What should my MPG be?

[Roger Bray]

Hi Guys,
It's been a long time since I've posted so I thought I'd stop by and run a question by y'all.

I have an E-M that I built about 10yrs ago and I'm getting about 11mpg.
I'm wondering if this sounds about right.
Here is my setup:

351 crate listed at 350 hp by ford.
Duel intake manifold with a 650cfm vac secondary holley.
Mild cam .. don't know the specs

Tremec 3550 5 speed.
Ford live axle with 3:73 gears.

17" rims

My driving is mostly on country roads at about 60mph.
Driving style is conservative.

The car is running well so I don't want to change anything unless the mileage sounds low.

Thanks for your help.

PS. I forgot what I have the timing set at.

[Rick Parker]

What rear tire diameter?

[jwd]

Something doesn't sound right. 11 MPG out of a very mild motor? My 351 dyno'd at 415HP and I got about 12 in town and 22 on the freeway. TKO 600 (.82 5th) and 3.27 gears.

Jim

[PeteF]

My 351W with 365HP and 750 Holley gets 13.2 around town. Richmond 5 speed with 2.88 rear, 295/50-15 rear tires.

[Dwight]

I have a stroker 302 to 354 and I get 18 to 22 mpg.

Dwight

[Bannon]

That does sound low. My first SPF with a 460 motor with 486hp/510ftlbs torque. Got 16mpg in all around driving... I was amazed, but chalked it up to the fact that if I wasn't putting my foot in it, moving only 2600lbs around town would not make that motor breath very hard...

[Cobrajames]

I have been getting 12 to 13 mpg in my SB 427 with 512hp/521ftlbs. It has a TKO600(.82 5th) and 3.46 BTR rear end with 295/50-15 tires.

[Rick Parker]

With 3.73 gears and a mild build it would appear the engine may be operating in the upper limits of the cam. What Rpm are you seeing at 60mph.

[Silversmith]

I have a 427 FE sideoiler, single 4, 4 speed & 350:1 gear. My tires are 15's & 26.5 dia. She's a thumper with 500+ hp. I did 200+ miles today in the white mountains N.H.. Up the Kangamangus hwy. & back home. My foot got a little heavy more than once I got 11 mpg average!! About the same as my snowmobile But way more fun Me thinks something is afoot with your sbf You might consider fuel injection! it usually improves milage alot Or a rebuild

[bobcowan]

Lots of things can effect your fuel mileage, so you need to do a little investigating to see what's going on.

The first thing to check is mixture. With modern unleaded fuels, oils, and plugs, reading the spark plugs is not nearly as accurate as it used to be. Don't waste your time trying to tune that way. Get an LM-1 and collect some real data.

What is your dynamic compression? If you havn't checked compression, then calculate your dynamic compression from one of the on-line sites, like K-B Pistons. If your compression is low, the engine won't be very efficient.

Take a look at your vacuum. It doesn't take much to temporarily run a vacuum line out the hood and around the windshield. Connect to a diagnostic vacuum gauge from Sears (or wherever). Low vacuum = high load = high fuel consumption.

Now take a look at your carb set up, especially the power valve and secondary springs. While cruising, I'd almost bet your vacuum is low enough to open the secondaries.

Don't forget to check fuel pressure and timing. If you have vacuum advance on your distributer, make sure it's adjusted properly.

Use a program like Desktop Dyno. If used properly, it's fairly accurate. Plug in all the right numbers, and take a look at the predicted torque curve. Your cruising rpm should be just to the right of where it starts to flatten out.

If the torque curve doesn't flatten out (relatively speaking), your cam doesn't match the rest of your engine bits.

And don't forget about normal stuff that effects every car - tire pressure, oil viscosity, trunk load, etc. 2-3 psi of air pressure in the tires can sometimes make a big differance.

Obviously, big power engines in small lightweight cars can get decent fuel mileage. But you have to be careful about how you set the engine up.

[Roger Bray]

Hey All,
Thanks for all the great info.
I have some investigating to do and I will report back.

[mikiec]

Not to Hi-jack, but I love the Kangamangus hwy.

Oh, by the way.. 302, 3:27 gears t-5 approx 24 on the highway.

[Roger Bray]

Well I've been reading my instruction manuals and have learned a few things.
First let me say that my main goal in building this car was to learn about cars and I still have a long way to go.

I've disscovered that I made some mistakes install my carb.

First. I didn't install a hose from carb to the Vacuum advance on the distributor.
Just a dumb mistake .... I didn't read the directions carefully.

The instructions go on to say that a line should be run from Full Manifold vacuum source to the air cleaner. Guess what? I didn't do this either.

Also, I could have very well have installed the incorrect spring to open the secondaries.

So, my plan of action is to address the issues above and see what results I get.

Thx,
Roger

[Roger Bray]

Correction.
I do in fact have a vacuum line connected to the dist.
Guess I'm not as dump as I thought

[xlr8or]

It's a Cobra. Gas mileage is not relevant, Only that it run properly.

[charles roybal]

MPG must be measured by SPG (Smiles per gal.) in a cobra.

charles

Drive with your lights on.

[bobcowan]

Originally Posted by xlr8or

It's a Cobra. Gas mileage is not relevant, Only that it run properly.

It is very relevant. To get good mileage, you have to select the proper bits that go together, assemble and tune the engine carefully, and make sure it's as efficient as possible.

An efficient engine makes good power everywhere, and gets good mileage at the same time.

If your mileage stinks (comparatively), then you're not making nearly as much power as you could be. Why spend all that money on cool shiny speed parts, then dumb it down with poor tuning?

I have a big displacement engine that makes well over 600 dyno proven hp. And yet it still gets 20mpg.

Besides, what's more fun - driving, or stopping at the gas station?

[Got the Bug]

Originally Posted by Cobrajames

I have been getting 12 to 13 mpg in my SB 427 with 512hp/521ftlbs. It has a TKO600(.82 5th) and 3.46 BTR rear end with 295/50-15 tires.

Pretty much the same thing with my 402 SR/TW Roush. SPF with the same set-up.

[YerDugliness]

My replica gets between 22 MPG and 25 MPG on the highway, putting along about 70 mph.

My drivetrain is (I THINK) a box-stock 5.0/HO engine with the T-5 transmission out of a 1989 Mustang GT, just converted from FI to carb.

3.70 gears, 0.63 overdrive, 2.95/50X15" tires on 15" Cragar Drag-star wheels.

Before I got the car it had a 780 CFM Holley on it, reportedly ran like crap and got very bad gas mileage. Right before I purchased the car the previous owner switched to the Edelbrock Thunder (575CFM, I think) and it runs great now.....you might want to check your carb.

...and, I'll second the recommendation that you spring for a good tune. It can be the cheapest horsepower you'll ever buy, and get you better mileage at the same time.

Cheers!

Dugly

[Roger Bray]

Geez ..... no it won't start.
Yesterday after posting I noticed the rubber plug used to block the brake booster vacuum port was cracked. I preplaced it with a new plug(cap) and now the car won't start. I tried again with the cap removed ... same result.
I've got fuel pressure and gas goes into the carb when I move the throttle linkage.
Last night I removed the plugs cleaned them, reinstalled being carefull to reconnect the wires in the correct order.

So my big fear now is than something has gone wrong with my MSD or coil.
Sorry to be such a dunce, but what is the best way to test for spark.
The only way I know is to pull a plug leave the wire attached and see if there is a spark.
Is this correct?