So what kind of mileage you getting SoCal?

 

12-15 on trips through mountains at speeds of 60mph or less not towing. 10-12 is my best guess for around town, since I don't commute in it much anymore. At speeds above 65mph, it's in the high single digits. Before the gear change it would get 15 at freeway speeds on flat ground but 10 in the mountains and under 10 around town or when towing. Basically anytime the going gets tough, with the new gear it gets better mileage.

 

I have a 89 F-150 with a 351 and 3.55 rear end and 3 speed auto tranny. It seems to run ok,but the gas mileage is horrendous. I dont think Im getting 10 mpg. I think a overdrive tranny would help,but It might be too expensive to invest in the truck. Any Ideas?

 

SoCal,

I am running a 351 with the 5 speed. Sounds like the 4.56 gears should work out ok for me with the OD.

Any opinions?

Steve

 

Yeah, I just got a new job today (finally, and 2 of them at that....) so I think I'll start saving my money for that 4.10 gear change and a small car to drive instead of the truck putting me in the poor house all the time. If I get the car first, I'll save the the money (thanks to less gas) for the gear change faster (and still have something to drive while my rear end's torn apart). Is a gearing change something a "driveway mechanic" can do, or would it need to go to the shop (again) for that??? If it's a "do-it-yourself", there's only one place I'll come for advice on it.

 

It really depends on how confident you are with setting backlash and pinion gear depth. You need a gauge that mounts to the housing and the patience to get it set right. If you don't get it set right they will howl and wear bad.

Steve

 

Wcmo, there are a few guys on here who have done gear changes themselves. I took mine in and had it done. It was worth it to me. On any of my old trucks, with a place to do involved work on them without hassle from the neighbors, I would likely have done it myself and learned as I went, and wouldn't have minded buying the tools needed to do it. That was not the case this time though. If you do decide to do it yourself, I'm sure you will get plenty of guidance from the experts on this site .

Wojo, the right gearing really depends on the tire size you run and what you like to do with the truck. 4.56's worked out great for me, not sure if they would be right for you or not. Something only you can decide. I would start out by writing down the relevant #'s - speed you like to drive on the highway, axle ratio, trans overdrive ratio, tire size, rpm at which your engine develops it's peak torque, and speed you like to climb hills at when towing in 4th/direct gear, if you tow. If you tow alot, find out what your 3rd gear ratio is too. Plug those numbers into the folowing formulas and use them to figure out what your engine rpms will be at different speeds in different trans gears for a certain axle ratio, match that to the power curve of your engine and how you like to drive the truck.

RPM = (MPH x AxleGearRatio x TransGearRatio x 336) / TireDiameter.

AxleGearRatio = (RPM x TireDiameter) / (MPH x 336).

To find the height of a metric tire in inches use:

TireDiameter = ((2 x SectionWidth x AspectRatio) / 2540) + WheelDiameter

For example, a 235/85-16 tire has a SectionWidth of 235mm, AspectRatio of 85% and WheelDiameter of 16".

For an easier method of calculating, there are tire size/gear ratio/mph charts at www.reiderracing.com and www.precisiongear.com . Most charts are set up to show information based on a vehicle speed of 60 or 65mph. For any other vehicle speed, the equations above are needed.

According to info on this site, my '92 truck 351 engine makes it's peak torque at 2800 rpm. This seems to agree with other sources I have found as well. Other year 351's may be different.

Your gas-engine ZF 5-speed's ratios are: 5.72, 2.94, 1.61, 1.00, 0.76. Some ZF's have a 1.26 3rd gear, not sure which ones.

Your engine would be spinning ~2300 rpms @ 65mph in OD with 4.56 gears and 33" tires; and ~2550 rpms @ 55mph in 4th gear.

Ratios in a C6 are: 2.47, 1.43, 1.00. Add ~250 engine rpms for torque converter slip in a C6.

I wanted to put my engine at or near it's peak torque rpm at around 55 mph, since that's the speed I wanted to be going when climbing a moderate grade while loaded/towing in 3rd. My equation looked like this: (55mph x 4.56 x 336) / 33" = ~2550rpm (plus 250rpm for torque converter slip = 2800rpm). I'm going 55mph right around my peak torque rpm. I'm at ~2800rpm in second gear at 40mph for climbing steep grades. If have to go slower than that, I probably shouldn't be pulling it .

BMGBluz, What size tires you run will make a difference as to whether or not the OD trans would help your mileage. On my truck, when it had 3.55 gears with the 33" tires, every time the trans shifted into direct 3rd gear (1.00:1 ratio), it felt like it was in overdrive. The engine was turning too few rpms and had no power. I spent alot of time in second gear because of the high axle ratio. For me the lower axle ratio got my engine up into it's powerband, and now it would be nice to have overdrive to go a little faster on the freeway. Before, with the 3.55 gears, I had no use for an overdrive gear in the trans but the tradeoff was no power in 3rd. With smaller tires, the situation changes...

 

SoCalDesertRider
Thanks for the awesome info. I am copying that whole post if you dont mind and saving to Word. I know my Stangs but this big tire size and big gearing is a lot different.

Thanks,
Steve

 

SoCalDesertRider:
Correction needed on your last post, C-6, 1st 2.47, 2nd 1.43. Should be 1st 2.46, 2nd
1.46 gear ratio. Converter slip can go from 200-400 rpm depending on how much torque your engine produces and mileage on converter, more slip with age. Sorry, Carl.
.................................................. .............................................o&o>. ..............

 

Correction noted on the ratios. Though I have to say I have seen them listed both ways. Possibly different years? I agree on the converter slip range. I posted 250 rpm for ease of calculation and because that's about what it is on my C6 with 45K orig miles on it . Anyways, thanks for the input!

 

yeah, sounds a little to complicated (and risky) for me to do it, even with you guys behind me. I'll just take it to the shop and pay to have someone do it. Possibly whenever I get the front fuel pump replaced (bad when we bought it). I've got a shop we're friends with and, if this is the next thing it has done to it, it'll be a while since it was in there last... Plus starting a job at an auto parts store on Monday can't hurt, either.........