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I have found a 6.50 gearing on a bluebird bus (1990). I presently have a 7.17. Is there much difference in the 2 ratios? Went on a 60 mile trip and got around 4 miles per gallon going 45 mph at 2900 to 3000 rpm..Not worried about the speed, just gas mileage...Thanks
6.50/1 is 90.6% of 7.17/1. The regear will allow you to cruise at 45mph at 2600 - 2700 rpm. If you still want to turn 2900-3000 rpm the 6.50 will increase your speed to 49.6 mph.
It's not really enough of a change to be worth the trouble.
As John says, a 4.86/1 might work better if you have the power to pull it. The 4.86 would theoretically get you out to 66.4mph at your 2900-3000 rpm. A more realistic cruise might be 55mph, which the 4.86/1 ration should do at 2400-2500 rpm. Unfortunately the higher gear will take away some of your takeoff and hill climbing ability.
You didn't mention what engine or trans you're running, so we can't really make an intelligent guess as to whether the 4.86/1 gear would work for you or not. If you're getting 4mpg and turning 3000 rpm I can assume you're burning gasoline (not diesel) and using an automatic transmission. If you have the horsepower to run with the taller gear it should improve your speed and your mpg, but without knowing what engine/trans you have, or how much manifold vacuum you're pulling, there is no way we can make a recommendation.
If you're running a smaller gas motor the poor little thing might be doing all it can do now. More info please. Manifold vacuum while cruising at 3000 rpm / 45 mph on flat, level road would be a big help.
The engine I am running is a 361 Dodge gas burner. I am running with a manual transmission. I have also found a 2 speed rear end also, but cant find out the gear ratios for this axle. This bus was never a school bus...it was a church bus with under storage and all. Its akso a 75 bluebird. Any idea how I can find out the gearing with it broke down and flat tires. I have looked for information plate inside the bus and under the hood. No plates on the differential either...Thanks for any help with my 4 mpg
I am thinking your 361 is going to be running about as hard as an engine can run already. I doubt it can pull much higher speeds.
Before you start spending a lot of time and $$$ making changes, invest in a manifold vacuum guage.
The maifold vacuum will tell you how hard your engine is working. If you are pulling less than 10" of manifold vacuum at full speed cruise, it is unlikely you will be able to go much faster, regardless of what rear end ratio you use. If you are running less than 5" of manifold vacuum at cruise you definitely will not be able to go any faster.
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