Hello, I want to change the old 235/75-15`s on my 87 f-150 4x4, L-6 221ci for new 31`s. The problem is that with stock tires she runs about 80 mph in 3rd gear. The question: What will happen with my final speed with the new 31`s?
Thanks to all of you

 

You'll be moving faster than what the speedo reads due to a larger circumference....There's a speed calculator somewhere out there, just a matter of finding it.
235/75-15= 235x.75=176.25mm sidewalls
176.25/25.4 mm/in= 6.94"
6.94"sidewall x 2 sidewalls=13.88"
13.88totallsidewall + 15" rim = 28.88"= old tire diameter to plug into calculator

Someone can check my math on that, but I'm pretty sure it's right....of course it'd be just as easy to measure your tire, lol




EDIT: Found it.... http://www.4lo.com/4LoCalc.htm
Calculator says 85.87 for your case

 

I already knew the spedoo thing, but what happens with the real speed, will my old ford "move" that extra radious or my final speed will come down?
Thanks the same for all that math

 

your down hill speed will increase but i think that think will be a pig otherwise. Probably need lower gear ratios ie.$$$$$$$
BTW whats a 221ci? I thought 300ci(bitchin truck motor) was the smallest in the F150 of that year.

 

My dear friend, your not mistaken, the thing is, that I´m in argentina, where Ford genius used a ****n l6 221ci with 120hp to make cheeper your work, the thing is, that running on 95 octane gas, truck goes fine, like 95 mph (unbrakeable); but also in argentina, we use to un our petrol cars with CNG, wich reduces nearly 20% of the petrol running power. So, running with this **** my truck has 96 HP!!!!!! I think its too poor for this truck.

Ah, for your interest, there was also a 188ci L-6 engine for the elder F-series produced in argentina.

221 ci specs: 221 ci - 3.6L; 120 HP; Compression ratio: 7.2 :1; Overall: very nice engine for a falcon but the f-series shoul use an 8 cyl.

Some f-series had a v8 but were the less.

Thanks for your interest.

 

That formula I beleive is for your actual speed, not the speedo reading, because of the bigger circumference. The speedometer will read slower than what you are actually moving. In other words, bigger tires=moving faster at same RPM.