I've got an 84 D-100 with the 225 Lean burn and I'm getting about 13 mpg! Should it be more? Also if I swap to HEI ignition would that help with power?

I got about the same with my 87. I converted it to super six and EI and get 22.
Frank

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Scrapple: Because a mind is a terrible thing to waste.
73 Duster - Race Car
66 Dart Wagon - DD
178" FED
82 D150
All Slant powered

Mine gets 14 around town, 16 on the highway. I am following Cuda's lead and doing the super six switch asap.

with my 100% stock 83 D-150 I have found the biggest impact on mpg is how I use the throttle. Keep it under 65 on the highway,, don't use the throttle like on on-off switch in stop and go driving and I'll get 18 mpg easy.
will be interesting to see what happens after the motor improvements are made

Buy a vacuum gauge and stay in the "green"

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'72 Duster 198 stock cam, 3:23's Hookers on jack stands for 8 years in the driveway
'79 Maxivan 360 Offy Qjet Comp RV cam/rusting in the driveway.
93 D350 160HP Cummins Auto Dually Clubcab needs a injector pump
2005 Golden Couch Buick