Gary Lewis Posted December 9, 2021 Share Posted December 9, 2021 3000 at 55??? shift to 3rd! Man, that's some low gears! And MPG is a consideration? You are going to be in the single digits with those gears. Our calculator says that 4.56 gears and 31" tires coupled to a C6 would give right at 3000 RPM at 55 MPH. I don't know what tranny you have, but a manual with a 1:1 top gear would give about 2750 because there's 250 RPM slippage dialed in for the torque converter. Link to comment Share on other sites More sharing options...
mat in tn Posted December 9, 2021 Share Posted December 9, 2021 Man, that's some low gears! And MPG is a consideration? You are going to be in the single digits with those gears. Our calculator says that 4.56 gears and 31" tires coupled to a C6 would give right at 3000 RPM at 55 MPH. I don't know what tranny you have, but a manual with a 1:1 top gear would give about 2750 because there's 250 RPM slippage dialed in for the torque converter. wow. 4.10 is the highest i have ever used and that was with an aod and 31" tires. did beautiful in my custom bronco on the highway and with low range would still crawl all you could need to. Link to comment Share on other sites More sharing options...
RenoHuskerDu Posted December 9, 2021 Share Posted December 9, 2021 Man, that's some low gears! And MPG is a consideration? You are going to be in the single digits with those gears. Our calculator says that 4.56 gears and 31" tires coupled to a C6 would give right at 3000 RPM at 55 MPH. I don't know what tranny you have, but a manual with a 1:1 top gear would give about 2750 because there's 250 RPM slippage dialed in for the torque converter. Hmmm, we may have an optimistic tachometer at play here. Poncho has 3.50 gears and the 4sp tranny is 1:1 in top gear. Link to comment Share on other sites More sharing options...
Gary Lewis Posted December 9, 2021 Share Posted December 9, 2021 Man, that's some low gears! And MPG is a consideration? You are going to be in the single digits with those gears. Our calculator says that 4.56 gears and 31" tires coupled to a C6 would give right at 3000 RPM at 55 MPH. I don't know what tranny you have, but a manual with a 1:1 top gear would give about 2750 because there's 250 RPM slippage dialed in for the torque converter. Hmmm, we may have an optimistic tachometer at play here. Poncho has 3.50 gears and the 4sp tranny is 1:1 in top gear. With 31" tires and 3.50 gears you should be turning 2086 RPM. Link to comment Share on other sites More sharing options...
mat in tn Posted December 9, 2021 Share Posted December 9, 2021 With 31" tires and 3.50 gears you should be turning 2086 RPM. yeah, that sounds much better. is it possible the tach came from a six cyl model originally? Link to comment Share on other sites More sharing options...
Gary Lewis Posted December 9, 2021 Share Posted December 9, 2021 yeah, that sounds much better. is it possible the tach came from a six cyl model originally? Actually, the tachs are interchangeable between 6 and 8 cylinder engines. But the 8 cylinder wiring harness grounds a terminal on the tach that puts it in 8 mode. So if the wiring harness has been changed... Link to comment Share on other sites More sharing options...
mat in tn Posted December 9, 2021 Share Posted December 9, 2021 Actually, the tachs are interchangeable between 6 and 8 cylinder engines. But the 8 cylinder wiring harness grounds a terminal on the tach that puts it in 8 mode. So if the wiring harness has been changed... exactly! one other possibility is a bad ground to the 8cyl connection Link to comment Share on other sites More sharing options...
RenoHuskerDu Posted December 10, 2021 Share Posted December 10, 2021 exactly! one other possibility is a bad ground to the 8cyl connection Kicking this idea around, I came up with yet another possibility. The stock Ford ignition is gone. I wonder if that was what fed the tach. The truck came with the GM HEI clone, coil in the cap. See below Perhaps that disty is feeding incorrect signal to the tach. Do we happen to have a pic of that 8cyl ground post? (This pic is from before we connected the vacuum advance and removed the mousetrap) Link to comment Share on other sites More sharing options...
mat in tn Posted December 10, 2021 Share Posted December 10, 2021 exactly! one other possibility is a bad ground to the 8cyl connection Kicking this idea around, I came up with yet another possibility. The stock Ford ignition is gone. I wonder if that was what fed the tach. The truck came with the GM HEI clone, coil in the cap. See below Perhaps that disty is feeding incorrect signal to the tach. Do we happen to have a pic of that 8cyl ground post? (This pic is from before we connected the vacuum advance and removed the mousetrap) that hei has a tach output which should go to the tach signal wire but that should not be confused with a bad ground. a bad ground to the 8 cyl point is a different matter Link to comment Share on other sites More sharing options...
Gary Lewis Posted December 10, 2021 Share Posted December 10, 2021 that hei has a tach output which should go to the tach signal wire but that should not be confused with a bad ground. a bad ground to the 8 cyl point is a different matter Right. But you are onto the problem. In the schematic below you can see the Bk/LG D wire that says "To Tachometer" and "8 Cyl Only". That is the ground wire to the tach, and the ground originate at the distributor. So if the yoyos that put a GM dizzy in a Ford didn't ground that wire you won't be in 8 cylinder mode on the tach. Just find that wire and ground it. Link to comment Share on other sites More sharing options...
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