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Hello all,
My Pantera has a 351C with a Holley 600 with vacuum secondaries. It makes 262bhp at wheels (approx 315 at the motor) - it could probably make a little more yet.
My problem is only get 12 miles to a UK gallon of fuel (thats 10 miles to a US gallon) - however fast or slow I drive.
That sounds pretty bad to me - what sort of gas mileage do you guys get?
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Thanks for the response guys.
The Holley is a new 1850S/4160 with a conversion kit for real secondary jets. I'm running 64 primary and 66 (or was it 68?) secondaries. The new carb replaces an old 700cfm Holley (a 4778-2) - I replaced that because it was getting only 10mpgUK/8.33mpgUS. Ignition timing is correct (Mallory dual point) and there are no obvious leaks or fuel fumes coming from the engine bay. The rolling road guy was happy with the mixture. From reading the other posts here I seem to have the worst fuel consumption for a stock (almost) car. I get 12mpg (10US) on country roads, and 12.9mpg (10.75US woopee!) on a motorway cruise at a steady 70mph.
Could it be the cam? (I have no idea whats in there!) Any other ideas?
Start with your plugs. Pull them to see what color the tips are: tan, black, oils or fuel soaked. There are sites that can help match the color of the plugs to a healthy engine. This helps narrow down the problems.

Next, try a compression test to make sure everything is sealing. Leak down test would be better but more expensive. If your rings or valve seats are shot, no carb tuning would help.

Look for any vacumn line leaks. Spray vac lines with carb cleaner or penetrating oil with the engine running, if the engine increases RPM you have a leak. Do not forget the massive vac line that runs from the engine to the brake booster up front in the trunk (boot).

If those check out, then comes the fun of carb tuning. Get a good holley book and try jet and vac value combo's. A few jet sizes can make a real difference in both performance and ecomony.

This is all trial and error stuff, so perhaps you can tap a friend who knows these things to speed up progress.
I have a 73L & on the old low compression engine it had in it when I purchased it I got 19 mpg on my last long trip (1350 mi.) My rebuilt engine I have in it now- approx. 425 FWHP- is giving me about 15-16 mpg. A lot better performance now though, so I'm happy to give up a few mpg!
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