since I live in the land of freeway parking lots in Southern California. If gas mileage is a major concern, then maybe you should swap out the 289 for a 6 cylinder![:@] If it's not a major concern and you are just "wondering", then your driving habits, styles, frequency will matter most. Just my $0.02.

 

let's look at it another way, how much in gas will it take to make up the cost of a new bellhousing, clutch kit, clutch fork, tranny, tranny crossmember and a modified drive shaft?

 

A very intelligent response - that's why I love this forum.

 

A smaller engine does not necessarily always mean better mileage. If your going up a hill your going to have to have you foot in the 6 cylinder, but a V-8 you would just have to barley crack the gas. I know this from experience.

 

Another thing to remember with small displacement engines is RPM. Lets say you have a carbed 302 with a mild cam, and it makes around less than 200ftlbs around 1500-2000 rpm. You really dont want to have a T5 infront of a 2.70-3.00 gear. Why? Because the rpm is so low at cruise that the engine cant make enough power to move the car efficiently, you run the risk of washing the cylinders and wasting the rings. With a large engine or an injected engine its not as bad as a carb. Newer engines are designed to run that low at cruise, and the fuel map is set up to allow for it. A carb has a tough time dealing with that aspect of operation. It will be in the idle circuit instead of the main, and it wont like it much especially when you need to pass someone.

That being said, an OD trans is a wonderfull thing and will save gas, just be sure to keep your cruise rpm around 1900-2200 rpm with a small ci carbed engine.

With your 3.25 gears and a 25-26" tire you should be okay with a T5. Go up to a taller tire like a 27 or 28 and then you will see what I mean the hard way. Figure on at least 3 mpg from the swap, provided you arent into the gas alot and have lots of highway time. It could be quite a bit more.

As an example I have a 70 GTO with a 455 (go figure right?) well with a 3 speed auto it was getting around 13-15 on highway drives. I installed an overdrive in it, and used a Q jet carb and picked up to 20-22mpg with it. Mind you it also had 2.93 gears, 28" tires and 500ftlbs at idle. If would grunt up any hill in 4th and I didnt have to move the throttle much at all. Its a matter of tuning and driving style that net you better mileage with the same parts, add an OD trans to that and you can really see an improvement. A smaller engine doesnt have the torque to pull that car around like that, but the big one worked alot less doing it and got better mileage.

That same car around town sees an average of 11mpg, really big engine in stop and go driving, and my heavy foot conspire to eat alot of gas. On the interstate it ran great.

I also had a 74 Firebird with a 350 in it, I could never get it over 10mpg no matter what I did with it, yet all my 400+ ci cars were getting much better. Not enough engine for the weight of the car, just like Ideal alluded too.

 

IMO, the Q-Jet is the most tunable and, unfortunately, misunderstood carburetor around. Glad to be in the company of someone who knows what they are talking about [sm=icon_cheers.gif]

You can get a T5 built with an 0.80:1 O.D.. Which is much more usable on a carbureted engine. Still, it would take a lot of driving to make up that kind of investment, even at $5.00/gallon.

 

I understood the cost factor, and I already need to replace the crossmember, clutch and flywheel, and various parts in the transmission itself(syncronizers, gears, and a few others). An adapter plate would serve me well on the bellhousing being that I don't use the car in any form of racing (this is a daily driver), and the current carb setup is more than adequate (750cfm Holley 4-V w/ Mechanical Secondaries so I control the back two barrels). I also understand the idea of putting the engine under strain by running too low on RPMs: to low on RPMs means not enough grunt to move the car properly. I only rev the engine occationally in order to remove some carbon from the pistons, other than that I take it easy on the car because I don't like paying for more Amoco Ultimate than I have to. I already average from 18-20 MPG and I go about 250 miles a week.

 

Yeah, ive been at a stop at the bottom of a hill and let off the brake and my 302 pulls me up the hill without even pushing the pedal in.....obviously u go like 2 mph and it wouldnt work on that steep of a hill but u get the point.......

 

I love my Q jets for daily driving and economy, I even have put them on some of my Fords. once you know how to make them work, they are tough to beat. I do run Holleys and Demons on my race cars though, limited number of good Qjet baseplates in my shop.