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Discussion Starter #1
Big problem. I just had new gears put in my rear diff after doing some math on a gear calculator and asking for all of your opinions. My top speed is 65 mph at 2300rpm, not much more above 2300. I was running 4.10 before and got it up to 70 with them, but cant seem to get above 65 now, thought I would be good up to 75.

My new set up is a 4bt ZF 5 speed hooked to an Np205 now running 3.70 gears on 33 inch BFG Allterrains in a 77 F150 Short bed.

Any thoughts? I am very disappointed in the outcome.
 

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You already had the 'BT swapped in with the ZF and '205, only swapped the gears and now it goes slower?

Are you sure the shop (or whomever) went the right way with the ratios (ie, did they put a 4.30 in by mistake?)?

If not, is the engine still putting out (good EGT at 2300, doesn't suddenly get cold or smoke alot)?

If it's a VE pump, do you have the 366 spring in it?
 

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if you have a zf5 and 33"" tires and 3.70 gears you have basically the same set up as me ecept i have 3.73 gears... i am at 1600 or so at 65 mph. if you truly are at 2300 rpm at 65 mph in 5th gear then you have like 4.56 gears.
 

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Discussion Starter #4
I am pretty sure they put my gears in, I ordered them from Jeff's Bronco Graveyard. When they arrived at my house I inspected them and counted the teeth to double check that they were 3.70. I guess I should look at the gears they gave back to me to make sure they swapped them out. My engine is putting out good EGT, consistantly around 600 to 800 degrees around 2300 rpm. It is a VE pump with the stock spring. I am really confused.
 

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Discussion Starter #6
It is after the turbo. I am completely lost, just confirmed that the new gears were put in. Does anyone have any ideas?
 

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Alright you guys. You are going to call me names and then laugh. After doing a google search and seeing a posting on another forum I realized the problem. I can't believe that I totally forgot, and didn't realize, that when you change the gearing in the rear end that it is going to throw off the speedo!
So I decided to grab the GPS and take the truck for a ride... So everything is good to go! I was wondering why I was going soo slow when I should have been wondering why I was passing a few people! Got it up to 75 mph so please disregard!
 

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Alright you guys. You are going to call me names and then laugh. After doing a google search and seeing a posting on another forum I realized the problem. I can't believe that I totally forgot, and didn't realize, that when you change the gearing in the rear end that it is going to throw off the speedo!
So I decided to grab the GPS and take the truck for a ride... So everything is good to go! I was wondering why I was going soo slow when I should have been wondering why I was passing a few people! Got it up to 75 mph so please disregard!
Glad you didnt have to explain that to the police.
 
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