So you have the non turbo diff? That would be a 346 ratio. Then you have a speedo designed for a turbo car, and a 307 ratio diff? So the speedo sees an engine speed as covering more distance than you are so to get a corrected reading you will need to multiply what it says by a factor of whatever the ratio is between 346 and 307. Not having a calculator handy I'll say that is about .8. So if it says you have covered lets say 100 miles, the distance needs to be multiplied by that fraction and you will find you are traveling less distance by the factor in my example it would be 80 miles.

So if your odometer (assuming it is correct) says you have gone 100 miles and you have burned 4 gallons of fuel you will show 25 mpg. But you need to mulitiply that by the factor and you will find you are getting something less than 20 mpg I imagine.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.