Micaiahfied

ok, when i had the stock 235/85's on the 91 (4.10 gears) i was going 78 accoring to the speedo and my brother behind (2001 civic) said he was reading just about 90.... i swapped up to 285/75 (should be about an inch taller) and was afraid i would be flying if i ran at the speed limit on the odometer.... but i passed like 10 of those things in construction zones, and they all said i was right on! the only thing i could think of what was i am keeping the 285's aired down a little to try to make them wear even (since they are on the stock 6 inch wide rim) anyone have thoughts?

DBF

Hmmm.... adjusting your tire size to make the speedometer read correctly is a new one.

Should do the trick, tho. Higher tire = greater distance per revolution, so the truck is going faster/farther at any speed. Seems to work out in your case.

My 92 with 3.54's goes the other way - at 65mph indicated, I can put a stopwatch on the milemarkers and get a mile a minute every time = 60mph real.

DBF

Micaiahfied

so a larger tire would fix yours (and make sence....)
the larger tire "fixed mine" and makes no sence, lol

DBF

One of us is thinking backwards, and it's entirely possible it's me.

Let me blather on for a bit & see if I find my problem.

Your speedometer was reading 78 when you think you were really doing closer to 90, in other words reading lower than real speed.

My speedometer reads 65 when it's really going 60, reading higher than real speed.

If I put on shorter tires, ones that don't go as far per revolution, the speedometer doesn't know the difference, and I go not quite as fast as I did before. So the speedometer might be right or at least closer to real speed.

Am I ok so far?

You put on taller tires, making the truck go faster than it did before, but the speedometer doesn't know this. Your speedometer now reads closer to real speed. The speed indicators you've run through seem to prove this.

Where am I going astray here?

DBF

kc8ksg

My only guess would be the Civic's speedo was off, ya might want to have him clock you again and see if it changed any from that. If you went to a larger tire and your speedo was already slow, it should have made it even slower.

Here's a gear calc it might help ya a little http://www.dfw-tx4wd.org/Tech-Dept/Tech-Dept/gears.html

Chris

Micaiahfied

DBF, the speedo (as far as i understand) reads the revolutions of the drive shaft. so the tach and speedo should stay the same no matter what tire you put on. if i put on a larger tire the motor does not have to spin as fast to attain the same speed... both tach and speedo would read slower at any given speed with larger tires on. on the other side of the coin, putting a smaller than stock tire on would mean the engine would have to spin faster to attain the same speed. therefor the speedo would read faster that the speed you are going.
i my case however, the speedo was already read slow (like the tires were larger than stock) and when i put the larger tires on it corrected the problem, the opposite of what it should have done. all that mess make sence?

kc8ksg, do you know if the getrag is listed on that page at all?

kc8ksg

I haven't seen it if it is, I usually just use the NV4500, I would think it would give a pretty good guesstiment.


Chris

tboling

My truck had 354.1 gears stock. The top speed is 92 to 95 mph, When the rear end went out, I found a good used set of 4.10's, the top speed was then 75 mph 80 was pushing it. That was with running 235/85/r16's, I now run 373's and the same tire size, the 410's are just to slow. By the way the rear end I'm running now is a Coreorata 14 bolt GM from a chevy dully. I have a dana 80 that's busted it's next after the rebuild.