I have a 93 B2600i with a 5speed manual tranny. 1st gear is so low that I rarely use it. I generaly don't use 4th gear either and go from 3rd to 5th without any lugging. So my 5 speed is basically a 3 or 4 speed depending on conditions. I also notice that 5th gear is not a true overdrive as it only drops the rpms by 200 to 300 rpms when shifting from 4th gear. Since the overall gearing is so low and I generally don't have any appreciable load, I'd like to go up in rim and tire size to more effectively use all 5 gears. I presently use the stock rims and tires P205/75R14 and am considering going up to a 16" rim. I don't care about the speedometer being off as the increase in gas mileage and overall driveability would off set it. Any advice on rim and tire sizes would be appreciated...

 

looked up on carpart.com
4x4 AT has gearing of 4.44
4x4 MT has 4.30
and the 4x2's has 3.727

also are you sure 14's my 91 4x4 (4.44 gears from an 88)has 15 inch rims running 33's and she lugs alittle in 1st.

if you do go with 16's what size tire? something 265/275 range to put it close to a 32/33?

 

Thanks MB,
I definitely have 14" alloy rims and the tires I'll probably use will be P215/70R16s as I don't want to increase the contact patch, just the diameter. I'm not sure what your reference to "33s" means though.

 

sizing of the tires.aspect ratio-overall diameter
a 31x10.5 r15 inch tire is approx 30-31 high where as a 235 75r15 may be closer to 29 inches.

example from bfg at's.
http://www.bfgoodrichtires.ca/specs/...t-a-ko/44.html

 

I'm not sure what you mean by "not a true overdrive". Why? Because the number of RPMs that an engine drops between gears has absolutely nothing to do with calling a gear as "overdrive". Consider the following:

• Any (and every) gear ratio that is "taller" than 1:1 is considered an "overdrive" gear
• Any gear ratio that is exactly 1:1 is considered a "drive" gear
• Any (and every) gear ratio that is "shorter" than 1:1 is considered an "underdrive" gear

FWIW, I've owned sever cars with 5-Speed transmissions where the top two gears were both "overdrive" gears.

My bet is that any difference in fuel economy would be so tiny that it would be virtually immeasurable.

 

question is: can i determine the rear gear ratio of my '87 b-2600 4x4 without dismantleing it? Manual trans, I converted it to 4.3 chevy powered, overdrive auto, put a nissan front axle in it, had the chevy driveshaft cut down 1 inch so it had room in the slip yoke. Speedometer is way off and I need the ratio of the rear end to get a new drive gear for the speedo.

 

If your transmission has a "Drive" gear (i.e. a 1:1 ratio), or of you know any or all of the specific ratios in the transmission, then you can work backwards to the final drive ratio.

 

Bill, whatever you do you have two different axles in your truck that are not likely compatable w/each other. Chances are pretty good that the Nissan axle and the Mazda axle are two different ratios, therefore, you need to determine which ratio is the one you want/need. Then worry about your speedo being off.
And remember that tire circumference will have a big affect on speedo readings too, so if you've installed bigger wheels/tires, you have to account for that as well.

What was the final drive ratio in the Chevy you got the drivetrain from? Then figure out tire dimensions, etc....

To me it sounds like you are going about this kinda' backwards. For one thing, are those skimpy axles going to be able to handle the extra power/torque from the V6 you put in? Do you plan on other power adding mods?

And you may have to settle for what speedo gears are available for the Chevy trans instead of exactly what you need.

 

Nissan ratio is 4.375' Mazda ratio is 4.30. Those are close enough to be operable. Speedometer is off 5 mph at 35 mph ( speedo reads 40 when traveling at 35). I checked the nissan ratio before I installed it so I had that information, several online stores have gears available for the speedo for around 30 dollars plus shipping. I selected the nissan axle because it is short enough to work and put the driveshaft on the drivers side where I needed it. If the axle ratios were too far off I do still have the nissan rear axle to use. the nissan motor was deystroyed or I would have fixed it instead of the mazda.

 

I would think you will still bind on solid ground or pavement. In a 4WD the front ratio is usually only slight taller than the rear, e.g. 4:32F-4:30R. Those gears are certainly not going to afford you very good mileage, but as an off-road toy it could be fun.

Also remember that speedo ratios are just that, ratios. If you are off by 5mph @ 35, the difference will be much greater when you reach 60-70mph.
I calculate when your speedo reads 60mph, your actual ground speed is 51.91mph. At 75mph, 64.86, and so on.....

You could also compensate by using different sized tires, in this case (edit: sorry) shorter tires?
I figure that by keeping the tread width the same, you would have to reduce the aspect ratio by approx 20%. A bizarre example would be: 205/80-17 - 205/55-17.