Ok, I already know the optimal final drive ratio is 10:1 but I just had this question I wanted to get verified.

The way I see it is if you keep the same final drive ratio for car A and car B and the cars are identical as far as weight goes and axle ratio and transmission but the only difference is the engines. Wouldnt the car with the more powerful engine accelerate faster and have better fuel economy than the other engine with less hp/trq?

This is why I am asking, I am having people constantly telling me that I should upgrade my rear axle when I overhaul my engine and build it up alittle cause the 2.50:1 axle ratio will be a slug and the new engine will get worse gas miliage as well as be slower than stock.

Well personally I dont see how if I maintain the same 6:1 final drive ratio and use the same engine and use street parts that makes power from 600 - 4800 rpm would perform worse than the stock engine and be slower.

Just need a quick verification on if I am viewing this right or if a higher hp/trq engine with the same final drive ratio will be slower than one with less hp/trq.

 

2.5 axle but you mention 6:1 "final drive ratio"????

But to your question:

The USUAL way to more power involves more RPM and torque and HP peaks at higher rpm. Thus, the "new", higher powered engine might be a slug with the old tall gears, at least until you get it up to the powerband. That powerband may be at engine speeds that aren't practical for street use, as you just can't drive 80-120 mph everywhere, even if your motor loves it.

But if your build is going to concentrate on better efficiency at lower revs, it would seem to me the tall gear will work just fine, and you should have a quite noticable improvement in performance. Fuel economy may improve also, but that depends on a lot, including how big a gas hog it is now--

 

Thats a first for me. Where did that "optimal" ratio come from? Second question, is the stock final drive really 2.5:1? That is highly unusual, especially in a truck. Yes, a 1965 Mercury with a 2-bbl 390 and Merc-O-Matic "Turnpike Cruiser" package had something like 2.56:1. The lowest numerical Ford or GM truck ratio I have seen is 3.08:1, with 3.31-3.73:1 range being the most common.

One method for selecting ratios is to take your normal cruising speed, say 70mph and the engines peak torque rpm, say 2400 and work out the tire rolling diameter, transmission overdrive gear ratio to determine final drive. If "drag racing" is your goal, you are more concerned with peak power and transmission shift points. If towing a heavy load, low gear plus torque converter multiplication from a standing start enter into the calculation.

In other words, what is the "mission" you wish to accomplish?

 

Same here, for trucks. I've never even heard of a 10:1 gear ratio!!!
At 30 mph, it seems that most (if not 'all') engines would be redlining!

I had an '88 Caprice Classic 'Brougham' with 2.48:1 gears in the rear.
That 'boat' was slow from a dead start, but, had a magnificient top-end! (Over 130 mph!, but don't tell anyone!!) And it got 26-28 mpg when driving like a 'sensible' man at hwy speeds.

 

That tripped me up too. I never heard of a 10:1 final drive ratio. Some trucks in the 80's and 90's came with 2.73 gears but I've never seen 2.50 gears. I would think the vehicle would be all but useless on any kind of hill.

If the car (or truck? what are we even talking about?) has 2.50 gears now I would for sure consider a gear change. It just won't be very fast with 2.50 gears.

 

I think what was meant was overall ratio in first gear, final drive x first gear x torque converter (if any). The thing that threw me is the word "optimal". It seems like that would vary a lot depending upon engine torque curve, loaded weight of rig and tire size.

 

Yes, in first gear the transmission is a 2.40:1 and the axle is a 2.50:1 which equals 6:1 final drive ratio in first gear. In third gear it would be 2.50:1 final drive ratio. Now I understand you cant run a high rpm camshaft, the camshaft I desided on is a street cam that makes good to great low/mid range trq and has a basic operating range of 600 to 4800 rpm. Currently the car kicks out of passing gear at 60 mph @ 4500 rpm. The miliage I am currently getting 18 city out of the car not sure on highway but the factory rating is supposed to be 12 city and 18 highway. But the engine is a stock 145hp/200trq 351W with a 351cfm 2V. I am looking at somewheres around 250 - 300hp and 300ish trq. Personally the way how I am seeing it going with stock performance replacement heads from edelbrock, the stock replacement manifold from edelbrock with that comp cams xtreme energy street cam along with a 600cfm 4V vac secondary carb I am thinking I should in theory see fuel milige better than 12 city and 18 highway for sure. I am also thinking that extra 100 ft lbs of trq should help the car get up and move better than it does now as well.

It came from a few articles I seen online about gear ratios in general. It says with the advent of overdrive transmissions the optimal over all ratio is 10:1 for the best economy as well as acceleration. Was talking about how you could basically run a 4.11 axle ratio and obtain 10:1 overall ratio in 1st gear for acceleration and pulling but the overdrive will drop the 4.11 down to a more stock like street friendly 3.30 - 3.73. Mine how ever is 6:1 overall ratio 2.40:1 first gear x 2.50:1 axle ratio = 6.00:1. In second gear my over all ratio would be 3.675:1 and in third gear 2.50:1. I know alot of websites tell me my 78 should be a 2.75:1 axle ratio or 2.90:1 but and heres the kicker my 1978 ford shop manual says that the door tag code is for a 2.50:1 axle ratio. I also believe the tag on the rear end I found when I was under the car still in place also said 2.50

The mission I want to accomplish is to build my engine up slightly with street parts to get more than the 145hp/200trq factory rating while keeping everythign else stock. In my view improving the power output of the engine should make the car move better and be slightly better on gas considering its a 4,000 lb vehicle. Reason I dont want to change the axle ratio is the engine turns very low rpm in the city as well as on the highway. The car also isnt real slow like alot of people think. My G-tech pro I used for 0 to 60 mph times I did four pulls, two in drive and two shifting manually best I got in drive was 6.72 seconds, worse 6.79 seconds. Shifting manually best I got was 6.20 seconds and worse was 6.36 seconds. So while yes the car is a heavy 4,000 lb tank for a everyday driver 0 to 60 mph in roughly 6 - 7 seconds isnt bad for a car with just 145hp but I would like to improve upon that some just by going with better flowing heads, a street camshaft that produces power low in the RPM through midrange.

I think what happened is a misunderstanding. When I say overall gear ratio I am not saying a 10:1 axle ratio. I am saying that is the first gear ratio multiplied by the axle ratio. Alot of the articles even pertaining to trucks say that alot of people get better performance out of their trucks with a 10:1 over all ratio where they are running around a 4.11:1 axle ratio but the overdrive transmission allows the axle ratio to be brought back down to a 3.40 or so for better highway driving. Mine is a 6:1 over all in 1st, and 2.50:1 in third which nets me 1500 RPM @ 45 mph in the city and 2500rpm @ 85-90 mph on the highway. I actually like this setup cause the engine isnt turning that fast but if you step down on it and kick down into 1st gear and back to 6:1 ratio even at 30 mph if you step on it the car gets up and moves fast for what it is.

Yep and thats what all the sites say my 78 came with was in the area of 2.70 - 2.90 axle gears but my ford shop manual for 1978 says the door tag code stands for a 2.50:1 axle ratio and I believe at one time I found the tag on the axle few years back that even said 2.50 on it. But the transmission allows the car to move pretty good and I can out accelerate most cars on the street that are making more hp and weigh less than mine. Its just as I said I planned on going with some slightly more performance based parts that are still listed for street use that should improve low and mid range power for city and highway use. Goind off what this one guy on fordmuscle forums said, he said the engine would make around 250 - 280 hp and low 300 trq. My DD2000 which when putting stock figures on my engine in comes back with 152hp when stock rating was 145bhp I think I might would trust DD2000 more. If I build it one way which is increasing the compression by use of custom built pistons to obtain a 0.008" deck clearance I should as far as the program says 334hp @ 5,000 rpm (not sure why 5,000 when the camshaft`s basic operating range is 600 - 4500 rpm) and 397 ft lbs @ 3,500 rpm @ 8.964:1 compression with the use of 22cc D shaped dished pistons. Now this is using the Edelbrock performer emission legal replacement aluminum heads @60cc chambers (same flow as the Edelbrock Performer RPM heads) with a 600CFM 4V carb and large tube headers with mufflers. If I build it with off the shelf pistons with 0.020" deck clearance with the same 22cc D shaped dished pistons @ 8.75:1 compression the power is still 330hp @ 5,000 and 393 @ 3,500 rpm with 380 @ 2,000 and 387 @ 2500 rpms.

But The thing that has me not fully trusting this program is its showing me a 5,000 rpm peak hp even though the camshaft as far as I know redlines at 5 grand with 4,800 being the max operational speed.

Instead of going on and on, I was just curious about the overall axle ratio because I thought increasing power matching the pieces in the engine to make power where the rear end needs it and make more than 145hp/200trq stock rating the car should have better peformance as well as being able to spin the tires easier than stock. But I constantly have people on the fordmuscle forums telling me the engine will be a slug even though all parts are street parts and being pieced together for a street driven car making power throughout the rpm range of idle @ 750 rpm (camshaft 600 - 4800 basic operating range) all the way up to 4,500 rpm when the transmission kicks out of passing gear @ 60 mph.

Personally I would hate to go to the trouble of spending extra money to change the axle ratio then change the transmission out just to obtain the low cruising rpm for city and highway driving that I like so much. I also like the fact that currently I dont have to press the throttle down more than 1/8 of the way to obtain a 1500rpm pull from a stop light and I still pull away from other cars that are giving more throttle. I dont know if this camshaft will be an improvement upon stock performance like that but I think comp cams told me once that i would see a improvement over stock performance with my vehicle with this camshaft. I would have to go check my saved emails and see if I still have the reply from compcams and see what they said exactly.