I have a 49 f1 with a 239 flathead and a 4 in the floor. I wanted to know if i change the rear to a 9" what is a good gear ratio. Looking at a rear out of a 65 f100 the guy said its a 3:70. Also would i need to change anything for it to work? I dont know about things like this never did a swap. Thanks again guys for any help.

Brian

 

I have a 52 with a HD 3 speed, with a 1 to 1 top gear. When I bought the truck it had a 9" with a 3 to 1 final. You could run over 80 on the freeway but it bogged down on every hill ( I live at 4000' elevation). I changed to a 3.5 to 1 and now the truck has a max of 75-80 and has much better power up the hills. This is with 27.7 inch tall tires. Hope this helps.

 

Thanks Mike. Sounds like what Im looking for. Any idea how much one might cost for my truck?

 

There are ratio calculators on the web where you can put in the tire diameter, rear axle ratio, transmission gear ratio (1:1 in high gear unless you have OD) and speed and find the engine RPM, or put in the desired engine RPM, speed, and tire diameter and solve for rear axle ratio desired. The Ford 9" has the most gear ratio ring and pinion sets available than any other rear axle. What ratio you want may depend on the type of driving you do. 3.7:1 is likely to be a good ratio for around town and occasional trips of 100 miles or less at 55-60 with moderate economy. Don't buy a used R&P set unless you get it in their original carrier.

 

Thanks AXracer. I'll look online for a ratio calculator.

 

Do an Advanced Search on here, there are hundreds of threads on this topic, including what years fit, and different people's experiences. I have a 9" and tried it with a 3.25 at first. I'm at 5300', and there are some good hills. It was awful. Taking off from a light took a lot of clutch slipping, going around corners in town in 2nd also required clutching. No guts on hills. Changed to a 3.50 (with 28" tires) and all is well.

 

Thanks ALBUQ F-1. I have 28" tires too . I am looking for a good gear so i can go down the highway if i want. Will do an advanced search thanks for the tip

 

Here is a good gear ratio calculator for you. Doesn't tell you what the power will feel like though, that's why I let you know how mine runs.

Engine RPM Calculator

 

Depending what you mean by "highway speed"... 3.50 with 28" tires gives 2300 RPM at 55. That is a good place for a flathead. If you want to go faster, you also need to make improvements in brake and suspension, pretty soon you're $15,000 lighter and for what?

BTW the only accurate way to measure your true rolling diameter is to make a mark at 6:00 on your tire and another next to it on the pavement. Roll the truck forward one wheel revolution and make a mark on the pavement there. Measure between pavement marks and divide by Pi (3.14159). Advertised or calculated diameters are not accurate at all.

 

Thanks for all the help guys. Thats why I come here you guys know what to do and how to do it.