Technical flathead to t5 speedometer

Hey guys,

I have recently had an S10 NWC T5 installed in my 40 ford with stock rearend and i'm running 6.00 - 16 ford script rear tyres.

I can't get to get my speedo to read properly, what is the best thing to do to get at least a semi accurate speedo.

Any advice would be great.

Cheers
John

 

On the T5 transmissions I sell the builder always asks me to get the tire size and rear end ratio so he can install the correct speedo gear set in the trans. Never had any complaints so far. I once went to my Chevy parts man who had a chart with the right gear combinations for various tire size and rear end ratios. That was several years ago when manual speedo drives were more common.

 

I thought that may have been the case, knew I should have done that before the install, I've found a calculator on line and based on tyre size and rear end I need an 18tooth driven gear for what I'm running


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Instead of taking the trans apart now to change out the speedo gear, they sell inline adapters that you can change the ratio's with. Search for Stewart Warner speedometer reducers on ebay. You can pick them up at swap meets for $5.

 

First you need the tire diameter Divide this by 20168. This will equal revolutions per mile. Next see what drive gear you have. Multiply drive gear teeth , gear ratio, and revs per mile. Take this total and divide by 1001. This will equal the drive. Gear tooth count you will need