Speed vs Revs question

[lard]

Hi,

Maybe it's the heat but I'm having a senior moment :lol: - logic tells me that speed to revs is fixed, i.e. as you are using a particular gearing at 3500 revs, spinning the axles and turning the wheels, you will always go xx mph?

Then my mind started thinking that doesn't sound right? so obviously you can be coasting downhill and the wheels/gears are turning faster than the engine is, and then I thought about going back up hill and got lost!

Relates to a question I was going to post about my gearbox - so at 3800 revs I'm doing 50-55 mph - seems low to me as I had thought 3800 revs is cruising and it should cruise at 55mph-60mph?

Then my brother told me some gearbox types were for commercials, and have a shorter top gear to give more power at sacrifice of top end?

Doesn't any of the above make sense? I swear it's the humidity! I had a couple of tight valves when adjusted last week and swear it cruises slightly quicker now.....

I do doubt I've got the OG gearbox - van's a westie but the engine was a recon and the box looks just as shiny - any pointers on checking if the gearing theory is correct?

Thanks!

 

[BrotherNumberOne]

Assuming your clutch isn't slipping and you're not spinning the wheels( unlikely in a bay), road speed will always equate to the same engine revs in a given gear. It sounds more up hill because the engine is labouring.
Check the bottom of your 'box for a code, should start with a 'C', you can then check the ratios to see if you've got a commercial one.

 

[67westy]

If you have smaller rear tires that will also affect your gearing

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[lard]

If you have smaller rear tires that will also affect your gearing

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Hi - I have standard 185 R14 Commercials on all round

 

[Nugsy]

I've got 205/70*14 on the back and using gps, my bus is doing around 50-55 @ 3000rpm. But I'm with you. if I keep at that constant and hit a hill it slows down then the revs start to drop or at least I think they do and as your going up hill its harder to build the revs back up

 

[jonboylaw]

Assuming your clutch isn't slipping and you're not spinning the wheels( unlikely in a bay), road speed will always equate to the same engine revs in a given gear.

True, unless the tyre pressure changes




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[lard]

Thanks guys - took it away this weekend and makes sense to me (also pumped my tyres up ) - I'll check the code as suggested and get to the bottom of what ratios are in it!