thats too high for me ill have to go with 3.73

 

Your tires roll at about 758 revs./mile (Bridgestone spec, other manufactuters may vary very slightly.) I plugged that into a spreadsheet I have for gear ratios. At 65 MPH in OD you'll be turning:

2357 with 4.10s
2144 with 3.73s
2041 with 3.55s

If you have 3.55s now there's NO point in changing to 3.73s. It's only a 5% gain, hardly worth the time and effort. It will make very nearly zero difference in your ETs and none in your launch. If you have 3.08s or 3.31s then go for it.

2350 is not a lot of highway RPM for a 302. It will put you right at your torque peak which will help minimize the tranny's habit of hunting back and forth between 3rd and OD. You'll lose at most 1-1.5 highway MPG vs. the 3.55s and lose no mileage at all around town.
Basically there's nothing that isn't a tradeoff. Everything that Ford did was a compromise among performance, economy, reliabilty, NVH and a dozen other factors. Everything you do to modify your truck is going to be a compromise that sacrifices one thing to gain another. If you want to be quicker with a near-stock engine, you need to pull some gears and you'll have to burn more gas to do it

 

Thats a good point you made and now that I see the #s I'm thinking. I have 3.31 gears right now. I really want to be able to spin my tires from the stop, but with the new 275s I dont have a chance so should I go for 3.73s or 4.10s to accomplish this?

 

Actually, for maximum launch you want to just miss being able to spin the tires. A little chirp right off the line is good but the weight transfer that you get from launch should prevent the tires from spinning. If they go into a full spin, your power is going up in smoke, quite literally, and your ET will be slower.

I've got 3.73s with 30" tires and a Trac-Lok in my truck with a 4.9 and E4OD. It will not spin the tires on dry pavement and will barely elicit a chirp. It's a real stone off the line, typicaly 2.60 sec 60' times. Your 302 is going to make slightly less torque off idle and need slightly more gear.

 

Took the truck out and at 65mph I turn 1650 rpms in OD So what will it be with 3.73s and 4.10s

 

I cannot make the data you've given me work. 1650 @65 in OD gives me:

1650 RPM / 0.70 OD ratio = 2375 driveshaft RPM

758 tire revs/mile x 65/60 miles per minute= 821 axle RPM

R&P ratio = 2357/821 = 2.87 which is not close to any of the available ratios but looks kind of like a 2.73.

There's several possibilities:

1) Your tach is way off.
2) Your R&P ratio is not 3.31
3) Your speedo is WAY off.
4) Two or more of the above

Did you re-calibrate your speedo when you installed the 28" tires? Your new conversion constant in the PSOM should be 10.23. If it still thinks you have 235/75-15s (conversion constant 9.72) then your speedo readings are going to be off by 720/758 = 0.95 and an indicated 65 is going to be just under 62 actual. A set of 3.08 gears and stock tires would give a tach reading of 1685 RPM. Go out on the interstate and drive at exactly 60 indicated MPH measure the time from mile marker to mile marker with a stopwatch. Divide 3600 by your time this will give you actual MPH. Repeat several times and average your answers.

Confirm your R&P ratio, I suspect you have a 3.08 (Axle code 18) not a 3.31 (Axle code 17.) Have you physically counted shaft revs to make a wheel rev? Since you have an open rear, jack up one rear wheel, make a chalk mark on the driveshaft and spin the jacked up wheel by hand. Count the number of driveshaft revs to make 2 wheel revs.

If neither of these tests show an anomaly, borrow an aftermarket tach and hook it up compare it's readings to yours.

You've got to know where you actually are with your equipment before you can start to modify it.

 

No I did not recalibrate my speedo because I do not know how to. How do I??? And I was going 65 because I had a friend drive next tom me with both of us on cruis control going exactly the same speed and we timed the miles and we were going 65. My tach might be off, but not by that much. Also my axle code (17) and the metal tag say 3.31, and when I got my tires changed I went and spun the right rear tire and it was something close to 3.3 as close as one can guess.

 

Something is strange here. At 65 with 3.31s in OD you should be turning right at 1900 RPM, not 1650. But if your confidant that your speed was an actual 65 (55.4 sec/mile), and you're sure that your R&P is a 3.31 then maybe the tach has a problem. Before you tear into it though you should recalibrate the speedo just to be sure.

The process is relatively simple, only takes a couple of minutes, but it's far too long for me to type out. Do you have a fax number? E-mail it to me and I'll send you the relevant pages out of the Ford shop manual. mgrove@nls.net

 

I do not have a fax machine, but if you could scan the pages or something you could email it to me at beastiel@bellsouth.net

Thanks for all the help strange. Regardless of my rpm / speedo stuff I want to order very soon: March power and amp pulleys superchip 3.73 or 4.10 gears gears come later