Here is what I have.. Tbucket 29” tires, 3.54 rear, world class t5 ford tranny .68 5th gear. At 2000 rpm 5th gear = 80mph... 55mph = 1100 rpm... 4th gear @ 4000 rpm = 100mph.. A good cruising speed is 50-65 mph. I’m looking to for about 1700 rpm in 5th gear 60-65mph.... any gear ratio ideas? 302 350hp 350torque. Current mpg 15.4
3.31:1 would net exactly 1700 rpm with a 0.68 5th and 29" tyre [at 65 mph] Your present combo would be 1812 rpm at 65mph [1673 rpm at 60mph] Don't change it! Change the cam instead
350 Hp from a 302 means its cammed up some. You typical cruising rpm should be high enough so the engine is up on the cam somewhat...
This is an excellent online calculator for this. https://spicerparts.com/calculators/transmission-ratio-rpm-calculator @Mimilan is correct. 29"tire / .68 gear ratio / 1700 rpm would be 65mph with a 3.31 rear end ratio.
Another fun option. Same 29"tire and same .68 overdrive ratio, but with a 3.9 rear end would mean 2000rpm at 65mph. Totally acceptable cruising revs, but it would really liven up the acceleration.......................
Never understood wanting, building, driving a hot rod and then worrying about every little bit of gas mileage. Glad the original hot rodders didn't, if they did we'd all be driving 4 cyl rice grinders....
There's a simple formula that you can use a pocket calculator [or phone] RPM x Diameter [inches] . then divided by Overall Ratio, then divided by 336 = MPH you can go the other way with this as well MPH x 336, then divided by Overall Ratio, then divided by Tyre Diameter = RPM To get a desired ratio for a set MPH to RPM RPM x Diameter [inches], then divided by MPH , then divided by 336 = Overall Ratio
It's an old age thing [like our hobby] We like to use them more with our "borrowed time".......and not beat our eardrums to death. I fully understand the OP here
Thanks everyone for the much needed info and that calculator link is cool..... from what I can tell from all of this I’m guessing my speedometer gear is off.. I’ll check this and let y’all know what I find.
Well spotted. I only just now ran your figures through the calculator, and something is definately off. The 5th gear ratio of a T5 is indeed .68:1. Assuming your tires are 29", and your rear ratio is 3.54:1, your speedo looks to be reading fast by about 8mph in 5th at 2000rpm.
Put a vacuum gauge on it...the engine must maintain enough vacuum on reasonably level ground to keep the power valve from opening or metering rods from going to full rich...This can cause lower rpm to increase fuel use...
I would caution not lugging your engine too much, it can have the opposite effect on what you're trying to achieve, all engines have a sweet spot where they run the most optimum in regards to power vs efficiency, you just need to find it. This is said on the basis that your ignition timing is on point and the fuel delivery system is dialed in and operating as designed (power valve only opening when its needed, etc.). Lower RPMs do not always equal more fuel efficiency.
I have 29" tall rear tires, and a 3.73 rear gear, with 700R4 OD that's .70:1 4th ratio. I cruise at 65 mph at around 1700-1750 rpm's. I think you need a 3.73 rear gear, or maybe 3.90 gear. And I knew my speedo was off, so I used my Garmin GPS to determine exactly what my true speed is.
http://www.grimmjeeper.com/gears.html This calculator will allow you to do side by side comparisons and allow you to change one variable at a time and compare the results.
And, I believe, if you're talking about your speedometer being off you have to deal in "percentage of error". Not how many mph...
TBucket, you provided no cam specs. However, if you're getting 350 hp from a 302, you must have a big cam. What is its rpm range? Having a light car/w manual transmission will let you get away with numerically lower rear gears, but then what's the point in having overdrive? If your car is most comfortable cruising between 55-65, either choose the gears that will put you there and still be on the cam, or, as suggested, replace your cam. Posting more info. regarding your engine combo will help to determine which would be the better route to take. All of your engine components should work together.
Then you have never had to carry a 5 gallon jug of gas in your truck or car for your buddy with the T bucket that got 10 Mpg on the highway on road trips and had a 6 gallon gas tank. A big T bucket tank is 10 gallons. I had friends in Texas with a guy with a T in their club with that six gallon tank and more than one of them carried a jug of gas to put gas in it on road trips. Even when gas stations were close together they had to pull over every 50 miles so he could fill it up.
It is helpful to know how far you can go, with the amount of gas you have, and plan accordingly. Hopefully fuel gauge and/or odometer are accurate.
If you are refering to my post #11, you may notice that I edited it to reflect this yesterday.............
Put a quick change under it and have real fun. 2.70-2.90 for going and coming from long drives; 4.10s for around town, 5.00 for real fun!
I'm usually way behind the curve on technology, but I found a free downloadable "speedometer" for my phone. I don't have a tach or speedometer in my T and had NO idea of how fast/slow I was going. Now, if I want to know, I use the phone speedometer, listen to the exhaust tone and try to use that to maintain a pace. Probably I'm still all over the place MPH-wise but I'm trying. Lynn
Speedo check : fast about 4mph at 60. 3 @40. Did the spin the wheel & driveshaft thingy. Definitely a 354 rear end. Pictured are the cam specks. I’m too stupid to understand them. The cam is a 351 Windsor. In my opinion 5th gear is just too high.. seriously thinking at least 390 gears, maybe even 411. Current fuel economy is 15.4 however if fuel economy was the goal, I would be driving a Prius (NOT) . This little bucket weighs in about 1200 lbs. Engine & Tranny are less than 600 of that. Thanks everyone for your help.
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