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SeventyOne · 12-02-2020, 09:45 PM

Let's get some actual numbers going on here...

-What RPM are you hitting on the highway at 65MPH?
-What size are your tires?
-Can you jack up the rear end, put the trans in neutral and spin the tires 1 revolution and count how many times your driveshaft spins? (Or spin the driveshaft X times to count 1 tire revolution) Then we'll know what gear you have.

I don't have a manual in my K5 but my '65 F100 has a granny gear (6.5:1 ratio) 4 speed and I start in second gear like you but it does not need all much RPM to get going. It doesn't have an over drive so in 4th (1:1) it pulls around 2500 at 65MPH. The truck has 37" tires and 4.10 gearing, though.

If you do swap in an automatic i'd recommend you get an overdrive trans or the high rpms on the highway will remain the same.

12-02-2020, 09:45 PM	#7
SeventyOne Registered User Join Date: Mar 2007 Location: Oakland, Ca Posts: 1,370	Re: 1970 Blazer Granny Gear Let's get some actual numbers going on here... -What RPM are you hitting on the highway at 65MPH? -What size are your tires? -Can you jack up the rear end, put the trans in neutral and spin the tires 1 revolution and count how many times your driveshaft spins? (Or spin the driveshaft X times to count 1 tire revolution) Then we'll know what gear you have. I don't have a manual in my K5 but my '65 F100 has a granny gear (6.5:1 ratio) 4 speed and I start in second gear like you but it does not need all much RPM to get going. It doesn't have an over drive so in 4th (1:1) it pulls around 2500 at 65MPH. The truck has 37" tires and 4.10 gearing, though. If you do swap in an automatic i'd recommend you get an overdrive trans or the high rpms on the highway will remain the same.