Diff ratio help

[66STNG]

Hi guys,
Need some advice on diff ratio.
302, BTR 4speed auto. The car goes great but not that quick off the line. When driving at 100ks the car is sitting on 1500rpm. What what diff ratio should I be running.....
Cheers.

[unilec5544]

I have a calculator buried somewhere, just need the rpm at 100Kph (60mph)with the gearbox at 1.00:1 ratio, which I believe is in 3rd and also the tyre diameter.

[Nuts]

I think the BTR 4th gear ratio is around .68m similar to am AOD. I'm running a 3.5 rear and could have gone 3.7 and still be ok for highway.

[hybrid]

I'm running 3.89 in mine with the AOD and it's fine on the highway.

[66STNG]

I think the BTR 4th gear ratio is around .68m similar to am AOD. I'm running a 3.5 rear and could have gone 3.7 and still be ok for highway.

.
3.5s is what I’m thunken..I’ve got 17 inch rims, I just want a little bit more poke off the lights, not a tire fryer....

[Chux]

Debut where is the torque range of your motor? There is some great calculators out there but if you drop it into third what RPM do you get at 100kph? What sort of RPM would you like at 100?
There is a fair bit of personal preference there as to what is comfortable...really you could go to 3.9 and get some great off the line and sit on just over 2000rpm at 60mph. If you changed at 5000rpm you would hit @ 60 in second and 100mph in 3rd and theoretically 150 in 4th.

[hybrid]

It also depends what the current ratio is.
If you're on say 3.23 or 3.25 or whatever stock is and it feels sluggish, I don't think 3.5 is going to feel that much different.

[66STNG]

It also depends what the current ratio is.
If you're on say 3.23 or 3.25 or whatever stock is and it feels sluggish, I don't think 3.5 is going to feel that much different.

.
Good point... I’ll check the ratio...
.
And where do I get new gears from?

[hybrid]

Most performance places should have them.
Rocket/VPW/EMA. Even ebay, but just make sure they're a reputable brand gearset.

It's an 8" set you're looking for - assuming you've still got the stock diff.

[66STNG]

Yes, stock 8”.

[StephenSLR]

I’ve got 17 inch rims

^ this is why it's slower off the mark, changing rim/tyre diameter affects your take off, the larger the rim the more 'highway' suited it is.

s

[Dwayne]

It's the tire diameter, not rim diameter that matters.

[StephenSLR]

It's the tire diameter, not rim diameter that matters.

True, I was thinking keeping the same profile with different sized rims. I'm not a fan of the elastic band tyre look.

s

[Pinto-Pete]

Try this..https://www.blocklayer.com/rpm-gear.aspx

[66STNG]

Try this..https://www.blocklayer.com/rpm-gear.aspx

.
Great site, cheers. Works out I need 3.7s, thank you for everyone’s input.